text: // SPDX-License-Identifier: GPL-2.0
  /*
   * INET		An implementation of the TCP/IP protocol suite for the LINUX
   *		operating system.  INET is implemented using the  BSD Socket
   *		interface as the means of communication with the user level.
   *
   *		Implementation of the Transmission Control Protocol(TCP).
   *
   * Authors:	Ross Biro
   *		Fred N. van Kempen, <waltje@uWalt.NL.Mugnet.ORG>
   *		Mark Evans, <evansmp@uhura.aston.ac.uk>
   *		Corey Minyard <wf-rch!minyard@relay.EU.net>
   *		Florian La Roche, <flla@stud.uni-sb.de>
   *		Charles Hedrick, <hedrick@klinzhai.rutgers.edu>
   *		Linus Torvalds, <torvalds@cs.helsinki.fi>
   *		Alan Cox, <gw4pts@gw4pts.ampr.org>
   *		Matthew Dillon, <dillon@apollo.west.oic.com>
   *		Arnt Gulbrandsen, <agulbra@nvg.unit.no>
   *		Jorge Cwik, <jorge@laser.satlink.net>
   */
  
  /*
   * Changes:
   *		Pedro Roque	:	Fast Retransmit/Recovery.
   *					Two receive queues.
   *					Retransmit queue handled by TCP.
   *					Better retransmit timer handling.
   *					New congestion avoidance.
   *					Header prediction.
   *					Variable renaming.
   *
   *		Eric		:	Fast Retransmit.
   *		Randy Scott	:	MSS option defines.
   *		Eric Schenk	:	Fixes to slow start algorithm.
   *		Eric Schenk	:	Yet another double ACK bug.
   *		Eric Schenk	:	Delayed ACK bug fixes.
   *		Eric Schenk	:	Floyd style fast retrans war avoidance.
   *		David S. Miller	:	Don't allow zero congestion window.
   *		Eric Schenk	:	Fix retransmitter so that it sends
   *					next packet on ack of previous packet.
   *		Andi Kleen	:	Moved open_request checking here
   *					and process RSTs for open_requests.
   *		Andi Kleen	:	Better prune_queue, and other fixes.
   *		Andrey Savochkin:	Fix RTT measurements in the presence of
   *					timestamps.
   *		Andrey Savochkin:	Check sequence numbers correctly when
   *					removing SACKs due to in sequence incoming
   *					data segments.
   *		Andi Kleen:		Make sure we never ack data there is not
   *					enough room for. Also make this condition
   *					a fatal error if it might still happen.
   *		Andi Kleen:		Add tcp_measure_rcv_mss to make
   *					connections with MSS<min(MTU,ann. MSS)
   *					work without delayed acks.
   *		Andi Kleen:		Process packets with PSH set in the
   *					fast path.
   *		J Hadi Salim:		ECN support
   *	 	Andrei Gurtov,
   *		Pasi Sarolahti,
   *		Panu Kuhlberg:		Experimental audit of TCP (re)transmission
   *					engine. Lots of bugs are found.
   *		Pasi Sarolahti:		F-RTO for dealing with spurious RTOs
   */
  
  #define pr_fmt(fmt) "TCP: " fmt
  
  #include <linux/mm.h>
  #include <linux/slab.h>
  #include <linux/module.h>
  #include <linux/sysctl.h>
  #include <linux/kernel.h>
  #include <linux/prefetch.h>
  #include <net/dst.h>
  #include <net/tcp.h>
  #include <net/inet_common.h>
  #include <linux/ipsec.h>
  #include <asm/unaligned.h>
  #include <linux/errqueue.h>
  #include <trace/events/tcp.h>
  #include <linux/static_key.h>
  
  int sysctl_tcp_max_orphans __read_mostly = NR_FILE;
  
  #define FLAG_DATA		0x01 /* Incoming frame contained data.		*/
  #define FLAG_WIN_UPDATE		0x02 /* Incoming ACK was a window update.	*/
  #define FLAG_DATA_ACKED		0x04 /* This ACK acknowledged new data.		*/
  #define FLAG_RETRANS_DATA_ACKED	0x08 /* "" "" some of which was retransmitted.	*/
  #define FLAG_SYN_ACKED		0x10 /* This ACK acknowledged SYN.		*/
  #define FLAG_DATA_SACKED	0x20 /* New SACK.				*/
  #define FLAG_ECE		0x40 /* ECE in this ACK				*/
  #define FLAG_LOST_RETRANS	0x80 /* This ACK marks some retransmission lost */
  #define FLAG_SLOWPATH		0x100 /* Do not skip RFC checks for window update.*/
  #define FLAG_ORIG_SACK_ACKED	0x200 /* Never retransmitted data are (s)acked	*/
  #define FLAG_SND_UNA_ADVANCED	0x400 /* Snd_una was changed (!= FLAG_DATA_ACKED) */
  #define FLAG_DSACKING_ACK	0x800 /* SACK blocks contained D-SACK info */
  #define FLAG_SET_XMIT_TIMER	0x1000 /* Set TLP or RTO timer */
  #define FLAG_SACK_RENEGING	0x2000 /* snd_una advanced to a sacked seq */
  #define FLAG_UPDATE_TS_RECENT	0x4000 /* tcp_replace_ts_recent() */
  #define FLAG_NO_CHALLENGE_ACK	0x8000 /* do not call tcp_send_challenge_ack()	*/
  #define FLAG_ACK_MAYBE_DELAYED	0x10000 /* Likely a delayed ACK */
  
  #define FLAG_ACKED		(FLAG_DATA_ACKED|FLAG_SYN_ACKED)
  #define FLAG_NOT_DUP		(FLAG_DATA|FLAG_WIN_UPDATE|FLAG_ACKED)
  #define FLAG_CA_ALERT		(FLAG_DATA_SACKED|FLAG_ECE|FLAG_DSACKING_ACK)
  #define FLAG_FORWARD_PROGRESS	(FLAG_ACKED|FLAG_DATA_SACKED)
  
  #define TCP_REMNANT (TCP_FLAG_FIN|TCP_FLAG_URG|TCP_FLAG_SYN|TCP_FLAG_PSH)
  #define TCP_HP_BITS (~(TCP_RESERVED_BITS|TCP_FLAG_PSH))
  
  #define REXMIT_NONE	0 /* no loss recovery to do */
  #define REXMIT_LOST	1 /* retransmit packets marked lost */
  #define REXMIT_NEW	2 /* FRTO-style transmit of unsent/new packets */
  
  #if IS_ENABLED(CONFIG_TLS_DEVICE)
  static DEFINE_STATIC_KEY_FALSE(clean_acked_data_enabled);
  
  void clean_acked_data_enable(struct inet_connection_sock *icsk,
  			     void (*cad)(struct sock *sk, u32 ack_seq))
  {
  	icsk->icsk_clean_acked = cad;
  	static_branch_inc(&clean_acked_data_enabled);
  }
  EXPORT_SYMBOL_GPL(clean_acked_data_enable);
  
  void clean_acked_data_disable(struct inet_connection_sock *icsk)
  {
  	static_branch_dec(&clean_acked_data_enabled);
  	icsk->icsk_clean_acked = NULL;
  }
  EXPORT_SYMBOL_GPL(clean_acked_data_disable);
  #endif
  
  static void tcp_gro_dev_warn(struct sock *sk, const struct sk_buff *skb,
  			     unsigned int len)
  {
  	static bool __once __read_mostly;
  
  	if (!__once) {
  		struct net_device *dev;
  
  		__once = true;
  
  		rcu_read_lock();
  		dev = dev_get_by_index_rcu(sock_net(sk), skb->skb_iif);
  		if (!dev || len >= dev->mtu)
  			pr_warn("%s: Driver has suspect GRO implementation, TCP performance may be compromised.\n",
  				dev ? dev->name : "Unknown driver");
  		rcu_read_unlock();
  	}
  }
  
  /* Adapt the MSS value used to make delayed ack decision to the
   * real world.
   */
  static void tcp_measure_rcv_mss(struct sock *sk, const struct sk_buff *skb)
  {
  	struct inet_connection_sock *icsk = inet_csk(sk);
  	const unsigned int lss = icsk->icsk_ack.last_seg_size;
  	unsigned int len;
  
  	icsk->icsk_ack.last_seg_size = 0;
  
  	/* skb->len may jitter because of SACKs, even if peer
  	 * sends good full-sized frames.
  	 */
  	len = skb_shinfo(skb)->gso_size ? : skb->len;
  	if (len >= icsk->icsk_ack.rcv_mss) {
  		icsk->icsk_ack.rcv_mss = min_t(unsigned int, len,
  					       tcp_sk(sk)->advmss);
  		/* Account for possibly-removed options */
  		if (unlikely(len > icsk->icsk_ack.rcv_mss +
  				   MAX_TCP_OPTION_SPACE))
  			tcp_gro_dev_warn(sk, skb, len);
  	} else {
  		/* Otherwise, we make more careful check taking into account,
  		 * that SACKs block is variable.
  		 *
  		 * "len" is invariant segment length, including TCP header.
  		 */
  		len += skb->data - skb_transport_header(skb);
  		if (len >= TCP_MSS_DEFAULT + sizeof(struct tcphdr) ||
  		    /* If PSH is not set, packet should be
  		     * full sized, provided peer TCP is not badly broken.
  		     * This observation (if it is correct 8)) allows
  		     * to handle super-low mtu links fairly.
  		     */
  		    (len >= TCP_MIN_MSS + sizeof(struct tcphdr) &&
  		     !(tcp_flag_word(tcp_hdr(skb)) & TCP_REMNANT))) {
  			/* Subtract also invariant (if peer is RFC compliant),
  			 * tcp header plus fixed timestamp option length.
  			 * Resulting "len" is MSS free of SACK jitter.
  			 */
  			len -= tcp_sk(sk)->tcp_header_len;
  			icsk->icsk_ack.last_seg_size = len;
  			if (len == lss) {
  				icsk->icsk_ack.rcv_mss = len;
  				return;
  			}
  		}
  		if (icsk->icsk_ack.pending & ICSK_ACK_PUSHED)
  			icsk->icsk_ack.pending |= ICSK_ACK_PUSHED2;
  		icsk->icsk_ack.pending |= ICSK_ACK_PUSHED;
  	}
  }
  
  static void tcp_incr_quickack(struct sock *sk, unsigned int max_quickacks)
  {
  	struct inet_connection_sock *icsk = inet_csk(sk);
  	unsigned int quickacks = tcp_sk(sk)->rcv_wnd / (2 * icsk->icsk_ack.rcv_mss);
  
  	if (quickacks == 0)
  		quickacks = 2;
  	quickacks = min(quickacks, max_quickacks);
  	if (quickacks > icsk->icsk_ack.quick)
  		icsk->icsk_ack.quick = quickacks;
  }
  
  static void tcp_enter_quickack_mode(struct sock *sk, unsigned int max_quickacks)
  {
  	struct inet_connection_sock *icsk = inet_csk(sk);
  
  	tcp_incr_quickack(sk, max_quickacks);
  	icsk->icsk_ack.pingpong = 0;
  	icsk->icsk_ack.ato = TCP_ATO_MIN;
  }
  
  /* Send ACKs quickly, if "quick" count is not exhausted
   * and the session is not interactive.
   */
  
  static bool tcp_in_quickack_mode(struct sock *sk)
  {
  	const struct inet_connection_sock *icsk = inet_csk(sk);
  	const struct dst_entry *dst = __sk_dst_get(sk);
  
  	return (dst && dst_metric(dst, RTAX_QUICKACK)) ||
  		(icsk->icsk_ack.quick && !icsk->icsk_ack.pingpong);
  }
  
  static void tcp_ecn_queue_cwr(struct tcp_sock *tp)
  {
  	if (tp->ecn_flags & TCP_ECN_OK)
  		tp->ecn_flags |= TCP_ECN_QUEUE_CWR;
  }
  
  static void tcp_ecn_accept_cwr(struct tcp_sock *tp, const struct sk_buff *skb)
  {
  	if (tcp_hdr(skb)->cwr)
  		tp->ecn_flags &= ~TCP_ECN_DEMAND_CWR;
  }
  
  static void tcp_ecn_withdraw_cwr(struct tcp_sock *tp)
  {
  	tp->ecn_flags &= ~TCP_ECN_DEMAND_CWR;
  }
  
  static void __tcp_ecn_check_ce(struct sock *sk, const struct sk_buff *skb)
  {
  	struct tcp_sock *tp = tcp_sk(sk);
  
  	switch (TCP_SKB_CB(skb)->ip_dsfield & INET_ECN_MASK) {
  	case INET_ECN_NOT_ECT:
  		/* Funny extension: if ECT is not set on a segment,
  		 * and we already seen ECT on a previous segment,
  		 * it is probably a retransmit.
  		 */
  		if (tp->ecn_flags & TCP_ECN_SEEN)
  			tcp_enter_quickack_mode(sk, 1);
  		break;
  	case INET_ECN_CE:
  		if (tcp_ca_needs_ecn(sk))
  			tcp_ca_event(sk, CA_EVENT_ECN_IS_CE);
  
  		if (!(tp->ecn_flags & TCP_ECN_DEMAND_CWR)) {
  			/* Better not delay acks, sender can have a very low cwnd */
  			tcp_enter_quickack_mode(sk, 1);
  			tp->ecn_flags |= TCP_ECN_DEMAND_CWR;
  		}
  		tp->ecn_flags |= TCP_ECN_SEEN;
  		break;
  	default:
  		if (tcp_ca_needs_ecn(sk))
  			tcp_ca_event(sk, CA_EVENT_ECN_NO_CE);
  		tp->ecn_flags |= TCP_ECN_SEEN;
  		break;
  	}
  }
  
  static void tcp_ecn_check_ce(struct sock *sk, const struct sk_buff *skb)
  {
  	if (tcp_sk(sk)->ecn_flags & TCP_ECN_OK)
  		__tcp_ecn_check_ce(sk, skb);
  }
  
  static void tcp_ecn_rcv_synack(struct tcp_sock *tp, const struct tcphdr *th)
  {
  	if ((tp->ecn_flags & TCP_ECN_OK) && (!th->ece || th->cwr))
  		tp->ecn_flags &= ~TCP_ECN_OK;
  }
  
  static void tcp_ecn_rcv_syn(struct tcp_sock *tp, const struct tcphdr *th)
  {
  	if ((tp->ecn_flags & TCP_ECN_OK) && (!th->ece || !th->cwr))
  		tp->ecn_flags &= ~TCP_ECN_OK;
  }
  
  static bool tcp_ecn_rcv_ecn_echo(const struct tcp_sock *tp, const struct tcphdr *th)
  {
  	if (th->ece && !th->syn && (tp->ecn_flags & TCP_ECN_OK))
  		return true;
  	return false;
  }
  
  /* Buffer size and advertised window tuning.
   *
   * 1. Tuning sk->sk_sndbuf, when connection enters established state.
   */
  
  static void tcp_sndbuf_expand(struct sock *sk)
  {
  	const struct tcp_sock *tp = tcp_sk(sk);
  	const struct tcp_congestion_ops *ca_ops = inet_csk(sk)->icsk_ca_ops;
  	int sndmem, per_mss;
  	u32 nr_segs;
  
  	/* Worst case is non GSO/TSO : each frame consumes one skb
  	 * and skb->head is kmalloced using power of two area of memory
  	 */
  	per_mss = max_t(u32, tp->rx_opt.mss_clamp, tp->mss_cache) +
  		  MAX_TCP_HEADER +
  		  SKB_DATA_ALIGN(sizeof(struct skb_shared_info));
  
  	per_mss = roundup_pow_of_two(per_mss) +
  		  SKB_DATA_ALIGN(sizeof(struct sk_buff));
  
  	nr_segs = max_t(u32, TCP_INIT_CWND, tp->snd_cwnd);
  	nr_segs = max_t(u32, nr_segs, tp->reordering + 1);
  
  	/* Fast Recovery (RFC 5681 3.2) :
  	 * Cubic needs 1.7 factor, rounded to 2 to include
  	 * extra cushion (application might react slowly to EPOLLOUT)
  	 */
  	sndmem = ca_ops->sndbuf_expand ? ca_ops->sndbuf_expand(sk) : 2;
  	sndmem *= nr_segs * per_mss;
  
  	if (sk->sk_sndbuf < sndmem)
  		sk->sk_sndbuf = min(sndmem, sock_net(sk)->ipv4.sysctl_tcp_wmem[2]);
  }
  
  /* 2. Tuning advertised window (window_clamp, rcv_ssthresh)
   *
   * All tcp_full_space() is split to two parts: "network" buffer, allocated
   * forward and advertised in receiver window (tp->rcv_wnd) and
   * "application buffer", required to isolate scheduling/application
   * latencies from network.
   * window_clamp is maximal advertised window. It can be less than
   * tcp_full_space(), in this case tcp_full_space() - window_clamp
   * is reserved for "application" buffer. The less window_clamp is
   * the smoother our behaviour from viewpoint of network, but the lower
   * throughput and the higher sensitivity of the connection to losses. 8)
   *
   * rcv_ssthresh is more strict window_clamp used at "slow start"
   * phase to predict further behaviour of this connection.
   * It is used for two goals:
   * - to enforce header prediction at sender, even when application
   *   requires some significant "application buffer". It is check #1.
   * - to prevent pruning of receive queue because of misprediction
   *   of receiver window. Check #2.
   *
   * The scheme does not work when sender sends good segments opening
   * window and then starts to feed us spaghetti. But it should work
   * in common situations. Otherwise, we have to rely on queue collapsing.
   */
  
  /* Slow part of check#2. */
  static int __tcp_grow_window(const struct sock *sk, const struct sk_buff *skb)
  {
  	struct tcp_sock *tp = tcp_sk(sk);
  	/* Optimize this! */
  	int truesize = tcp_win_from_space(sk, skb->truesize) >> 1;
  	int window = tcp_win_from_space(sk, sock_net(sk)->ipv4.sysctl_tcp_rmem[2]) >> 1;
  
  	while (tp->rcv_ssthresh <= window) {
  		if (truesize <= skb->len)
  			return 2 * inet_csk(sk)->icsk_ack.rcv_mss;
  
  		truesize >>= 1;
  		window >>= 1;
  	}
  	return 0;
  }
  
  static void tcp_grow_window(struct sock *sk, const struct sk_buff *skb)
  {
  	struct tcp_sock *tp = tcp_sk(sk);
  
  	/* Check #1 */
  	if (tp->rcv_ssthresh < tp->window_clamp &&
  	    (int)tp->rcv_ssthresh < tcp_space(sk) &&
  	    !tcp_under_memory_pressure(sk)) {
  		int incr;
  
  		/* Check #2. Increase window, if skb with such overhead
  		 * will fit to rcvbuf in future.
  		 */
  		if (tcp_win_from_space(sk, skb->truesize) <= skb->len)
  			incr = 2 * tp->advmss;
  		else
  			incr = __tcp_grow_window(sk, skb);
  
  		if (incr) {
  			incr = max_t(int, incr, 2 * skb->len);
  			tp->rcv_ssthresh = min(tp->rcv_ssthresh + incr,
  					       tp->window_clamp);
  			inet_csk(sk)->icsk_ack.quick |= 1;
  		}
  	}
  }
  
  /* 3. Tuning rcvbuf, when connection enters established state. */
  static void tcp_fixup_rcvbuf(struct sock *sk)
  {
  	u32 mss = tcp_sk(sk)->advmss;
  	int rcvmem;
  
  	rcvmem = 2 * SKB_TRUESIZE(mss + MAX_TCP_HEADER) *
  		 tcp_default_init_rwnd(mss);
  
  	/* Dynamic Right Sizing (DRS) has 2 to 3 RTT latency
  	 * Allow enough cushion so that sender is not limited by our window
  	 */
  	if (sock_net(sk)->ipv4.sysctl_tcp_moderate_rcvbuf)
  		rcvmem <<= 2;
  
  	if (sk->sk_rcvbuf < rcvmem)
  		sk->sk_rcvbuf = min(rcvmem, sock_net(sk)->ipv4.sysctl_tcp_rmem[2]);
  }
  
  /* 4. Try to fixup all. It is made immediately after connection enters
   *    established state.
   */
  void tcp_init_buffer_space(struct sock *sk)
  {
  	int tcp_app_win = sock_net(sk)->ipv4.sysctl_tcp_app_win;
  	struct tcp_sock *tp = tcp_sk(sk);
  	int maxwin;
  
  	if (!(sk->sk_userlocks & SOCK_RCVBUF_LOCK))
  		tcp_fixup_rcvbuf(sk);
  	if (!(sk->sk_userlocks & SOCK_SNDBUF_LOCK))
  		tcp_sndbuf_expand(sk);
  
  	tp->rcvq_space.space = tp->rcv_wnd;
  	tcp_mstamp_refresh(tp);
  	tp->rcvq_space.time = tp->tcp_mstamp;
  	tp->rcvq_space.seq = tp->copied_seq;
  
  	maxwin = tcp_full_space(sk);
  
  	if (tp->window_clamp >= maxwin) {
  		tp->window_clamp = maxwin;
  
  		if (tcp_app_win && maxwin > 4 * tp->advmss)
  			tp->window_clamp = max(maxwin -
  					       (maxwin >> tcp_app_win),
  					       4 * tp->advmss);
  	}
  
  	/* Force reservation of one segment. */
  	if (tcp_app_win &&
  	    tp->window_clamp > 2 * tp->advmss &&
  	    tp->window_clamp + tp->advmss > maxwin)
  		tp->window_clamp = max(2 * tp->advmss, maxwin - tp->advmss);
  
  	tp->rcv_ssthresh = min(tp->rcv_ssthresh, tp->window_clamp);
  	tp->snd_cwnd_stamp = tcp_jiffies32;
  }
  
  /* 5. Recalculate window clamp after socket hit its memory bounds. */
  static void tcp_clamp_window(struct sock *sk)
  {
  	struct tcp_sock *tp = tcp_sk(sk);
  	struct inet_connection_sock *icsk = inet_csk(sk);
  	struct net *net = sock_net(sk);
  
  	icsk->icsk_ack.quick = 0;
  
  	if (sk->sk_rcvbuf < net->ipv4.sysctl_tcp_rmem[2] &&
  	    !(sk->sk_userlocks & SOCK_RCVBUF_LOCK) &&
  	    !tcp_under_memory_pressure(sk) &&
  	    sk_memory_allocated(sk) < sk_prot_mem_limits(sk, 0)) {
  		sk->sk_rcvbuf = min(atomic_read(&sk->sk_rmem_alloc),
  				    net->ipv4.sysctl_tcp_rmem[2]);
  	}
  	if (atomic_read(&sk->sk_rmem_alloc) > sk->sk_rcvbuf)
  		tp->rcv_ssthresh = min(tp->window_clamp, 2U * tp->advmss);
  }
  
  /* Initialize RCV_MSS value.
   * RCV_MSS is an our guess about MSS used by the peer.
   * We haven't any direct information about the MSS.
   * It's better to underestimate the RCV_MSS rather than overestimate.
   * Overestimations make us ACKing less frequently than needed.
   * Underestimations are more easy to detect and fix by tcp_measure_rcv_mss().
   */
  void tcp_initialize_rcv_mss(struct sock *sk)
  {
  	const struct tcp_sock *tp = tcp_sk(sk);
  	unsigned int hint = min_t(unsigned int, tp->advmss, tp->mss_cache);
  
  	hint = min(hint, tp->rcv_wnd / 2);
  	hint = min(hint, TCP_MSS_DEFAULT);
  	hint = max(hint, TCP_MIN_MSS);
  
  	inet_csk(sk)->icsk_ack.rcv_mss = hint;
  }
  EXPORT_SYMBOL(tcp_initialize_rcv_mss);
  
  /* Receiver "autotuning" code.
   *
   * The algorithm for RTT estimation w/o timestamps is based on
   * Dynamic Right-Sizing (DRS) by Wu Feng and Mike Fisk of LANL.
   * <http://public.lanl.gov/radiant/pubs.html#DRS>
   *
   * More detail on this code can be found at
   * <http://staff.psc.edu/jheffner/>,
   * though this reference is out of date.  A new paper
   * is pending.
   */
  static void tcp_rcv_rtt_update(struct tcp_sock *tp, u32 sample, int win_dep)
  {
  	u32 new_sample = tp->rcv_rtt_est.rtt_us;
  	long m = sample;
  
  	if (new_sample != 0) {
  		/* If we sample in larger samples in the non-timestamp
  		 * case, we could grossly overestimate the RTT especially
  		 * with chatty applications or bulk transfer apps which
  		 * are stalled on filesystem I/O.
  		 *
  		 * Also, since we are only going for a minimum in the
  		 * non-timestamp case, we do not smooth things out
  		 * else with timestamps disabled convergence takes too
  		 * long.
  		 */
  		if (!win_dep) {
  			m -= (new_sample >> 3);
  			new_sample += m;
  		} else {
  			m <<= 3;
  			if (m < new_sample)
  				new_sample = m;
  		}
  	} else {
  		/* No previous measure. */
  		new_sample = m << 3;
  	}
  
  	tp->rcv_rtt_est.rtt_us = new_sample;
  }
  
  static inline void tcp_rcv_rtt_measure(struct tcp_sock *tp)
  {
  	u32 delta_us;
  
  	if (tp->rcv_rtt_est.time == 0)
  		goto new_measure;
  	if (before(tp->rcv_nxt, tp->rcv_rtt_est.seq))
  		return;
  	delta_us = tcp_stamp_us_delta(tp->tcp_mstamp, tp->rcv_rtt_est.time);
  	if (!delta_us)
  		delta_us = 1;
  	tcp_rcv_rtt_update(tp, delta_us, 1);
  
  new_measure:
  	tp->rcv_rtt_est.seq = tp->rcv_nxt + tp->rcv_wnd;
  	tp->rcv_rtt_est.time = tp->tcp_mstamp;
  }
  
  static inline void tcp_rcv_rtt_measure_ts(struct sock *sk,
  					  const struct sk_buff *skb)
  {
  	struct tcp_sock *tp = tcp_sk(sk);
  
  	if (tp->rx_opt.rcv_tsecr &&
  	    (TCP_SKB_CB(skb)->end_seq -
  	     TCP_SKB_CB(skb)->seq >= inet_csk(sk)->icsk_ack.rcv_mss)) {
  		u32 delta = tcp_time_stamp(tp) - tp->rx_opt.rcv_tsecr;
  		u32 delta_us;
  
  		if (!delta)
  			delta = 1;
  		delta_us = delta * (USEC_PER_SEC / TCP_TS_HZ);
  		tcp_rcv_rtt_update(tp, delta_us, 0);
  	}
  }
  
  /*
   * This function should be called every time data is copied to user space.
   * It calculates the appropriate TCP receive buffer space.
   */
  void tcp_rcv_space_adjust(struct sock *sk)
  {
  	struct tcp_sock *tp = tcp_sk(sk);
  	u32 copied;
  	int time;
  
  	trace_tcp_rcv_space_adjust(sk);
  
  	tcp_mstamp_refresh(tp);
  	time = tcp_stamp_us_delta(tp->tcp_mstamp, tp->rcvq_space.time);
  	if (time < (tp->rcv_rtt_est.rtt_us >> 3) || tp->rcv_rtt_est.rtt_us == 0)
  		return;
  
  	/* Number of bytes copied to user in last RTT */
  	copied = tp->copied_seq - tp->rcvq_space.seq;
  	if (copied <= tp->rcvq_space.space)
  		goto new_measure;
  
  	/* A bit of theory :
  	 * copied = bytes received in previous RTT, our base window
  	 * To cope with packet losses, we need a 2x factor
  	 * To cope with slow start, and sender growing its cwin by 100 %
  	 * every RTT, we need a 4x factor, because the ACK we are sending
  	 * now is for the next RTT, not the current one :
  	 * <prev RTT . ><current RTT .. ><next RTT .... >
  	 */
  
  	if (sock_net(sk)->ipv4.sysctl_tcp_moderate_rcvbuf &&
  	    !(sk->sk_userlocks & SOCK_RCVBUF_LOCK)) {
  		int rcvmem, rcvbuf;
  		u64 rcvwin, grow;
  
  		/* minimal window to cope with packet losses, assuming
  		 * steady state. Add some cushion because of small variations.
  		 */
  		rcvwin = ((u64)copied << 1) + 16 * tp->advmss;
  
  		/* Accommodate for sender rate increase (eg. slow start) */
  		grow = rcvwin * (copied - tp->rcvq_space.space);
  		do_div(grow, tp->rcvq_space.space);
  		rcvwin += (grow << 1);
  
  		rcvmem = SKB_TRUESIZE(tp->advmss + MAX_TCP_HEADER);
  		while (tcp_win_from_space(sk, rcvmem) < tp->advmss)
  			rcvmem += 128;
  
  		do_div(rcvwin, tp->advmss);
  		rcvbuf = min_t(u64, rcvwin * rcvmem,
  			       sock_net(sk)->ipv4.sysctl_tcp_rmem[2]);
  		if (rcvbuf > sk->sk_rcvbuf) {
  			sk->sk_rcvbuf = rcvbuf;
  
  			/* Make the window clamp follow along.  */
  			tp->window_clamp = tcp_win_from_space(sk, rcvbuf);
  		}
  	}
  	tp->rcvq_space.space = copied;
  
  new_measure:
  	tp->rcvq_space.seq = tp->copied_seq;
  	tp->rcvq_space.time = tp->tcp_mstamp;
  }
  
  /* There is something which you must keep in mind when you analyze the
   * behavior of the tp->ato delayed ack timeout interval.  When a
   * connection starts up, we want to ack as quickly as possible.  The
   * problem is that "good" TCP's do slow start at the beginning of data
   * transmission.  The means that until we send the first few ACK's the
   * sender will sit on his end and only queue most of his data, because
   * he can only send snd_cwnd unacked packets at any given time.  For
   * each ACK we send, he increments snd_cwnd and transmits more of his
   * queue.  -DaveM
   */
  static void tcp_event_data_recv(struct sock *sk, struct sk_buff *skb)
  {
  	struct tcp_sock *tp = tcp_sk(sk);
  	struct inet_connection_sock *icsk = inet_csk(sk);
  	u32 now;
  
  	inet_csk_schedule_ack(sk);
  
  	tcp_measure_rcv_mss(sk, skb);
  
  	tcp_rcv_rtt_measure(tp);
  
  	now = tcp_jiffies32;
  
  	if (!icsk->icsk_ack.ato) {
  		/* The _first_ data packet received, initialize
  		 * delayed ACK engine.
  		 */
  		tcp_incr_quickack(sk, TCP_MAX_QUICKACKS);
  		icsk->icsk_ack.ato = TCP_ATO_MIN;
  	} else {
  		int m = now - icsk->icsk_ack.lrcvtime;
  
  		if (m <= TCP_ATO_MIN / 2) {
  			/* The fastest case is the first. */
  			icsk->icsk_ack.ato = (icsk->icsk_ack.ato >> 1) + TCP_ATO_MIN / 2;
  		} else if (m < icsk->icsk_ack.ato) {
  			icsk->icsk_ack.ato = (icsk->icsk_ack.ato >> 1) + m;
  			if (icsk->icsk_ack.ato > icsk->icsk_rto)
  				icsk->icsk_ack.ato = icsk->icsk_rto;
  		} else if (m > icsk->icsk_rto) {
  			/* Too long gap. Apparently sender failed to
  			 * restart window, so that we send ACKs quickly.
  			 */
  			tcp_incr_quickack(sk, TCP_MAX_QUICKACKS);
  			sk_mem_reclaim(sk);
  		}
  	}
  	icsk->icsk_ack.lrcvtime = now;
  
  	tcp_ecn_check_ce(sk, skb);
  
  	if (skb->len >= 128)
  		tcp_grow_window(sk, skb);
  }
  
  /* Called to compute a smoothed rtt estimate. The data fed to this
   * routine either comes from timestamps, or from segments that were
   * known _not_ to have been retransmitted [see Karn/Partridge
   * Proceedings SIGCOMM 87]. The algorithm is from the SIGCOMM 88
   * piece by Van Jacobson.
   * NOTE: the next three routines used to be one big routine.
   * To save cycles in the RFC 1323 implementation it was better to break
   * it up into three procedures. -- erics
   */
  static void tcp_rtt_estimator(struct sock *sk, long mrtt_us)
  {
  	struct tcp_sock *tp = tcp_sk(sk);
  	long m = mrtt_us; /* RTT */
  	u32 srtt = tp->srtt_us;
  
  	/*	The following amusing code comes from Jacobson's
  	 *	article in SIGCOMM '88.  Note that rtt and mdev
  	 *	are scaled versions of rtt and mean deviation.
  	 *	This is designed to be as fast as possible
  	 *	m stands for "measurement".
  	 *
  	 *	On a 1990 paper the rto value is changed to:
  	 *	RTO = rtt + 4 * mdev
  	 *
  	 * Funny. This algorithm seems to be very broken.
  	 * These formulae increase RTO, when it should be decreased, increase
  	 * too slowly, when it should be increased quickly, decrease too quickly
  	 * etc. I guess in BSD RTO takes ONE value, so that it is absolutely
  	 * does not matter how to _calculate_ it. Seems, it was trap
  	 * that VJ failed to avoid. 8)
  	 */
  	if (srtt != 0) {
  		m -= (srtt >> 3);	/* m is now error in rtt est */
  		srtt += m;		/* rtt = 7/8 rtt + 1/8 new */
  		if (m < 0) {
  			m = -m;		/* m is now abs(error) */
  			m -= (tp->mdev_us >> 2);   /* similar update on mdev */
  			/* This is similar to one of Eifel findings.
  			 * Eifel blocks mdev updates when rtt decreases.
  			 * This solution is a bit different: we use finer gain
  			 * for mdev in this case (alpha*beta).
  			 * Like Eifel it also prevents growth of rto,
  			 * but also it limits too fast rto decreases,
  			 * happening in pure Eifel.
  			 */
  			if (m > 0)
  				m >>= 3;
  		} else {
  			m -= (tp->mdev_us >> 2);   /* similar update on mdev */
  		}
  		tp->mdev_us += m;		/* mdev = 3/4 mdev + 1/4 new */
  		if (tp->mdev_us > tp->mdev_max_us) {
  			tp->mdev_max_us = tp->mdev_us;
  			if (tp->mdev_max_us > tp->rttvar_us)
  				tp->rttvar_us = tp->mdev_max_us;
  		}
  		if (after(tp->snd_una, tp->rtt_seq)) {
  			if (tp->mdev_max_us < tp->rttvar_us)
  				tp->rttvar_us -= (tp->rttvar_us - tp->mdev_max_us) >> 2;
  			tp->rtt_seq = tp->snd_nxt;
  			tp->mdev_max_us = tcp_rto_min_us(sk);
  		}
  	} else {
  		/* no previous measure. */
  		srtt = m << 3;		/* take the measured time to be rtt */
  		tp->mdev_us = m << 1;	/* make sure rto = 3*rtt */
  		tp->rttvar_us = max(tp->mdev_us, tcp_rto_min_us(sk));
  		tp->mdev_max_us = tp->rttvar_us;
  		tp->rtt_seq = tp->snd_nxt;
  	}
  	tp->srtt_us = max(1U, srtt);
  }
  
  static void tcp_update_pacing_rate(struct sock *sk)
  {
  	const struct tcp_sock *tp = tcp_sk(sk);
  	u64 rate;
  
  	/* set sk_pacing_rate to 200 % of current rate (mss * cwnd / srtt) */
  	rate = (u64)tp->mss_cache * ((USEC_PER_SEC / 100) << 3);
  
  	/* current rate is (cwnd * mss) / srtt
  	 * In Slow Start [1], set sk_pacing_rate to 200 % the current rate.
  	 * In Congestion Avoidance phase, set it to 120 % the current rate.
  	 *
  	 * [1] : Normal Slow Start condition is (tp->snd_cwnd < tp->snd_ssthresh)
  	 *	 If snd_cwnd >= (tp->snd_ssthresh / 2), we are approaching
  	 *	 end of slow start and should slow down.
  	 */
  	if (tp->snd_cwnd < tp->snd_ssthresh / 2)
  		rate *= sock_net(sk)->ipv4.sysctl_tcp_pacing_ss_ratio;
  	else
  		rate *= sock_net(sk)->ipv4.sysctl_tcp_pacing_ca_ratio;
  
  	rate *= max(tp->snd_cwnd, tp->packets_out);
  
  	if (likely(tp->srtt_us))
  		do_div(rate, tp->srtt_us);
  
  	/* WRITE_ONCE() is needed because sch_fq fetches sk_pacing_rate
  	 * without any lock. We want to make sure compiler wont store
  	 * intermediate values in this location.
  	 */
  	WRITE_ONCE(sk->sk_pacing_rate, min_t(u64, rate,
  					     sk->sk_max_pacing_rate));
  }
  
  /* Calculate rto without backoff.  This is the second half of Van Jacobson's
   * routine referred to above.
   */
  static void tcp_set_rto(struct sock *sk)
  {
  	const struct tcp_sock *tp = tcp_sk(sk);
  	/* Old crap is replaced with new one. 8)
  	 *
  	 * More seriously:
  	 * 1. If rtt variance happened to be less 50msec, it is hallucination.
  	 *    It cannot be less due to utterly erratic ACK generation made
  	 *    at least by solaris and freebsd. "Erratic ACKs" has _nothing_
  	 *    to do with delayed acks, because at cwnd>2 true delack timeout
  	 *    is invisible. Actually, Linux-2.4 also generates erratic
  	 *    ACKs in some circumstances.
  	 */
  	inet_csk(sk)->icsk_rto = __tcp_set_rto(tp);
  
  	/* 2. Fixups made earlier cannot be right.
  	 *    If we do not estimate RTO correctly without them,
  	 *    all the algo is pure shit and should be replaced
  	 *    with correct one. It is exactly, which we pretend to do.
  	 */
  
  	/* NOTE: clamping at TCP_RTO_MIN is not required, current algo
  	 * guarantees that rto is higher.
  	 */
  	tcp_bound_rto(sk);
  }
  
  __u32 tcp_init_cwnd(const struct tcp_sock *tp, const struct dst_entry *dst)
  {
  	__u32 cwnd = (dst ? dst_metric(dst, RTAX_INITCWND) : 0);
  
  	if (!cwnd)
  		cwnd = TCP_INIT_CWND;
  	return min_t(__u32, cwnd, tp->snd_cwnd_clamp);
  }
  
  /* Take a notice that peer is sending D-SACKs */
  static void tcp_dsack_seen(struct tcp_sock *tp)
  {
  	tp->rx_opt.sack_ok |= TCP_DSACK_SEEN;
  	tp->rack.dsack_seen = 1;
  }
  
  /* It's reordering when higher sequence was delivered (i.e. sacked) before
   * some lower never-retransmitted sequence ("low_seq"). The maximum reordering
   * distance is approximated in full-mss packet distance ("reordering").
   */
  static void tcp_check_sack_reordering(struct sock *sk, const u32 low_seq,
  				      const int ts)
  {
  	struct tcp_sock *tp = tcp_sk(sk);
  	const u32 mss = tp->mss_cache;
  	u32 fack, metric;
  
  	fack = tcp_highest_sack_seq(tp);
  	if (!before(low_seq, fack))
  		return;
  
  	metric = fack - low_seq;
  	if ((metric > tp->reordering * mss) && mss) {
  #if FASTRETRANS_DEBUG > 1
  		pr_debug("Disorder%d %d %u f%u s%u rr%d\n",
  			 tp->rx_opt.sack_ok, inet_csk(sk)->icsk_ca_state,
  			 tp->reordering,
  			 0,
  			 tp->sacked_out,
  			 tp->undo_marker ? tp->undo_retrans : 0);
  #endif
  		tp->reordering = min_t(u32, (metric + mss - 1) / mss,
  				       sock_net(sk)->ipv4.sysctl_tcp_max_reordering);
  	}
  
  	tp->rack.reord = 1;
  	/* This exciting event is worth to be remembered. 8) */
  	NET_INC_STATS(sock_net(sk),
  		      ts ? LINUX_MIB_TCPTSREORDER : LINUX_MIB_TCPSACKREORDER);
  }
  
  /* This must be called before lost_out is incremented */
  static void tcp_verify_retransmit_hint(struct tcp_sock *tp, struct sk_buff *skb)
  {
  	if (!tp->retransmit_skb_hint ||
  	    before(TCP_SKB_CB(skb)->seq,
  		   TCP_SKB_CB(tp->retransmit_skb_hint)->seq))
  		tp->retransmit_skb_hint = skb;
  }
  
  /* Sum the number of packets on the wire we have marked as lost.
   * There are two cases we care about here:
   * a) Packet hasn't been marked lost (nor retransmitted),
   *    and this is the first loss.
   * b) Packet has been marked both lost and retransmitted,
   *    and this means we think it was lost again.
   */
  static void tcp_sum_lost(struct tcp_sock *tp, struct sk_buff *skb)
  {
  	__u8 sacked = TCP_SKB_CB(skb)->sacked;
  
  	if (!(sacked & TCPCB_LOST) ||
  	    ((sacked & TCPCB_LOST) && (sacked & TCPCB_SACKED_RETRANS)))
  		tp->lost += tcp_skb_pcount(skb);
  }
  
  static void tcp_skb_mark_lost(struct tcp_sock *tp, struct sk_buff *skb)
  {
  	if (!(TCP_SKB_CB(skb)->sacked & (TCPCB_LOST|TCPCB_SACKED_ACKED))) {
  		tcp_verify_retransmit_hint(tp, skb);
  
  		tp->lost_out += tcp_skb_pcount(skb);
  		tcp_sum_lost(tp, skb);
  		TCP_SKB_CB(skb)->sacked |= TCPCB_LOST;
  	}
  }
  
  void tcp_skb_mark_lost_uncond_verify(struct tcp_sock *tp, struct sk_buff *skb)
  {
  	tcp_verify_retransmit_hint(tp, skb);
  
  	tcp_sum_lost(tp, skb);
  	if (!(TCP_SKB_CB(skb)->sacked & (TCPCB_LOST|TCPCB_SACKED_ACKED))) {
  		tp->lost_out += tcp_skb_pcount(skb);
  		TCP_SKB_CB(skb)->sacked |= TCPCB_LOST;
  	}
  }
  
  /* This procedure tags the retransmission queue when SACKs arrive.
   *
   * We have three tag bits: SACKED(S), RETRANS(R) and LOST(L).
   * Packets in queue with these bits set are counted in variables
   * sacked_out, retrans_out and lost_out, correspondingly.
   *
   * Valid combinations are:
   * Tag  InFlight	Description
   * 0	1		- orig segment is in flight.
   * S	0		- nothing flies, orig reached receiver.
   * L	0		- nothing flies, orig lost by net.
   * R	2		- both orig and retransmit are in flight.
   * L|R	1		- orig is lost, retransmit is in flight.
   * S|R  1		- orig reached receiver, retrans is still in flight.
   * (L|S|R is logically valid, it could occur when L|R is sacked,
   *  but it is equivalent to plain S and code short-curcuits it to S.
   *  L|S is logically invalid, it would mean -1 packet in flight 8))
   *
   * These 6 states form finite state machine, controlled by the following events:
   * 1. New ACK (+SACK) arrives. (tcp_sacktag_write_queue())
   * 2. Retransmission. (tcp_retransmit_skb(), tcp_xmit_retransmit_queue())
   * 3. Loss detection event of two flavors:
   *	A. Scoreboard estimator decided the packet is lost.
   *	   A'. Reno "three dupacks" marks head of queue lost.
   *	B. SACK arrives sacking SND.NXT at the moment, when the
   *	   segment was retransmitted.
   * 4. D-SACK added new rule: D-SACK changes any tag to S.
   *
   * It is pleasant to note, that state diagram turns out to be commutative,
   * so that we are allowed not to be bothered by order of our actions,
   * when multiple events arrive simultaneously. (see the function below).
   *
   * Reordering detection.
   * --------------------
   * Reordering metric is maximal distance, which a packet can be displaced
   * in packet stream. With SACKs we can estimate it:
   *
   * 1. SACK fills old hole and the corresponding segment was not
   *    ever retransmitted -> reordering. Alas, we cannot use it
   *    when segment was retransmitted.
   * 2. The last flaw is solved with D-SACK. D-SACK arrives
   *    for retransmitted and already SACKed segment -> reordering..
   * Both of these heuristics are not used in Loss state, when we cannot
   * account for retransmits accurately.
   *
   * SACK block validation.
   * ----------------------
   *
   * SACK block range validation checks that the received SACK block fits to
   * the expected sequence limits, i.e., it is between SND.UNA and SND.NXT.
   * Note that SND.UNA is not included to the range though being valid because
   * it means that the receiver is rather inconsistent with itself reporting
   * SACK reneging when it should advance SND.UNA. Such SACK block this is
   * perfectly valid, however, in light of RFC2018 which explicitly states
   * that "SACK block MUST reflect the newest segment.  Even if the newest
   * segment is going to be discarded ...", not that it looks very clever
   * in case of head skb. Due to potentional receiver driven attacks, we
   * choose to avoid immediate execution of a walk in write queue due to
   * reneging and defer head skb's loss recovery to standard loss recovery
   * procedure that will eventually trigger (nothing forbids us doing this).
   *
   * Implements also blockage to start_seq wrap-around. Problem lies in the
   * fact that though start_seq (s) is before end_seq (i.e., not reversed),
   * there's no guarantee that it will be before snd_nxt (n). The problem
   * happens when start_seq resides between end_seq wrap (e_w) and snd_nxt
   * wrap (s_w):
   *
   *         <- outs wnd ->                          <- wrapzone ->
   *         u     e      n                         u_w   e_w  s n_w
   *         |     |      |                          |     |   |  |
   * |<------------+------+----- TCP seqno space --------------+---------->|
   * ...-- <2^31 ->|                                           |<--------...
   * ...---- >2^31 ------>|                                    |<--------...
   *
   * Current code wouldn't be vulnerable but it's better still to discard such
   * crazy SACK blocks. Doing this check for start_seq alone closes somewhat
   * similar case (end_seq after snd_nxt wrap) as earlier reversed check in
   * snd_nxt wrap -> snd_una region will then become "well defined", i.e.,
   * equal to the ideal case (infinite seqno space without wrap caused issues).
   *
   * With D-SACK the lower bound is extended to cover sequence space below
   * SND.UNA down to undo_marker, which is the last point of interest. Yet
   * again, D-SACK block must not to go across snd_una (for the same reason as
   * for the normal SACK blocks, explained above). But there all simplicity
   * ends, TCP might receive valid D-SACKs below that. As long as they reside
   * fully below undo_marker they do not affect behavior in anyway and can
   * therefore be safely ignored. In rare cases (which are more or less
   * theoretical ones), the D-SACK will nicely cross that boundary due to skb
   * fragmentation and packet reordering past skb's retransmission. To consider
   * them correctly, the acceptable range must be extended even more though
   * the exact amount is rather hard to quantify. However, tp->max_window can
   * be used as an exaggerated estimate.
   */
  static bool tcp_is_sackblock_valid(struct tcp_sock *tp, bool is_dsack,
  				   u32 start_seq, u32 end_seq)
  {
  	/* Too far in future, or reversed (interpretation is ambiguous) */
  	if (after(end_seq, tp->snd_nxt) || !before(start_seq, end_seq))
  		return false;
  
  	/* Nasty start_seq wrap-around check (see comments above) */
  	if (!before(start_seq, tp->snd_nxt))
  		return false;
  
  	/* In outstanding window? ...This is valid exit for D-SACKs too.
  	 * start_seq == snd_una is non-sensical (see comments above)
  	 */
  	if (after(start_seq, tp->snd_una))
  		return true;
  
  	if (!is_dsack || !tp->undo_marker)
  		return false;
  
  	/* ...Then it's D-SACK, and must reside below snd_una completely */
  	if (after(end_seq, tp->snd_una))
  		return false;
  
  	if (!before(start_seq, tp->undo_marker))
  		return true;
  
  	/* Too old */
  	if (!after(end_seq, tp->undo_marker))
  		return false;
  
  	/* Undo_marker boundary crossing (overestimates a lot). Known already:
  	 *   start_seq < undo_marker and end_seq >= undo_marker.
  	 */
  	return !before(start_seq, end_seq - tp->max_window);
  }
  
  static bool tcp_check_dsack(struct sock *sk, const struct sk_buff *ack_skb,
  			    struct tcp_sack_block_wire *sp, int num_sacks,
  			    u32 prior_snd_una)
  {
  	struct tcp_sock *tp = tcp_sk(sk);
  	u32 start_seq_0 = get_unaligned_be32(&sp[0].start_seq);
  	u32 end_seq_0 = get_unaligned_be32(&sp[0].end_seq);
  	bool dup_sack = false;
  
  	if (before(start_seq_0, TCP_SKB_CB(ack_skb)->ack_seq)) {
  		dup_sack = true;
  		tcp_dsack_seen(tp);
  		NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPDSACKRECV);
  	} else if (num_sacks > 1) {
  		u32 end_seq_1 = get_unaligned_be32(&sp[1].end_seq);
  		u32 start_seq_1 = get_unaligned_be32(&sp[1].start_seq);
  
  		if (!after(end_seq_0, end_seq_1) &&
  		    !before(start_seq_0, start_seq_1)) {
  			dup_sack = true;
  			tcp_dsack_seen(tp);
  			NET_INC_STATS(sock_net(sk),
  					LINUX_MIB_TCPDSACKOFORECV);
  		}
  	}
  
  	/* D-SACK for already forgotten data... Do dumb counting. */
  	if (dup_sack && tp->undo_marker && tp->undo_retrans > 0 &&
  	    !after(end_seq_0, prior_snd_una) &&
  	    after(end_seq_0, tp->undo_marker))
  		tp->undo_retrans--;
  
  	return dup_sack;
  }
  
  struct tcp_sacktag_state {
  	u32	reord;
  	/* Timestamps for earliest and latest never-retransmitted segment
  	 * that was SACKed. RTO needs the earliest RTT to stay conservative,
  	 * but congestion control should still get an accurate delay signal.
  	 */
  	u64	first_sackt;
  	u64	last_sackt;
  	struct rate_sample *rate;
  	int	flag;
  	unsigned int mss_now;
  };
  
  /* Check if skb is fully within the SACK block. In presence of GSO skbs,
   * the incoming SACK may not exactly match but we can find smaller MSS
   * aligned portion of it that matches. Therefore we might need to fragment
   * which may fail and creates some hassle (caller must handle error case
   * returns).
   *
   * FIXME: this could be merged to shift decision code
   */
  static int tcp_match_skb_to_sack(struct sock *sk, struct sk_buff *skb,
  				  u32 start_seq, u32 end_seq)
  {
  	int err;
  	bool in_sack;
  	unsigned int pkt_len;
  	unsigned int mss;
  
  	in_sack = !after(start_seq, TCP_SKB_CB(skb)->seq) &&
  		  !before(end_seq, TCP_SKB_CB(skb)->end_seq);
  
  	if (tcp_skb_pcount(skb) > 1 && !in_sack &&
  	    after(TCP_SKB_CB(skb)->end_seq, start_seq)) {
  		mss = tcp_skb_mss(skb);
  		in_sack = !after(start_seq, TCP_SKB_CB(skb)->seq);
  
  		if (!in_sack) {
  			pkt_len = start_seq - TCP_SKB_CB(skb)->seq;
  			if (pkt_len < mss)
  				pkt_len = mss;
  		} else {
  			pkt_len = end_seq - TCP_SKB_CB(skb)->seq;
  			if (pkt_len < mss)
  				return -EINVAL;
  		}
  
  		/* Round if necessary so that SACKs cover only full MSSes
  		 * and/or the remaining small portion (if present)
  		 */
  		if (pkt_len > mss) {
  			unsigned int new_len = (pkt_len / mss) * mss;
  			if (!in_sack && new_len < pkt_len)
  				new_len += mss;
  			pkt_len = new_len;
  		}
  
  		if (pkt_len >= skb->len && !in_sack)
  			return 0;
  
  		err = tcp_fragment(sk, TCP_FRAG_IN_RTX_QUEUE, skb,
  				   pkt_len, mss, GFP_ATOMIC);
  		if (err < 0)
  			return err;
  	}
  
  	return in_sack;
  }
  
  /* Mark the given newly-SACKed range as such, adjusting counters and hints. */
  static u8 tcp_sacktag_one(struct sock *sk,
  			  struct tcp_sacktag_state *state, u8 sacked,
  			  u32 start_seq, u32 end_seq,
  			  int dup_sack, int pcount,
  			  u64 xmit_time)
  {
  	struct tcp_sock *tp = tcp_sk(sk);
  
  	/* Account D-SACK for retransmitted packet. */
  	if (dup_sack && (sacked & TCPCB_RETRANS)) {
  		if (tp->undo_marker && tp->undo_retrans > 0 &&
  		    after(end_seq, tp->undo_marker))
  			tp->undo_retrans--;
  		if ((sacked & TCPCB_SACKED_ACKED) &&
  		    before(start_seq, state->reord))
  				state->reord = start_seq;
  	}
  
  	/* Nothing to do; acked frame is about to be dropped (was ACKed). */
  	if (!after(end_seq, tp->snd_una))
  		return sacked;
  
  	if (!(sacked & TCPCB_SACKED_ACKED)) {
  		tcp_rack_advance(tp, sacked, end_seq, xmit_time);
  
  		if (sacked & TCPCB_SACKED_RETRANS) {
  			/* If the segment is not tagged as lost,
  			 * we do not clear RETRANS, believing
  			 * that retransmission is still in flight.
  			 */
  			if (sacked & TCPCB_LOST) {
  				sacked &= ~(TCPCB_LOST|TCPCB_SACKED_RETRANS);
  				tp->lost_out -= pcount;
  				tp->retrans_out -= pcount;
  			}
  		} else {
  			if (!(sacked & TCPCB_RETRANS)) {
  				/* New sack for not retransmitted frame,
  				 * which was in hole. It is reordering.
  				 */
  				if (before(start_seq,
  					   tcp_highest_sack_seq(tp)) &&
  				    before(start_seq, state->reord))
  					state->reord = start_seq;
  
  				if (!after(end_seq, tp->high_seq))
  					state->flag |= FLAG_ORIG_SACK_ACKED;
  				if (state->first_sackt == 0)
  					state->first_sackt = xmit_time;
  				state->last_sackt = xmit_time;
  			}
  
  			if (sacked & TCPCB_LOST) {
  				sacked &= ~TCPCB_LOST;
  				tp->lost_out -= pcount;
  			}
  		}
  
  		sacked |= TCPCB_SACKED_ACKED;
  		state->flag |= FLAG_DATA_SACKED;
  		tp->sacked_out += pcount;
  		tp->delivered += pcount;  /* Out-of-order packets delivered */
  
  		/* Lost marker hint past SACKed? Tweak RFC3517 cnt */
  		if (tp->lost_skb_hint &&
  		    before(start_seq, TCP_SKB_CB(tp->lost_skb_hint)->seq))
  			tp->lost_cnt_hint += pcount;
  	}
  
  	/* D-SACK. We can detect redundant retransmission in S|R and plain R
  	 * frames and clear it. undo_retrans is decreased above, L|R frames
  	 * are accounted above as well.
  	 */
  	if (dup_sack && (sacked & TCPCB_SACKED_RETRANS)) {
  		sacked &= ~TCPCB_SACKED_RETRANS;
  		tp->retrans_out -= pcount;
  	}
  
  	return sacked;
  }
  
  /* Shift newly-SACKed bytes from this skb to the immediately previous
   * already-SACKed sk_buff. Mark the newly-SACKed bytes as such.
   */
  static bool tcp_shifted_skb(struct sock *sk, struct sk_buff *prev,
  			    struct sk_buff *skb,
  			    struct tcp_sacktag_state *state,
  			    unsigned int pcount, int shifted, int mss,
  			    bool dup_sack)
  {
  	struct tcp_sock *tp = tcp_sk(sk);
  	u32 start_seq = TCP_SKB_CB(skb)->seq;	/* start of newly-SACKed */
  	u32 end_seq = start_seq + shifted;	/* end of newly-SACKed */
  
  	BUG_ON(!pcount);
  
  	/* Adjust counters and hints for the newly sacked sequence
  	 * range but discard the return value since prev is already
  	 * marked. We must tag the range first because the seq
  	 * advancement below implicitly advances
  	 * tcp_highest_sack_seq() when skb is highest_sack.
  	 */
  	tcp_sacktag_one(sk, state, TCP_SKB_CB(skb)->sacked,
  			start_seq, end_seq, dup_sack, pcount,
  			skb->skb_mstamp);
  	tcp_rate_skb_delivered(sk, skb, state->rate);
  
  	if (skb == tp->lost_skb_hint)
  		tp->lost_cnt_hint += pcount;
  
  	TCP_SKB_CB(prev)->end_seq += shifted;
  	TCP_SKB_CB(skb)->seq += shifted;
  
  	tcp_skb_pcount_add(prev, pcount);
  	BUG_ON(tcp_skb_pcount(skb) < pcount);
  	tcp_skb_pcount_add(skb, -pcount);
  
  	/* When we're adding to gso_segs == 1, gso_size will be zero,
  	 * in theory this shouldn't be necessary but as long as DSACK
  	 * code can come after this skb later on it's better to keep
  	 * setting gso_size to something.
  	 */
  	if (!TCP_SKB_CB(prev)->tcp_gso_size)
  		TCP_SKB_CB(prev)->tcp_gso_size = mss;
  
  	/* CHECKME: To clear or not to clear? Mimics normal skb currently */
  	if (tcp_skb_pcount(skb) <= 1)
  		TCP_SKB_CB(skb)->tcp_gso_size = 0;
  
  	/* Difference in this won't matter, both ACKed by the same cumul. ACK */
  	TCP_SKB_CB(prev)->sacked |= (TCP_SKB_CB(skb)->sacked & TCPCB_EVER_RETRANS);
  
  	if (skb->len > 0) {
  		BUG_ON(!tcp_skb_pcount(skb));
  		NET_INC_STATS(sock_net(sk), LINUX_MIB_SACKSHIFTED);
  		return false;
  	}
  
  	/* Whole SKB was eaten :-) */
  
  	if (skb == tp->retransmit_skb_hint)
  		tp->retransmit_skb_hint = prev;
  	if (skb == tp->lost_skb_hint) {
  		tp->lost_skb_hint = prev;
  		tp->lost_cnt_hint -= tcp_skb_pcount(prev);
  	}
  
  	TCP_SKB_CB(prev)->tcp_flags |= TCP_SKB_CB(skb)->tcp_flags;
  	TCP_SKB_CB(prev)->eor = TCP_SKB_CB(skb)->eor;
  	if (TCP_SKB_CB(skb)->tcp_flags & TCPHDR_FIN)
  		TCP_SKB_CB(prev)->end_seq++;
  
  	if (skb == tcp_highest_sack(sk))
  		tcp_advance_highest_sack(sk, skb);
  
  	tcp_skb_collapse_tstamp(prev, skb);
  	if (unlikely(TCP_SKB_CB(prev)->tx.delivered_mstamp))
  		TCP_SKB_CB(prev)->tx.delivered_mstamp = 0;
  
  	tcp_rtx_queue_unlink_and_free(skb, sk);
  
  	NET_INC_STATS(sock_net(sk), LINUX_MIB_SACKMERGED);
  
  	return true;
  }
  
  /* I wish gso_size would have a bit more sane initialization than
   * something-or-zero which complicates things
   */
  static int tcp_skb_seglen(const struct sk_buff *skb)
  {
  	return tcp_skb_pcount(skb) == 1 ? skb->len : tcp_skb_mss(skb);
  }
  
  /* Shifting pages past head area doesn't work */
  static int skb_can_shift(const struct sk_buff *skb)
  {
  	return !skb_headlen(skb) && skb_is_nonlinear(skb);
  }
  
  /* Try collapsing SACK blocks spanning across multiple skbs to a single
   * skb.
   */
  static struct sk_buff *tcp_shift_skb_data(struct sock *sk, struct sk_buff *skb,
  					  struct tcp_sacktag_state *state,
  					  u32 start_seq, u32 end_seq,
  					  bool dup_sack)
  {
  	struct tcp_sock *tp = tcp_sk(sk);
  	struct sk_buff *prev;
  	int mss;
  	int pcount = 0;
  	int len;
  	int in_sack;
  
  	/* Normally R but no L won't result in plain S */
  	if (!dup_sack &&
  	    (TCP_SKB_CB(skb)->sacked & (TCPCB_LOST|TCPCB_SACKED_RETRANS)) == TCPCB_SACKED_RETRANS)
  		goto fallback;
  	if (!skb_can_shift(skb))
  		goto fallback;
  	/* This frame is about to be dropped (was ACKed). */
  	if (!after(TCP_SKB_CB(skb)->end_seq, tp->snd_una))
  		goto fallback;
  
  	/* Can only happen with delayed DSACK + discard craziness */
  	prev = skb_rb_prev(skb);
  	if (!prev)
  		goto fallback;
  
  	if ((TCP_SKB_CB(prev)->sacked & TCPCB_TAGBITS) != TCPCB_SACKED_ACKED)
  		goto fallback;
  
  	if (!tcp_skb_can_collapse_to(prev))
  		goto fallback;
  
  	in_sack = !after(start_seq, TCP_SKB_CB(skb)->seq) &&
  		  !before(end_seq, TCP_SKB_CB(skb)->end_seq);
  
  	if (in_sack) {
  		len = skb->len;
  		pcount = tcp_skb_pcount(skb);
  		mss = tcp_skb_seglen(skb);
  
  		/* TODO: Fix DSACKs to not fragment already SACKed and we can
  		 * drop this restriction as unnecessary
  		 */
  		if (mss != tcp_skb_seglen(prev))
  			goto fallback;
  	} else {
  		if (!after(TCP_SKB_CB(skb)->end_seq, start_seq))
  			goto noop;
  		/* CHECKME: This is non-MSS split case only?, this will
  		 * cause skipped skbs due to advancing loop btw, original
  		 * has that feature too
  		 */
  		if (tcp_skb_pcount(skb) <= 1)
  			goto noop;
  
  		in_sack = !after(start_seq, TCP_SKB_CB(skb)->seq);
  		if (!in_sack) {
  			/* TODO: head merge to next could be attempted here
  			 * if (!after(TCP_SKB_CB(skb)->end_seq, end_seq)),
  			 * though it might not be worth of the additional hassle
  			 *
  			 * ...we can probably just fallback to what was done
  			 * previously. We could try merging non-SACKed ones
  			 * as well but it probably isn't going to buy off
  			 * because later SACKs might again split them, and
  			 * it would make skb timestamp tracking considerably
  			 * harder problem.
  			 */
  			goto fallback;
  		}
  
  		len = end_seq - TCP_SKB_CB(skb)->seq;
  		BUG_ON(len < 0);
  		BUG_ON(len > skb->len);
  
  		/* MSS boundaries should be honoured or else pcount will
  		 * severely break even though it makes things bit trickier.
  		 * Optimize common case to avoid most of the divides
  		 */
  		mss = tcp_skb_mss(skb);
  
  		/* TODO: Fix DSACKs to not fragment already SACKed and we can
  		 * drop this restriction as unnecessary
  		 */
  		if (mss != tcp_skb_seglen(prev))
  			goto fallback;
  
  		if (len == mss) {
  			pcount = 1;
  		} else if (len < mss) {
  			goto noop;
  		} else {
  			pcount = len / mss;
  			len = pcount * mss;
  		}
  	}
  
  	/* tcp_sacktag_one() won't SACK-tag ranges below snd_una */
  	if (!after(TCP_SKB_CB(skb)->seq + len, tp->snd_una))
  		goto fallback;
  
  	if (!skb_shift(prev, skb, len))
  		goto fallback;
  	if (!tcp_shifted_skb(sk, prev, skb, state, pcount, len, mss, dup_sack))
  		goto out;
  
  	/* Hole filled allows collapsing with the next as well, this is very
  	 * useful when hole on every nth skb pattern happens
  	 */
  	skb = skb_rb_next(prev);
  	if (!skb)
  		goto out;
  
  	if (!skb_can_shift(skb) ||
  	    ((TCP_SKB_CB(skb)->sacked & TCPCB_TAGBITS) != TCPCB_SACKED_ACKED) ||
  	    (mss != tcp_skb_seglen(skb)))
  		goto out;
  
  	len = skb->len;
  	if (skb_shift(prev, skb, len)) {
  		pcount += tcp_skb_pcount(skb);
  		tcp_shifted_skb(sk, prev, skb, state, tcp_skb_pcount(skb),
  				len, mss, 0);
  	}
  
  out:
  	return prev;
  
  noop:
  	return skb;
  
  fallback:
  	NET_INC_STATS(sock_net(sk), LINUX_MIB_SACKSHIFTFALLBACK);
  	return NULL;
  }
  
  static struct sk_buff *tcp_sacktag_walk(struct sk_buff *skb, struct sock *sk,
  					struct tcp_sack_block *next_dup,
  					struct tcp_sacktag_state *state,
  					u32 start_seq, u32 end_seq,
  					bool dup_sack_in)
  {
  	struct tcp_sock *tp = tcp_sk(sk);
  	struct sk_buff *tmp;
  
  	skb_rbtree_walk_from(skb) {
  		int in_sack = 0;
  		bool dup_sack = dup_sack_in;
  
  		/* queue is in-order => we can short-circuit the walk early */
  		if (!before(TCP_SKB_CB(skb)->seq, end_seq))
  			break;
  
  		if (next_dup  &&
  		    before(TCP_SKB_CB(skb)->seq, next_dup->end_seq)) {
  			in_sack = tcp_match_skb_to_sack(sk, skb,
  							next_dup->start_seq,
  							next_dup->end_seq);
  			if (in_sack > 0)
  				dup_sack = true;
  		}
  
  		/* skb reference here is a bit tricky to get right, since
  		 * shifting can eat and free both this skb and the next,
  		 * so not even _safe variant of the loop is enough.
  		 */
  		if (in_sack <= 0) {
  			tmp = tcp_shift_skb_data(sk, skb, state,
  						 start_seq, end_seq, dup_sack);
  			if (tmp) {
  				if (tmp != skb) {
  					skb = tmp;
  					continue;
  				}
  
  				in_sack = 0;
  			} else {
  				in_sack = tcp_match_skb_to_sack(sk, skb,
  								start_seq,
  								end_seq);
  			}
  		}
  
  		if (unlikely(in_sack < 0))
  			break;
  
  		if (in_sack) {
  			TCP_SKB_CB(skb)->sacked =
  				tcp_sacktag_one(sk,
  						state,
  						TCP_SKB_CB(skb)->sacked,
  						TCP_SKB_CB(skb)->seq,
  						TCP_SKB_CB(skb)->end_seq,
  						dup_sack,
  						tcp_skb_pcount(skb),
  						skb->skb_mstamp);
  			tcp_rate_skb_delivered(sk, skb, state->rate);
  			if (TCP_SKB_CB(skb)->sacked & TCPCB_SACKED_ACKED)
  				list_del_init(&skb->tcp_tsorted_anchor);
  
  			if (!before(TCP_SKB_CB(skb)->seq,
  				    tcp_highest_sack_seq(tp)))
  				tcp_advance_highest_sack(sk, skb);
  		}
  	}
  	return skb;
  }
  
  static struct sk_buff *tcp_sacktag_bsearch(struct sock *sk,
  					   struct tcp_sacktag_state *state,
  					   u32 seq)
  {
  	struct rb_node *parent, **p = &sk->tcp_rtx_queue.rb_node;
  	struct sk_buff *skb;
  
  	while (*p) {
  		parent = *p;
  		skb = rb_to_skb(parent);
  		if (before(seq, TCP_SKB_CB(skb)->seq)) {
  			p = &parent->rb_left;
  			continue;
  		}
  		if (!before(seq, TCP_SKB_CB(skb)->end_seq)) {
  			p = &parent->rb_right;
  			continue;
  		}
  		return skb;
  	}
  	return NULL;
  }
  
  static struct sk_buff *tcp_sacktag_skip(struct sk_buff *skb, struct sock *sk,
  					struct tcp_sacktag_state *state,
  					u32 skip_to_seq)
  {
  	if (skb && after(TCP_SKB_CB(skb)->seq, skip_to_seq))
  		return skb;
  
  	return tcp_sacktag_bsearch(sk, state, skip_to_seq);
  }
  
  static struct sk_buff *tcp_maybe_skipping_dsack(struct sk_buff *skb,
  						struct sock *sk,
  						struct tcp_sack_block *next_dup,
  						struct tcp_sacktag_state *state,
  						u32 skip_to_seq)
  {
  	if (!next_dup)
  		return skb;
  
  	if (before(next_dup->start_seq, skip_to_seq)) {
  		skb = tcp_sacktag_skip(skb, sk, state, next_dup->start_seq);
  		skb = tcp_sacktag_walk(skb, sk, NULL, state,
  				       next_dup->start_seq, next_dup->end_seq,
  				       1);
  	}
  
  	return skb;
  }
  
  static int tcp_sack_cache_ok(const struct tcp_sock *tp, const struct tcp_sack_block *cache)
  {
  	return cache < tp->recv_sack_cache + ARRAY_SIZE(tp->recv_sack_cache);
  }
  
  static int
  tcp_sacktag_write_queue(struct sock *sk, const struct sk_buff *ack_skb,
  			u32 prior_snd_una, struct tcp_sacktag_state *state)
  {
  	struct tcp_sock *tp = tcp_sk(sk);
  	const unsigned char *ptr = (skb_transport_header(ack_skb) +
  				    TCP_SKB_CB(ack_skb)->sacked);
  	struct tcp_sack_block_wire *sp_wire = (struct tcp_sack_block_wire *)(ptr+2);
  	struct tcp_sack_block sp[TCP_NUM_SACKS];
  	struct tcp_sack_block *cache;
  	struct sk_buff *skb;
  	int num_sacks = min(TCP_NUM_SACKS, (ptr[1] - TCPOLEN_SACK_BASE) >> 3);
  	int used_sacks;
  	bool found_dup_sack = false;
  	int i, j;
  	int first_sack_index;
  
  	state->flag = 0;
  	state->reord = tp->snd_nxt;
  
  	if (!tp->sacked_out)
  		tcp_highest_sack_reset(sk);
  
  	found_dup_sack = tcp_check_dsack(sk, ack_skb, sp_wire,
  					 num_sacks, prior_snd_una);
  	if (found_dup_sack) {
  		state->flag |= FLAG_DSACKING_ACK;
  		tp->delivered++; /* A spurious retransmission is delivered */
  	}
  
  	/* Eliminate too old ACKs, but take into
  	 * account more or less fresh ones, they can
  	 * contain valid SACK info.
  	 */
  	if (before(TCP_SKB_CB(ack_skb)->ack_seq, prior_snd_una - tp->max_window))
  		return 0;
  
  	if (!tp->packets_out)
  		goto out;
  
  	used_sacks = 0;
  	first_sack_index = 0;
  	for (i = 0; i < num_sacks; i++) {
  		bool dup_sack = !i && found_dup_sack;
  
  		sp[used_sacks].start_seq = get_unaligned_be32(&sp_wire[i].start_seq);
  		sp[used_sacks].end_seq = get_unaligned_be32(&sp_wire[i].end_seq);
  
  		if (!tcp_is_sackblock_valid(tp, dup_sack,
  					    sp[used_sacks].start_seq,
  					    sp[used_sacks].end_seq)) {
  			int mib_idx;
  
  			if (dup_sack) {
  				if (!tp->undo_marker)
  					mib_idx = LINUX_MIB_TCPDSACKIGNOREDNOUNDO;
  				else
  					mib_idx = LINUX_MIB_TCPDSACKIGNOREDOLD;
  			} else {
  				/* Don't count olds caused by ACK reordering */
  				if ((TCP_SKB_CB(ack_skb)->ack_seq != tp->snd_una) &&
  				    !after(sp[used_sacks].end_seq, tp->snd_una))
  					continue;
  				mib_idx = LINUX_MIB_TCPSACKDISCARD;
  			}
  
  			NET_INC_STATS(sock_net(sk), mib_idx);
  			if (i == 0)
  				first_sack_index = -1;
  			continue;
  		}
  
  		/* Ignore very old stuff early */
  		if (!after(sp[used_sacks].end_seq, prior_snd_una))
  			continue;
  
  		used_sacks++;
  	}
  
  	/* order SACK blocks to allow in order walk of the retrans queue */
  	for (i = used_sacks - 1; i > 0; i--) {
  		for (j = 0; j < i; j++) {
  			if (after(sp[j].start_seq, sp[j + 1].start_seq)) {
  				swap(sp[j], sp[j + 1]);
  
  				/* Track where the first SACK block goes to */
  				if (j == first_sack_index)
  					first_sack_index = j + 1;
  			}
  		}
  	}
  
  	state->mss_now = tcp_current_mss(sk);
  	skb = NULL;
  	i = 0;
  
  	if (!tp->sacked_out) {
  		/* It's already past, so skip checking against it */
  		cache = tp->recv_sack_cache + ARRAY_SIZE(tp->recv_sack_cache);
  	} else {
  		cache = tp->recv_sack_cache;
  		/* Skip empty blocks in at head of the cache */
  		while (tcp_sack_cache_ok(tp, cache) && !cache->start_seq &&
  		       !cache->end_seq)
  			cache++;
  	}
  
  	while (i < used_sacks) {
  		u32 start_seq = sp[i].start_seq;
  		u32 end_seq = sp[i].end_seq;
  		bool dup_sack = (found_dup_sack && (i == first_sack_index));
  		struct tcp_sack_block *next_dup = NULL;
  
  		if (found_dup_sack && ((i + 1) == first_sack_index))
  			next_dup = &sp[i + 1];
  
  		/* Skip too early cached blocks */
  		while (tcp_sack_cache_ok(tp, cache) &&
  		       !before(start_seq, cache->end_seq))
  			cache++;
  
  		/* Can skip some work by looking recv_sack_cache? */
  		if (tcp_sack_cache_ok(tp, cache) && !dup_sack &&
  		    after(end_seq, cache->start_seq)) {
  
  			/* Head todo? */
  			if (before(start_seq, cache->start_seq)) {
  				skb = tcp_sacktag_skip(skb, sk, state,
  						       start_seq);
  				skb = tcp_sacktag_walk(skb, sk, next_dup,
  						       state,
  						       start_seq,
  						       cache->start_seq,
  						       dup_sack);
  			}
  
  			/* Rest of the block already fully processed? */
  			if (!after(end_seq, cache->end_seq))
  				goto advance_sp;
  
  			skb = tcp_maybe_skipping_dsack(skb, sk, next_dup,
  						       state,
  						       cache->end_seq);
  
  			/* ...tail remains todo... */
  			if (tcp_highest_sack_seq(tp) == cache->end_seq) {
  				/* ...but better entrypoint exists! */
  				skb = tcp_highest_sack(sk);
  				if (!skb)
  					break;
  				cache++;
  				goto walk;
  			}
  
  			skb = tcp_sacktag_skip(skb, sk, state, cache->end_seq);
  			/* Check overlap against next cached too (past this one already) */
  			cache++;
  			continue;
  		}
  
  		if (!before(start_seq, tcp_highest_sack_seq(tp))) {
  			skb = tcp_highest_sack(sk);
  			if (!skb)
  				break;
  		}
  		skb = tcp_sacktag_skip(skb, sk, state, start_seq);
  
  walk:
  		skb = tcp_sacktag_walk(skb, sk, next_dup, state,
  				       start_seq, end_seq, dup_sack);
  
  advance_sp:
  		i++;
  	}
  
  	/* Clear the head of the cache sack blocks so we can skip it next time */
  	for (i = 0; i < ARRAY_SIZE(tp->recv_sack_cache) - used_sacks; i++) {
  		tp->recv_sack_cache[i].start_seq = 0;
  		tp->recv_sack_cache[i].end_seq = 0;
  	}
  	for (j = 0; j < used_sacks; j++)
  		tp->recv_sack_cache[i++] = sp[j];
  
  	if (inet_csk(sk)->icsk_ca_state != TCP_CA_Loss || tp->undo_marker)
  		tcp_check_sack_reordering(sk, state->reord, 0);
  
  	tcp_verify_left_out(tp);
  out:
  
  #if FASTRETRANS_DEBUG > 0
  	WARN_ON((int)tp->sacked_out < 0);
  	WARN_ON((int)tp->lost_out < 0);
  	WARN_ON((int)tp->retrans_out < 0);
  	WARN_ON((int)tcp_packets_in_flight(tp) < 0);
  #endif
  	return state->flag;
  }
  
  /* Limits sacked_out so that sum with lost_out isn't ever larger than
   * packets_out. Returns false if sacked_out adjustement wasn't necessary.
   */
  static bool tcp_limit_reno_sacked(struct tcp_sock *tp)
  {
  	u32 holes;
  
  	holes = max(tp->lost_out, 1U);
  	holes = min(holes, tp->packets_out);
  
  	if ((tp->sacked_out + holes) > tp->packets_out) {
  		tp->sacked_out = tp->packets_out - holes;
  		return true;
  	}
  	return false;
  }
  
  /* If we receive more dupacks than we expected counting segments
   * in assumption of absent reordering, interpret this as reordering.
   * The only another reason could be bug in receiver TCP.
   */
  static void tcp_check_reno_reordering(struct sock *sk, const int addend)
  {
  	struct tcp_sock *tp = tcp_sk(sk);
  
  	if (!tcp_limit_reno_sacked(tp))
  		return;
  
  	tp->reordering = min_t(u32, tp->packets_out + addend,
  			       sock_net(sk)->ipv4.sysctl_tcp_max_reordering);
  	NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPRENOREORDER);
  }
  
  /* Emulate SACKs for SACKless connection: account for a new dupack. */
  
  static void tcp_add_reno_sack(struct sock *sk)
  {
  	struct tcp_sock *tp = tcp_sk(sk);
  	u32 prior_sacked = tp->sacked_out;
  
  	tp->sacked_out++;
  	tcp_check_reno_reordering(sk, 0);
  	if (tp->sacked_out > prior_sacked)
  		tp->delivered++; /* Some out-of-order packet is delivered */
  	tcp_verify_left_out(tp);
  }
  
  /* Account for ACK, ACKing some data in Reno Recovery phase. */
  
  static void tcp_remove_reno_sacks(struct sock *sk, int acked)
  {
  	struct tcp_sock *tp = tcp_sk(sk);
  
  	if (acked > 0) {
  		/* One ACK acked hole. The rest eat duplicate ACKs. */
  		tp->delivered += max_t(int, acked - tp->sacked_out, 1);
  		if (acked - 1 >= tp->sacked_out)
  			tp->sacked_out = 0;
  		else
  			tp->sacked_out -= acked - 1;
  	}
  	tcp_check_reno_reordering(sk, acked);
  	tcp_verify_left_out(tp);
  }
  
  static inline void tcp_reset_reno_sack(struct tcp_sock *tp)
  {
  	tp->sacked_out = 0;
  }
  
  void tcp_clear_retrans(struct tcp_sock *tp)
  {
  	tp->retrans_out = 0;
  	tp->lost_out = 0;
  	tp->undo_marker = 0;
  	tp->undo_retrans = -1;
  	tp->sacked_out = 0;
  }
  
  static inline void tcp_init_undo(struct tcp_sock *tp)
  {
  	tp->undo_marker = tp->snd_una;
  	/* Retransmission still in flight may cause DSACKs later. */
  	tp->undo_retrans = tp->retrans_out ? : -1;
  }
  
  static bool tcp_is_rack(const struct sock *sk)
  {
  	return sock_net(sk)->ipv4.sysctl_tcp_recovery & TCP_RACK_LOSS_DETECTION;
  }
  
  /* If we detect SACK reneging, forget all SACK information
   * and reset tags completely, otherwise preserve SACKs. If receiver
   * dropped its ofo queue, we will know this due to reneging detection.
   */
  static void tcp_timeout_mark_lost(struct sock *sk)
  {
  	struct tcp_sock *tp = tcp_sk(sk);
  	struct sk_buff *skb, *head;
  	bool is_reneg;			/* is receiver reneging on SACKs? */
  
  	head = tcp_rtx_queue_head(sk);
  	is_reneg = head && (TCP_SKB_CB(head)->sacked & TCPCB_SACKED_ACKED);
  	if (is_reneg) {
  		NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPSACKRENEGING);
  		tp->sacked_out = 0;
  		/* Mark SACK reneging until we recover from this loss event. */
  		tp->is_sack_reneg = 1;
  	} else if (tcp_is_reno(tp)) {
  		tcp_reset_reno_sack(tp);
  	}
  
  	skb = head;
  	skb_rbtree_walk_from(skb) {
  		if (is_reneg)
  			TCP_SKB_CB(skb)->sacked &= ~TCPCB_SACKED_ACKED;
  		else if (tcp_is_rack(sk) && skb != head &&
  			 tcp_rack_skb_timeout(tp, skb, 0) > 0)
  			continue; /* Don't mark recently sent ones lost yet */
  		tcp_mark_skb_lost(sk, skb);
  	}
  	tcp_verify_left_out(tp);
  	tcp_clear_all_retrans_hints(tp);
  }
  
  /* Enter Loss state. */
  void tcp_enter_loss(struct sock *sk)
  {
  	const struct inet_connection_sock *icsk = inet_csk(sk);
  	struct tcp_sock *tp = tcp_sk(sk);
  	struct net *net = sock_net(sk);
  	bool new_recovery = icsk->icsk_ca_state < TCP_CA_Recovery;
  
  	tcp_timeout_mark_lost(sk);
  
  	/* Reduce ssthresh if it has not yet been made inside this window. */
  	if (icsk->icsk_ca_state <= TCP_CA_Disorder ||
  	    !after(tp->high_seq, tp->snd_una) ||
  	    (icsk->icsk_ca_state == TCP_CA_Loss && !icsk->icsk_retransmits)) {
  		tp->prior_ssthresh = tcp_current_ssthresh(sk);
  		tp->prior_cwnd = tp->snd_cwnd;
  		tp->snd_ssthresh = icsk->icsk_ca_ops->ssthresh(sk);
  		tcp_ca_event(sk, CA_EVENT_LOSS);
  		tcp_init_undo(tp);
  	}
  	tp->snd_cwnd	   = tcp_packets_in_flight(tp) + 1;
  	tp->snd_cwnd_cnt   = 0;
  	tp->snd_cwnd_stamp = tcp_jiffies32;
  
  	/* Timeout in disordered state after receiving substantial DUPACKs
  	 * suggests that the degree of reordering is over-estimated.
  	 */
  	if (icsk->icsk_ca_state <= TCP_CA_Disorder &&
  	    tp->sacked_out >= net->ipv4.sysctl_tcp_reordering)
  		tp->reordering = min_t(unsigned int, tp->reordering,
  				       net->ipv4.sysctl_tcp_reordering);
  	tcp_set_ca_state(sk, TCP_CA_Loss);
  	tp->high_seq = tp->snd_nxt;
  	tcp_ecn_queue_cwr(tp);
  
  	/* F-RTO RFC5682 sec 3.1 step 1: retransmit SND.UNA if no previous
  	 * loss recovery is underway except recurring timeout(s) on
  	 * the same SND.UNA (sec 3.2). Disable F-RTO on path MTU probing
  	 */
  	tp->frto = net->ipv4.sysctl_tcp_frto &&
  		   (new_recovery || icsk->icsk_retransmits) &&
  		   !inet_csk(sk)->icsk_mtup.probe_size;
  }
  
  /* If ACK arrived pointing to a remembered SACK, it means that our
   * remembered SACKs do not reflect real state of receiver i.e.
   * receiver _host_ is heavily congested (or buggy).
   *
   * To avoid big spurious retransmission bursts due to transient SACK
   * scoreboard oddities that look like reneging, we give the receiver a
   * little time (max(RTT/2, 10ms)) to send us some more ACKs that will
   * restore sanity to the SACK scoreboard. If the apparent reneging
   * persists until this RTO then we'll clear the SACK scoreboard.
   */
  static bool tcp_check_sack_reneging(struct sock *sk, int flag)
  {
  	if (flag & FLAG_SACK_RENEGING) {
  		struct tcp_sock *tp = tcp_sk(sk);
  		unsigned long delay = max(usecs_to_jiffies(tp->srtt_us >> 4),
  					  msecs_to_jiffies(10));
  
  		inet_csk_reset_xmit_timer(sk, ICSK_TIME_RETRANS,
  					  delay, TCP_RTO_MAX);
  		return true;
  	}
  	return false;
  }
  
  /* Heurestics to calculate number of duplicate ACKs. There's no dupACKs
   * counter when SACK is enabled (without SACK, sacked_out is used for
   * that purpose).
   *
   * With reordering, holes may still be in flight, so RFC3517 recovery
   * uses pure sacked_out (total number of SACKed segments) even though
   * it violates the RFC that uses duplicate ACKs, often these are equal
   * but when e.g. out-of-window ACKs or packet duplication occurs,
   * they differ. Since neither occurs due to loss, TCP should really
   * ignore them.
   */
  static inline int tcp_dupack_heuristics(const struct tcp_sock *tp)
  {
  	return tp->sacked_out + 1;
  }
  
  /* Linux NewReno/SACK/ECN state machine.
   * --------------------------------------
   *
   * "Open"	Normal state, no dubious events, fast path.
   * "Disorder"   In all the respects it is "Open",
   *		but requires a bit more attention. It is entered when
   *		we see some SACKs or dupacks. It is split of "Open"
   *		mainly to move some processing from fast path to slow one.
   * "CWR"	CWND was reduced due to some Congestion Notification event.
   *		It can be ECN, ICMP source quench, local device congestion.
   * "Recovery"	CWND was reduced, we are fast-retransmitting.
   * "Loss"	CWND was reduced due to RTO timeout or SACK reneging.
   *
   * tcp_fastretrans_alert() is entered:
   * - each incoming ACK, if state is not "Open"
   * - when arrived ACK is unusual, namely:
   *	* SACK
   *	* Duplicate ACK.
   *	* ECN ECE.
   *
   * Counting packets in flight is pretty simple.
   *
   *	in_flight = packets_out - left_out + retrans_out
   *
   *	packets_out is SND.NXT-SND.UNA counted in packets.
   *
   *	retrans_out is number of retransmitted segments.
   *
   *	left_out is number of segments left network, but not ACKed yet.
   *
   *		left_out = sacked_out + lost_out
   *
   *     sacked_out: Packets, which arrived to receiver out of order
   *		   and hence not ACKed. With SACKs this number is simply
   *		   amount of SACKed data. Even without SACKs
   *		   it is easy to give pretty reliable estimate of this number,
   *		   counting duplicate ACKs.
   *
   *       lost_out: Packets lost by network. TCP has no explicit
   *		   "loss notification" feedback from network (for now).
   *		   It means that this number can be only _guessed_.
   *		   Actually, it is the heuristics to predict lossage that
   *		   distinguishes different algorithms.
   *
   *	F.e. after RTO, when all the queue is considered as lost,
   *	lost_out = packets_out and in_flight = retrans_out.
   *
   *		Essentially, we have now a few algorithms detecting
   *		lost packets.
   *
   *		If the receiver supports SACK:
   *
   *		RFC6675/3517: It is the conventional algorithm. A packet is
   *		considered lost if the number of higher sequence packets
   *		SACKed is greater than or equal the DUPACK thoreshold
   *		(reordering). This is implemented in tcp_mark_head_lost and
   *		tcp_update_scoreboard.
   *
   *		RACK (draft-ietf-tcpm-rack-01): it is a newer algorithm
   *		(2017-) that checks timing instead of counting DUPACKs.
   *		Essentially a packet is considered lost if it's not S/ACKed
   *		after RTT + reordering_window, where both metrics are
   *		dynamically measured and adjusted. This is implemented in
   *		tcp_rack_mark_lost.
   *
   *		If the receiver does not support SACK:
   *
   *		NewReno (RFC6582): in Recovery we assume that one segment
   *		is lost (classic Reno). While we are in Recovery and
   *		a partial ACK arrives, we assume that one more packet
   *		is lost (NewReno). This heuristics are the same in NewReno
   *		and SACK.
   *
   * Really tricky (and requiring careful tuning) part of algorithm
   * is hidden in functions tcp_time_to_recover() and tcp_xmit_retransmit_queue().
   * The first determines the moment _when_ we should reduce CWND and,
   * hence, slow down forward transmission. In fact, it determines the moment
   * when we decide that hole is caused by loss, rather than by a reorder.
   *
   * tcp_xmit_retransmit_queue() decides, _what_ we should retransmit to fill
   * holes, caused by lost packets.
   *
   * And the most logically complicated part of algorithm is undo
   * heuristics. We detect false retransmits due to both too early
   * fast retransmit (reordering) and underestimated RTO, analyzing
   * timestamps and D-SACKs. When we detect that some segments were
   * retransmitted by mistake and CWND reduction was wrong, we undo
   * window reduction and abort recovery phase. This logic is hidden
   * inside several functions named tcp_try_undo_<something>.
   */
  
  /* This function decides, when we should leave Disordered state
   * and enter Recovery phase, reducing congestion window.
   *
   * Main question: may we further continue forward transmission
   * with the same cwnd?
   */
  static bool tcp_time_to_recover(struct sock *sk, int flag)
  {
  	struct tcp_sock *tp = tcp_sk(sk);
  
  	/* Trick#1: The loss is proven. */
  	if (tp->lost_out)
  		return true;
  
  	/* Not-A-Trick#2 : Classic rule... */
  	if (!tcp_is_rack(sk) && tcp_dupack_heuristics(tp) > tp->reordering)
  		return true;
  
  	return false;
  }
  
  /* Detect loss in event "A" above by marking head of queue up as lost.
   * For non-SACK(Reno) senders, the first "packets" number of segments
   * are considered lost. For RFC3517 SACK, a segment is considered lost if it
   * has at least tp->reordering SACKed seqments above it; "packets" refers to
   * the maximum SACKed segments to pass before reaching this limit.
   */
  static void tcp_mark_head_lost(struct sock *sk, int packets, int mark_head)
  {
  	struct tcp_sock *tp = tcp_sk(sk);
  	struct sk_buff *skb;
  	int cnt, oldcnt, lost;
  	unsigned int mss;
  	/* Use SACK to deduce losses of new sequences sent during recovery */
  	const u32 loss_high = tcp_is_sack(tp) ?  tp->snd_nxt : tp->high_seq;
  
  	WARN_ON(packets > tp->packets_out);
  	skb = tp->lost_skb_hint;
  	if (skb) {
  		/* Head already handled? */
  		if (mark_head && after(TCP_SKB_CB(skb)->seq, tp->snd_una))
  			return;
  		cnt = tp->lost_cnt_hint;
  	} else {
  		skb = tcp_rtx_queue_head(sk);
  		cnt = 0;
  	}
  
  	skb_rbtree_walk_from(skb) {
  		/* TODO: do this better */
  		/* this is not the most efficient way to do this... */
  		tp->lost_skb_hint = skb;
  		tp->lost_cnt_hint = cnt;
  
  		if (after(TCP_SKB_CB(skb)->end_seq, loss_high))
  			break;
  
  		oldcnt = cnt;
  		if (tcp_is_reno(tp) ||
  		    (TCP_SKB_CB(skb)->sacked & TCPCB_SACKED_ACKED))
  			cnt += tcp_skb_pcount(skb);
  
  		if (cnt > packets) {
  			if (tcp_is_sack(tp) ||
  			    (TCP_SKB_CB(skb)->sacked & TCPCB_SACKED_ACKED) ||
  			    (oldcnt >= packets))
  				break;
  
  			mss = tcp_skb_mss(skb);
  			/* If needed, chop off the prefix to mark as lost. */
  			lost = (packets - oldcnt) * mss;
  			if (lost < skb->len &&
  			    tcp_fragment(sk, TCP_FRAG_IN_RTX_QUEUE, skb,
  					 lost, mss, GFP_ATOMIC) < 0)
  				break;
  			cnt = packets;
  		}
  
  		tcp_skb_mark_lost(tp, skb);
  
  		if (mark_head)
  			break;
  	}
  	tcp_verify_left_out(tp);
  }
  
  /* Account newly detected lost packet(s) */
  
  static void tcp_update_scoreboard(struct sock *sk, int fast_rexmit)
  {
  	struct tcp_sock *tp = tcp_sk(sk);
  
  	if (tcp_is_sack(tp)) {
  		int sacked_upto = tp->sacked_out - tp->reordering;
  		if (sacked_upto >= 0)
  			tcp_mark_head_lost(sk, sacked_upto, 0);
  		else if (fast_rexmit)
  			tcp_mark_head_lost(sk, 1, 1);
  	}
  }
  
  static bool tcp_tsopt_ecr_before(const struct tcp_sock *tp, u32 when)
  {
  	return tp->rx_opt.saw_tstamp && tp->rx_opt.rcv_tsecr &&
  	       before(tp->rx_opt.rcv_tsecr, when);
  }
  
  /* skb is spurious retransmitted if the returned timestamp echo
   * reply is prior to the skb transmission time
   */
  static bool tcp_skb_spurious_retrans(const struct tcp_sock *tp,
  				     const struct sk_buff *skb)
  {
  	return (TCP_SKB_CB(skb)->sacked & TCPCB_RETRANS) &&
  	       tcp_tsopt_ecr_before(tp, tcp_skb_timestamp(skb));
  }
  
  /* Nothing was retransmitted or returned timestamp is less
   * than timestamp of the first retransmission.
   */
  static inline bool tcp_packet_delayed(const struct tcp_sock *tp)
  {
  	return !tp->retrans_stamp ||
  	       tcp_tsopt_ecr_before(tp, tp->retrans_stamp);
  }
  
  /* Undo procedures. */
  
  /* We can clear retrans_stamp when there are no retransmissions in the
   * window. It would seem that it is trivially available for us in
   * tp->retrans_out, however, that kind of assumptions doesn't consider
   * what will happen if errors occur when sending retransmission for the
   * second time. ...It could the that such segment has only
   * TCPCB_EVER_RETRANS set at the present time. It seems that checking
   * the head skb is enough except for some reneging corner cases that
   * are not worth the effort.
   *
   * Main reason for all this complexity is the fact that connection dying
   * time now depends on the validity of the retrans_stamp, in particular,
   * that successive retransmissions of a segment must not advance
   * retrans_stamp under any conditions.
   */
  static bool tcp_any_retrans_done(const struct sock *sk)
  {
  	const struct tcp_sock *tp = tcp_sk(sk);
  	struct sk_buff *skb;
  
  	if (tp->retrans_out)
  		return true;
  
  	skb = tcp_rtx_queue_head(sk);
  	if (unlikely(skb && TCP_SKB_CB(skb)->sacked & TCPCB_EVER_RETRANS))
  		return true;
  
  	return false;
  }
  
  static void DBGUNDO(struct sock *sk, const char *msg)
  {
  #if FASTRETRANS_DEBUG > 1
  	struct tcp_sock *tp = tcp_sk(sk);
  	struct inet_sock *inet = inet_sk(sk);
  
  	if (sk->sk_family == AF_INET) {
  		pr_debug("Undo %s %pI4/%u c%u l%u ss%u/%u p%u\n",
  			 msg,
  			 &inet->inet_daddr, ntohs(inet->inet_dport),
  			 tp->snd_cwnd, tcp_left_out(tp),
  			 tp->snd_ssthresh, tp->prior_ssthresh,
  			 tp->packets_out);
  	}
  #if IS_ENABLED(CONFIG_IPV6)
  	else if (sk->sk_family == AF_INET6) {
  		pr_debug("Undo %s %pI6/%u c%u l%u ss%u/%u p%u\n",
  			 msg,
  			 &sk->sk_v6_daddr, ntohs(inet->inet_dport),
  			 tp->snd_cwnd, tcp_left_out(tp),
  			 tp->snd_ssthresh, tp->prior_ssthresh,
  			 tp->packets_out);
  	}
  #endif
  #endif
  }
  
  static void tcp_undo_cwnd_reduction(struct sock *sk, bool unmark_loss)
  {
  	struct tcp_sock *tp = tcp_sk(sk);
  
  	if (unmark_loss) {
  		struct sk_buff *skb;
  
  		skb_rbtree_walk(skb, &sk->tcp_rtx_queue) {
  			TCP_SKB_CB(skb)->sacked &= ~TCPCB_LOST;
  		}
  		tp->lost_out = 0;
  		tcp_clear_all_retrans_hints(tp);
  	}
  
  	if (tp->prior_ssthresh) {
  		const struct inet_connection_sock *icsk = inet_csk(sk);
  
  		tp->snd_cwnd = icsk->icsk_ca_ops->undo_cwnd(sk);
  
  		if (tp->prior_ssthresh > tp->snd_ssthresh) {
  			tp->snd_ssthresh = tp->prior_ssthresh;
  			tcp_ecn_withdraw_cwr(tp);
  		}
  	}
  	tp->snd_cwnd_stamp = tcp_jiffies32;
  	tp->undo_marker = 0;
  	tp->rack.advanced = 1; /* Force RACK to re-exam losses */
  }
  
  static inline bool tcp_may_undo(const struct tcp_sock *tp)
  {
  	return tp->undo_marker && (!tp->undo_retrans || tcp_packet_delayed(tp));
  }
  
  /* People celebrate: "We love our President!" */
  static bool tcp_try_undo_recovery(struct sock *sk)
  {
  	struct tcp_sock *tp = tcp_sk(sk);
  
  	if (tcp_may_undo(tp)) {
  		int mib_idx;
  
  		/* Happy end! We did not retransmit anything
  		 * or our original transmission succeeded.
  		 */
  		DBGUNDO(sk, inet_csk(sk)->icsk_ca_state == TCP_CA_Loss ? "loss" : "retrans");
  		tcp_undo_cwnd_reduction(sk, false);
  		if (inet_csk(sk)->icsk_ca_state == TCP_CA_Loss)
  			mib_idx = LINUX_MIB_TCPLOSSUNDO;
  		else
  			mib_idx = LINUX_MIB_TCPFULLUNDO;
  
  		NET_INC_STATS(sock_net(sk), mib_idx);
  	} else if (tp->rack.reo_wnd_persist) {
  		tp->rack.reo_wnd_persist--;
  	}
  	if (tp->snd_una == tp->high_seq && tcp_is_reno(tp)) {
  		/* Hold old state until something *above* high_seq
  		 * is ACKed. For Reno it is MUST to prevent false
  		 * fast retransmits (RFC2582). SACK TCP is safe. */
  		if (!tcp_any_retrans_done(sk))
  			tp->retrans_stamp = 0;
  		return true;
  	}
  	tcp_set_ca_state(sk, TCP_CA_Open);
  	tp->is_sack_reneg = 0;
  	return false;
  }
  
  /* Try to undo cwnd reduction, because D-SACKs acked all retransmitted data */
  static bool tcp_try_undo_dsack(struct sock *sk)
  {
  	struct tcp_sock *tp = tcp_sk(sk);
  
  	if (tp->undo_marker && !tp->undo_retrans) {
  		tp->rack.reo_wnd_persist = min(TCP_RACK_RECOVERY_THRESH,
  					       tp->rack.reo_wnd_persist + 1);
  		DBGUNDO(sk, "D-SACK");
  		tcp_undo_cwnd_reduction(sk, false);
  		NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPDSACKUNDO);
  		return true;
  	}
  	return false;
  }
  
  /* Undo during loss recovery after partial ACK or using F-RTO. */
  static bool tcp_try_undo_loss(struct sock *sk, bool frto_undo)
  {
  	struct tcp_sock *tp = tcp_sk(sk);
  
  	if (frto_undo || tcp_may_undo(tp)) {
  		tcp_undo_cwnd_reduction(sk, true);
  
  		DBGUNDO(sk, "partial loss");
  		NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPLOSSUNDO);
  		if (frto_undo)
  			NET_INC_STATS(sock_net(sk),
  					LINUX_MIB_TCPSPURIOUSRTOS);
  		inet_csk(sk)->icsk_retransmits = 0;
  		if (frto_undo || tcp_is_sack(tp)) {
  			tcp_set_ca_state(sk, TCP_CA_Open);
  			tp->is_sack_reneg = 0;
  		}
  		return true;
  	}
  	return false;
  }
  
  /* The cwnd reduction in CWR and Recovery uses the PRR algorithm in RFC 6937.
   * It computes the number of packets to send (sndcnt) based on packets newly
   * delivered:
   *   1) If the packets in flight is larger than ssthresh, PRR spreads the
   *	cwnd reductions across a full RTT.
   *   2) Otherwise PRR uses packet conservation to send as much as delivered.
   *      But when the retransmits are acked without further losses, PRR
   *      slow starts cwnd up to ssthresh to speed up the recovery.
   */
  static void tcp_init_cwnd_reduction(struct sock *sk)
  {
  	struct tcp_sock *tp = tcp_sk(sk);
  
  	tp->high_seq = tp->snd_nxt;
  	tp->tlp_high_seq = 0;
  	tp->snd_cwnd_cnt = 0;
  	tp->prior_cwnd = tp->snd_cwnd;
  	tp->prr_delivered = 0;
  	tp->prr_out = 0;
  	tp->snd_ssthresh = inet_csk(sk)->icsk_ca_ops->ssthresh(sk);
  	tcp_ecn_queue_cwr(tp);
  }
  
  void tcp_cwnd_reduction(struct sock *sk, int newly_acked_sacked, int flag)
  {
  	struct tcp_sock *tp = tcp_sk(sk);
  	int sndcnt = 0;
  	int delta = tp->snd_ssthresh - tcp_packets_in_flight(tp);
  
  	if (newly_acked_sacked <= 0 || WARN_ON_ONCE(!tp->prior_cwnd))
  		return;
  
  	tp->prr_delivered += newly_acked_sacked;
  	if (delta < 0) {
  		u64 dividend = (u64)tp->snd_ssthresh * tp->prr_delivered +
  			       tp->prior_cwnd - 1;
  		sndcnt = div_u64(dividend, tp->prior_cwnd) - tp->prr_out;
  	} else if ((flag & FLAG_RETRANS_DATA_ACKED) &&
  		   !(flag & FLAG_LOST_RETRANS)) {
  		sndcnt = min_t(int, delta,
  			       max_t(int, tp->prr_delivered - tp->prr_out,
  				     newly_acked_sacked) + 1);
  	} else {
  		sndcnt = min(delta, newly_acked_sacked);
  	}
  	/* Force a fast retransmit upon entering fast recovery */
  	sndcnt = max(sndcnt, (tp->prr_out ? 0 : 1));
  	tp->snd_cwnd = tcp_packets_in_flight(tp) + sndcnt;
  }
  
  static inline void tcp_end_cwnd_reduction(struct sock *sk)
  {
  	struct tcp_sock *tp = tcp_sk(sk);
  
  	if (inet_csk(sk)->icsk_ca_ops->cong_control)
  		return;
  
  	/* Reset cwnd to ssthresh in CWR or Recovery (unless it's undone) */
  	if (tp->snd_ssthresh < TCP_INFINITE_SSTHRESH &&
  	    (inet_csk(sk)->icsk_ca_state == TCP_CA_CWR || tp->undo_marker)) {
  		tp->snd_cwnd = tp->snd_ssthresh;
  		tp->snd_cwnd_stamp = tcp_jiffies32;
  	}
  	tcp_ca_event(sk, CA_EVENT_COMPLETE_CWR);
  }
  
  /* Enter CWR state. Disable cwnd undo since congestion is proven with ECN */
  void tcp_enter_cwr(struct sock *sk)
  {
  	struct tcp_sock *tp = tcp_sk(sk);
  
  	tp->prior_ssthresh = 0;
  	if (inet_csk(sk)->icsk_ca_state < TCP_CA_CWR) {
  		tp->undo_marker = 0;
  		tcp_init_cwnd_reduction(sk);
  		tcp_set_ca_state(sk, TCP_CA_CWR);
  	}
  }
  EXPORT_SYMBOL(tcp_enter_cwr);
  
  static void tcp_try_keep_open(struct sock *sk)
  {
  	struct tcp_sock *tp = tcp_sk(sk);
  	int state = TCP_CA_Open;
  
  	if (tcp_left_out(tp) || tcp_any_retrans_done(sk))
  		state = TCP_CA_Disorder;
  
  	if (inet_csk(sk)->icsk_ca_state != state) {
  		tcp_set_ca_state(sk, state);
  		tp->high_seq = tp->snd_nxt;
  	}
  }
  
  static void tcp_try_to_open(struct sock *sk, int flag)
  {
  	struct tcp_sock *tp = tcp_sk(sk);
  
  	tcp_verify_left_out(tp);
  
  	if (!tcp_any_retrans_done(sk))
  		tp->retrans_stamp = 0;
  
  	if (flag & FLAG_ECE)
  		tcp_enter_cwr(sk);
  
  	if (inet_csk(sk)->icsk_ca_state != TCP_CA_CWR) {
  		tcp_try_keep_open(sk);
  	}
  }
  
  static void tcp_mtup_probe_failed(struct sock *sk)
  {
  	struct inet_connection_sock *icsk = inet_csk(sk);
  
  	icsk->icsk_mtup.search_high = icsk->icsk_mtup.probe_size - 1;
  	icsk->icsk_mtup.probe_size = 0;
  	NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPMTUPFAIL);
  }
  
  static void tcp_mtup_probe_success(struct sock *sk)
  {
  	struct tcp_sock *tp = tcp_sk(sk);
  	struct inet_connection_sock *icsk = inet_csk(sk);
  
  	/* FIXME: breaks with very large cwnd */
  	tp->prior_ssthresh = tcp_current_ssthresh(sk);
  	tp->snd_cwnd = tp->snd_cwnd *
  		       tcp_mss_to_mtu(sk, tp->mss_cache) /
  		       icsk->icsk_mtup.probe_size;
  	tp->snd_cwnd_cnt = 0;
  	tp->snd_cwnd_stamp = tcp_jiffies32;
  	tp->snd_ssthresh = tcp_current_ssthresh(sk);
  
  	icsk->icsk_mtup.search_low = icsk->icsk_mtup.probe_size;
  	icsk->icsk_mtup.probe_size = 0;
  	tcp_sync_mss(sk, icsk->icsk_pmtu_cookie);
  	NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPMTUPSUCCESS);
  }
  
  /* Do a simple retransmit without using the backoff mechanisms in
   * tcp_timer. This is used for path mtu discovery.
   * The socket is already locked here.
   */
  void tcp_simple_retransmit(struct sock *sk)
  {
  	const struct inet_connection_sock *icsk = inet_csk(sk);
  	struct tcp_sock *tp = tcp_sk(sk);
  	struct sk_buff *skb;
  	unsigned int mss = tcp_current_mss(sk);
  
  	skb_rbtree_walk(skb, &sk->tcp_rtx_queue) {
  		if (tcp_skb_seglen(skb) > mss &&
  		    !(TCP_SKB_CB(skb)->sacked & TCPCB_SACKED_ACKED)) {
  			if (TCP_SKB_CB(skb)->sacked & TCPCB_SACKED_RETRANS) {
  				TCP_SKB_CB(skb)->sacked &= ~TCPCB_SACKED_RETRANS;
  				tp->retrans_out -= tcp_skb_pcount(skb);
  			}
  			tcp_skb_mark_lost_uncond_verify(tp, skb);
  		}
  	}
  
  	tcp_clear_retrans_hints_partial(tp);
  
  	if (!tp->lost_out)
  		return;
  
  	if (tcp_is_reno(tp))
  		tcp_limit_reno_sacked(tp);
  
  	tcp_verify_left_out(tp);
  
  	/* Don't muck with the congestion window here.
  	 * Reason is that we do not increase amount of _data_
  	 * in network, but units changed and effective
  	 * cwnd/ssthresh really reduced now.
  	 */
  	if (icsk->icsk_ca_state != TCP_CA_Loss) {
  		tp->high_seq = tp->snd_nxt;
  		tp->snd_ssthresh = tcp_current_ssthresh(sk);
  		tp->prior_ssthresh = 0;
  		tp->undo_marker = 0;
  		tcp_set_ca_state(sk, TCP_CA_Loss);
  	}
  	tcp_xmit_retransmit_queue(sk);
  }
  EXPORT_SYMBOL(tcp_simple_retransmit);
  
  void tcp_enter_recovery(struct sock *sk, bool ece_ack)
  {
  	struct tcp_sock *tp = tcp_sk(sk);
  	int mib_idx;
  
  	if (tcp_is_reno(tp))
  		mib_idx = LINUX_MIB_TCPRENORECOVERY;
  	else
  		mib_idx = LINUX_MIB_TCPSACKRECOVERY;
  
  	NET_INC_STATS(sock_net(sk), mib_idx);
  
  	tp->prior_ssthresh = 0;
  	tcp_init_undo(tp);
  
  	if (!tcp_in_cwnd_reduction(sk)) {
  		if (!ece_ack)
  			tp->prior_ssthresh = tcp_current_ssthresh(sk);
  		tcp_init_cwnd_reduction(sk);
  	}
  	tcp_set_ca_state(sk, TCP_CA_Recovery);
  }
  
  /* Process an ACK in CA_Loss state. Move to CA_Open if lost data are
   * recovered or spurious. Otherwise retransmits more on partial ACKs.
   */
  static void tcp_process_loss(struct sock *sk, int flag, bool is_dupack,
  			     int *rexmit)
  {
  	struct tcp_sock *tp = tcp_sk(sk);
  	bool recovered = !before(tp->snd_una, tp->high_seq);
  
  	if ((flag & FLAG_SND_UNA_ADVANCED) &&
  	    tcp_try_undo_loss(sk, false))
  		return;
  
  	if (tp->frto) { /* F-RTO RFC5682 sec 3.1 (sack enhanced version). */
  		/* Step 3.b. A timeout is spurious if not all data are
  		 * lost, i.e., never-retransmitted data are (s)acked.
  		 */
  		if ((flag & FLAG_ORIG_SACK_ACKED) &&
  		    tcp_try_undo_loss(sk, true))
  			return;
  
  		if (after(tp->snd_nxt, tp->high_seq)) {
  			if (flag & FLAG_DATA_SACKED || is_dupack)
  				tp->frto = 0; /* Step 3.a. loss was real */
  		} else if (flag & FLAG_SND_UNA_ADVANCED && !recovered) {
  			tp->high_seq = tp->snd_nxt;
  			/* Step 2.b. Try send new data (but deferred until cwnd
  			 * is updated in tcp_ack()). Otherwise fall back to
  			 * the conventional recovery.
  			 */
  			if (!tcp_write_queue_empty(sk) &&
  			    after(tcp_wnd_end(tp), tp->snd_nxt)) {
  				*rexmit = REXMIT_NEW;
  				return;
  			}
  			tp->frto = 0;
  		}
  	}
  
  	if (recovered) {
  		/* F-RTO RFC5682 sec 3.1 step 2.a and 1st part of step 3.a */
  		tcp_try_undo_recovery(sk);
  		return;
  	}
  	if (tcp_is_reno(tp)) {
  		/* A Reno DUPACK means new data in F-RTO step 2.b above are
  		 * delivered. Lower inflight to clock out (re)tranmissions.
  		 */
  		if (after(tp->snd_nxt, tp->high_seq) && is_dupack)
  			tcp_add_reno_sack(sk);
  		else if (flag & FLAG_SND_UNA_ADVANCED)
  			tcp_reset_reno_sack(tp);
  	}
  	*rexmit = REXMIT_LOST;
  }
  
  /* Undo during fast recovery after partial ACK. */
  static bool tcp_try_undo_partial(struct sock *sk, u32 prior_snd_una)
  {
  	struct tcp_sock *tp = tcp_sk(sk);
  
  	if (tp->undo_marker && tcp_packet_delayed(tp)) {
  		/* Plain luck! Hole if filled with delayed
  		 * packet, rather than with a retransmit. Check reordering.
  		 */
  		tcp_check_sack_reordering(sk, prior_snd_una, 1);
  
  		/* We are getting evidence that the reordering degree is higher
  		 * than we realized. If there are no retransmits out then we
  		 * can undo. Otherwise we clock out new packets but do not
  		 * mark more packets lost or retransmit more.
  		 */
  		if (tp->retrans_out)
  			return true;
  
  		if (!tcp_any_retrans_done(sk))
  			tp->retrans_stamp = 0;
  
  		DBGUNDO(sk, "partial recovery");
  		tcp_undo_cwnd_reduction(sk, true);
  		NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPPARTIALUNDO);
  		tcp_try_keep_open(sk);
  		return true;
  	}
  	return false;
  }
  
  static void tcp_identify_packet_loss(struct sock *sk, int *ack_flag)
  {
  	struct tcp_sock *tp = tcp_sk(sk);
  
  	if (tcp_rtx_queue_empty(sk))
  		return;
  
  	if (unlikely(tcp_is_reno(tp))) {
  		tcp_newreno_mark_lost(sk, *ack_flag & FLAG_SND_UNA_ADVANCED);
  	} else if (tcp_is_rack(sk)) {
  		u32 prior_retrans = tp->retrans_out;
  
  		tcp_rack_mark_lost(sk);
  		if (prior_retrans > tp->retrans_out)
  			*ack_flag |= FLAG_LOST_RETRANS;
  	}
  }
  
  static bool tcp_force_fast_retransmit(struct sock *sk)
  {
  	struct tcp_sock *tp = tcp_sk(sk);
  
  	return after(tcp_highest_sack_seq(tp),
  		     tp->snd_una + tp->reordering * tp->mss_cache);
  }
  
  /* Process an event, which can update packets-in-flight not trivially.
   * Main goal of this function is to calculate new estimate for left_out,
   * taking into account both packets sitting in receiver's buffer and
   * packets lost by network.
   *
   * Besides that it updates the congestion state when packet loss or ECN
   * is detected. But it does not reduce the cwnd, it is done by the
   * congestion control later.
   *
   * It does _not_ decide what to send, it is made in function
   * tcp_xmit_retransmit_queue().
   */
  static void tcp_fastretrans_alert(struct sock *sk, const u32 prior_snd_una,
  				  bool is_dupack, int *ack_flag, int *rexmit)
  {
  	struct inet_connection_sock *icsk = inet_csk(sk);
  	struct tcp_sock *tp = tcp_sk(sk);
  	int fast_rexmit = 0, flag = *ack_flag;
  	bool do_lost = is_dupack || ((flag & FLAG_DATA_SACKED) &&
  				     tcp_force_fast_retransmit(sk));
  
  	if (!tp->packets_out && tp->sacked_out)
  		tp->sacked_out = 0;
  
  	/* Now state machine starts.
  	 * A. ECE, hence prohibit cwnd undoing, the reduction is required. */
  	if (flag & FLAG_ECE)
  		tp->prior_ssthresh = 0;
  
  	/* B. In all the states check for reneging SACKs. */
  	if (tcp_check_sack_reneging(sk, flag))
  		return;
  
  	/* C. Check consistency of the current state. */
  	tcp_verify_left_out(tp);
  
  	/* D. Check state exit conditions. State can be terminated
  	 *    when high_seq is ACKed. */
  	if (icsk->icsk_ca_state == TCP_CA_Open) {
  		WARN_ON(tp->retrans_out != 0);
  		tp->retrans_stamp = 0;
  	} else if (!before(tp->snd_una, tp->high_seq)) {
  		switch (icsk->icsk_ca_state) {
  		case TCP_CA_CWR:
  			/* CWR is to be held something *above* high_seq
  			 * is ACKed for CWR bit to reach receiver. */
  			if (tp->snd_una != tp->high_seq) {
  				tcp_end_cwnd_reduction(sk);
  				tcp_set_ca_state(sk, TCP_CA_Open);
  			}
  			break;
  
  		case TCP_CA_Recovery:
  			if (tcp_is_reno(tp))
  				tcp_reset_reno_sack(tp);
  			if (tcp_try_undo_recovery(sk))
  				return;
  			tcp_end_cwnd_reduction(sk);
  			break;
  		}
  	}
  
  	/* E. Process state. */
  	switch (icsk->icsk_ca_state) {
  	case TCP_CA_Recovery:
  		if (!(flag & FLAG_SND_UNA_ADVANCED)) {
  			if (tcp_is_reno(tp) && is_dupack)
  				tcp_add_reno_sack(sk);
  		} else {
  			if (tcp_try_undo_partial(sk, prior_snd_una))
  				return;
  			/* Partial ACK arrived. Force fast retransmit. */
  			do_lost = tcp_is_reno(tp) ||
  				  tcp_force_fast_retransmit(sk);
  		}
  		if (tcp_try_undo_dsack(sk)) {
  			tcp_try_keep_open(sk);
  			return;
  		}
  		tcp_identify_packet_loss(sk, ack_flag);
  		break;
  	case TCP_CA_Loss:
  		tcp_process_loss(sk, flag, is_dupack, rexmit);
  		tcp_identify_packet_loss(sk, ack_flag);
  		if (!(icsk->icsk_ca_state == TCP_CA_Open ||
  		      (*ack_flag & FLAG_LOST_RETRANS)))
  			return;
  		/* Change state if cwnd is undone or retransmits are lost */
  		/* fall through */
  	default:
  		if (tcp_is_reno(tp)) {
  			if (flag & FLAG_SND_UNA_ADVANCED)
  				tcp_reset_reno_sack(tp);
  			if (is_dupack)
  				tcp_add_reno_sack(sk);
  		}
  
  		if (icsk->icsk_ca_state <= TCP_CA_Disorder)
  			tcp_try_undo_dsack(sk);
  
  		tcp_identify_packet_loss(sk, ack_flag);
  		if (!tcp_time_to_recover(sk, flag)) {
  			tcp_try_to_open(sk, flag);
  			return;
  		}
  
  		/* MTU probe failure: don't reduce cwnd */
  		if (icsk->icsk_ca_state < TCP_CA_CWR &&
  		    icsk->icsk_mtup.probe_size &&
  		    tp->snd_una == tp->mtu_probe.probe_seq_start) {
  			tcp_mtup_probe_failed(sk);
  			/* Restores the reduction we did in tcp_mtup_probe() */
  			tp->snd_cwnd++;
  			tcp_simple_retransmit(sk);
  			return;
  		}
  
  		/* Otherwise enter Recovery state */
  		tcp_enter_recovery(sk, (flag & FLAG_ECE));
  		fast_rexmit = 1;
  	}
  
  	if (!tcp_is_rack(sk) && do_lost)
  		tcp_update_scoreboard(sk, fast_rexmit);
  	*rexmit = REXMIT_LOST;
  }
  
  static void tcp_update_rtt_min(struct sock *sk, u32 rtt_us, const int flag)
  {
  	u32 wlen = sock_net(sk)->ipv4.sysctl_tcp_min_rtt_wlen * HZ;
  	struct tcp_sock *tp = tcp_sk(sk);
  
  	if ((flag & FLAG_ACK_MAYBE_DELAYED) && rtt_us > tcp_min_rtt(tp)) {
  		/* If the remote keeps returning delayed ACKs, eventually
  		 * the min filter would pick it up and overestimate the
  		 * prop. delay when it expires. Skip suspected delayed ACKs.
  		 */
  		return;
  	}
  	minmax_running_min(&tp->rtt_min, wlen, tcp_jiffies32,
  			   rtt_us ? : jiffies_to_usecs(1));
  }
  
  static bool tcp_ack_update_rtt(struct sock *sk, const int flag,
  			       long seq_rtt_us, long sack_rtt_us,
  			       long ca_rtt_us, struct rate_sample *rs)
  {
  	const struct tcp_sock *tp = tcp_sk(sk);
  
  	/* Prefer RTT measured from ACK's timing to TS-ECR. This is because
  	 * broken middle-boxes or peers may corrupt TS-ECR fields. But
  	 * Karn's algorithm forbids taking RTT if some retransmitted data
  	 * is acked (RFC6298).
  	 */
  	if (seq_rtt_us < 0)
  		seq_rtt_us = sack_rtt_us;
  
  	/* RTTM Rule: A TSecr value received in a segment is used to
  	 * update the averaged RTT measurement only if the segment
  	 * acknowledges some new data, i.e., only if it advances the
  	 * left edge of the send window.
  	 * See draft-ietf-tcplw-high-performance-00, section 3.3.
  	 */
  	if (seq_rtt_us < 0 && tp->rx_opt.saw_tstamp && tp->rx_opt.rcv_tsecr &&
  	    flag & FLAG_ACKED) {
  		u32 delta = tcp_time_stamp(tp) - tp->rx_opt.rcv_tsecr;
  		u32 delta_us = delta * (USEC_PER_SEC / TCP_TS_HZ);
  
  		seq_rtt_us = ca_rtt_us = delta_us;
  	}
  	rs->rtt_us = ca_rtt_us; /* RTT of last (S)ACKed packet (or -1) */
  	if (seq_rtt_us < 0)
  		return false;
  
  	/* ca_rtt_us >= 0 is counting on the invariant that ca_rtt_us is
  	 * always taken together with ACK, SACK, or TS-opts. Any negative
  	 * values will be skipped with the seq_rtt_us < 0 check above.
  	 */
  	tcp_update_rtt_min(sk, ca_rtt_us, flag);
  	tcp_rtt_estimator(sk, seq_rtt_us);
  	tcp_set_rto(sk);
  
  	/* RFC6298: only reset backoff on valid RTT measurement. */
  	inet_csk(sk)->icsk_backoff = 0;
  	return true;
  }
  
  /* Compute time elapsed between (last) SYNACK and the ACK completing 3WHS. */
  void tcp_synack_rtt_meas(struct sock *sk, struct request_sock *req)
  {
  	struct rate_sample rs;
  	long rtt_us = -1L;
  
  	if (req && !req->num_retrans && tcp_rsk(req)->snt_synack)
  		rtt_us = tcp_stamp_us_delta(tcp_clock_us(), tcp_rsk(req)->snt_synack);
  
  	tcp_ack_update_rtt(sk, FLAG_SYN_ACKED, rtt_us, -1L, rtt_us, &rs);
  }
  
  
  static void tcp_cong_avoid(struct sock *sk, u32 ack, u32 acked)
  {
  	const struct inet_connection_sock *icsk = inet_csk(sk);
  
  	icsk->icsk_ca_ops->cong_avoid(sk, ack, acked);
  	tcp_sk(sk)->snd_cwnd_stamp = tcp_jiffies32;
  }
  
  /* Restart timer after forward progress on connection.
   * RFC2988 recommends to restart timer to now+rto.
   */
  void tcp_rearm_rto(struct sock *sk)
  {
  	const struct inet_connection_sock *icsk = inet_csk(sk);
  	struct tcp_sock *tp = tcp_sk(sk);
  
  	/* If the retrans timer is currently being used by Fast Open
  	 * for SYN-ACK retrans purpose, stay put.
  	 */
  	if (tp->fastopen_rsk)
  		return;
  
  	if (!tp->packets_out) {
  		inet_csk_clear_xmit_timer(sk, ICSK_TIME_RETRANS);
  	} else {
  		u32 rto = inet_csk(sk)->icsk_rto;
  		/* Offset the time elapsed after installing regular RTO */
  		if (icsk->icsk_pending == ICSK_TIME_REO_TIMEOUT ||
  		    icsk->icsk_pending == ICSK_TIME_LOSS_PROBE) {
  			s64 delta_us = tcp_rto_delta_us(sk);
  			/* delta_us may not be positive if the socket is locked
  			 * when the retrans timer fires and is rescheduled.
  			 */
  			rto = usecs_to_jiffies(max_t(int, delta_us, 1));
  		}
  		inet_csk_reset_xmit_timer(sk, ICSK_TIME_RETRANS, rto,
  					  TCP_RTO_MAX);
  	}
  }
  
  /* Try to schedule a loss probe; if that doesn't work, then schedule an RTO. */
  static void tcp_set_xmit_timer(struct sock *sk)
  {
  	if (!tcp_schedule_loss_probe(sk, true))
  		tcp_rearm_rto(sk);
  }
  
  /* If we get here, the whole TSO packet has not been acked. */
  static u32 tcp_tso_acked(struct sock *sk, struct sk_buff *skb)
  {
  	struct tcp_sock *tp = tcp_sk(sk);
  	u32 packets_acked;
  
  	BUG_ON(!after(TCP_SKB_CB(skb)->end_seq, tp->snd_una));
  
  	packets_acked = tcp_skb_pcount(skb);
  	if (tcp_trim_head(sk, skb, tp->snd_una - TCP_SKB_CB(skb)->seq))
  		return 0;
  	packets_acked -= tcp_skb_pcount(skb);
  
  	if (packets_acked) {
  		BUG_ON(tcp_skb_pcount(skb) == 0);
  		BUG_ON(!before(TCP_SKB_CB(skb)->seq, TCP_SKB_CB(skb)->end_seq));
  	}
  
  	return packets_acked;
  }
  
  static void tcp_ack_tstamp(struct sock *sk, struct sk_buff *skb,
  			   u32 prior_snd_una)
  {
  	const struct skb_shared_info *shinfo;
  
  	/* Avoid cache line misses to get skb_shinfo() and shinfo->tx_flags */
  	if (likely(!TCP_SKB_CB(skb)->txstamp_ack))
  		return;
  
  	shinfo = skb_shinfo(skb);
  	if (!before(shinfo->tskey, prior_snd_una) &&
  	    before(shinfo->tskey, tcp_sk(sk)->snd_una)) {
  		tcp_skb_tsorted_save(skb) {
  			__skb_tstamp_tx(skb, NULL, sk, SCM_TSTAMP_ACK);
  		} tcp_skb_tsorted_restore(skb);
  	}
  }
  
  /* Remove acknowledged frames from the retransmission queue. If our packet
   * is before the ack sequence we can discard it as it's confirmed to have
   * arrived at the other end.
   */
  static int tcp_clean_rtx_queue(struct sock *sk, u32 prior_fack,
  			       u32 prior_snd_una,
  			       struct tcp_sacktag_state *sack)
  {
  	const struct inet_connection_sock *icsk = inet_csk(sk);
  	u64 first_ackt, last_ackt;
  	struct tcp_sock *tp = tcp_sk(sk);
  	u32 prior_sacked = tp->sacked_out;
  	u32 reord = tp->snd_nxt; /* lowest acked un-retx un-sacked seq */
  	struct sk_buff *skb, *next;
  	bool fully_acked = true;
  	long sack_rtt_us = -1L;
  	long seq_rtt_us = -1L;
  	long ca_rtt_us = -1L;
  	u32 pkts_acked = 0;
  	u32 last_in_flight = 0;
  	bool rtt_update;
  	int flag = 0;
  
  	first_ackt = 0;
  
  	for (skb = skb_rb_first(&sk->tcp_rtx_queue); skb; skb = next) {
  		struct tcp_skb_cb *scb = TCP_SKB_CB(skb);
  		const u32 start_seq = scb->seq;
  		u8 sacked = scb->sacked;
  		u32 acked_pcount;
  
  		tcp_ack_tstamp(sk, skb, prior_snd_una);
  
  		/* Determine how many packets and what bytes were acked, tso and else */
  		if (after(scb->end_seq, tp->snd_una)) {
  			if (tcp_skb_pcount(skb) == 1 ||
  			    !after(tp->snd_una, scb->seq))
  				break;
  
  			acked_pcount = tcp_tso_acked(sk, skb);
  			if (!acked_pcount)
  				break;
  			fully_acked = false;
  		} else {
  			acked_pcount = tcp_skb_pcount(skb);
  		}
  
  		if (unlikely(sacked & TCPCB_RETRANS)) {
  			if (sacked & TCPCB_SACKED_RETRANS)
  				tp->retrans_out -= acked_pcount;
  			flag |= FLAG_RETRANS_DATA_ACKED;
  		} else if (!(sacked & TCPCB_SACKED_ACKED)) {
  			last_ackt = skb->skb_mstamp;
  			WARN_ON_ONCE(last_ackt == 0);
  			if (!first_ackt)
  				first_ackt = last_ackt;
  
  			last_in_flight = TCP_SKB_CB(skb)->tx.in_flight;
  			if (before(start_seq, reord))
  				reord = start_seq;
  			if (!after(scb->end_seq, tp->high_seq))
  				flag |= FLAG_ORIG_SACK_ACKED;
  		}
  
  		if (sacked & TCPCB_SACKED_ACKED) {
  			tp->sacked_out -= acked_pcount;
  		} else if (tcp_is_sack(tp)) {
  			tp->delivered += acked_pcount;
  			if (!tcp_skb_spurious_retrans(tp, skb))
  				tcp_rack_advance(tp, sacked, scb->end_seq,
  						 skb->skb_mstamp);
  		}
  		if (sacked & TCPCB_LOST)
  			tp->lost_out -= acked_pcount;
  
  		tp->packets_out -= acked_pcount;
  		pkts_acked += acked_pcount;
  		tcp_rate_skb_delivered(sk, skb, sack->rate);
  
  		/* Initial outgoing SYN's get put onto the write_queue
  		 * just like anything else we transmit.  It is not
  		 * true data, and if we misinform our callers that
  		 * this ACK acks real data, we will erroneously exit
  		 * connection startup slow start one packet too
  		 * quickly.  This is severely frowned upon behavior.
  		 */
  		if (likely(!(scb->tcp_flags & TCPHDR_SYN))) {
  			flag |= FLAG_DATA_ACKED;
  		} else {
  			flag |= FLAG_SYN_ACKED;
  			tp->retrans_stamp = 0;
  		}
  
  		if (!fully_acked)
  			break;
  
  		next = skb_rb_next(skb);
  		if (unlikely(skb == tp->retransmit_skb_hint))
  			tp->retransmit_skb_hint = NULL;
  		if (unlikely(skb == tp->lost_skb_hint))
  			tp->lost_skb_hint = NULL;
  		tcp_rtx_queue_unlink_and_free(skb, sk);
  	}
  
  	if (!skb)
  		tcp_chrono_stop(sk, TCP_CHRONO_BUSY);
  
  	if (likely(between(tp->snd_up, prior_snd_una, tp->snd_una)))
  		tp->snd_up = tp->snd_una;
  
  	if (skb && (TCP_SKB_CB(skb)->sacked & TCPCB_SACKED_ACKED))
  		flag |= FLAG_SACK_RENEGING;
  
  	if (likely(first_ackt) && !(flag & FLAG_RETRANS_DATA_ACKED)) {
  		seq_rtt_us = tcp_stamp_us_delta(tp->tcp_mstamp, first_ackt);
  		ca_rtt_us = tcp_stamp_us_delta(tp->tcp_mstamp, last_ackt);
  
  		if (pkts_acked == 1 && last_in_flight < tp->mss_cache &&
  		    last_in_flight && !prior_sacked && fully_acked &&
  		    sack->rate->prior_delivered + 1 == tp->delivered &&
  		    !(flag & (FLAG_CA_ALERT | FLAG_SYN_ACKED))) {
  			/* Conservatively mark a delayed ACK. It's typically
  			 * from a lone runt packet over the round trip to
  			 * a receiver w/o out-of-order or CE events.
  			 */
  			flag |= FLAG_ACK_MAYBE_DELAYED;
  		}
  	}
  	if (sack->first_sackt) {
  		sack_rtt_us = tcp_stamp_us_delta(tp->tcp_mstamp, sack->first_sackt);
  		ca_rtt_us = tcp_stamp_us_delta(tp->tcp_mstamp, sack->last_sackt);
  	}
  	rtt_update = tcp_ack_update_rtt(sk, flag, seq_rtt_us, sack_rtt_us,
  					ca_rtt_us, sack->rate);
  
  	if (flag & FLAG_ACKED) {
  		flag |= FLAG_SET_XMIT_TIMER;  /* set TLP or RTO timer */
  		if (unlikely(icsk->icsk_mtup.probe_size &&
  			     !after(tp->mtu_probe.probe_seq_end, tp->snd_una))) {
  			tcp_mtup_probe_success(sk);
  		}
  
  		if (tcp_is_reno(tp)) {
  			tcp_remove_reno_sacks(sk, pkts_acked);
  		} else {
  			int delta;
  
  			/* Non-retransmitted hole got filled? That's reordering */
  			if (before(reord, prior_fack))
  				tcp_check_sack_reordering(sk, reord, 0);
  
  			delta = prior_sacked - tp->sacked_out;
  			tp->lost_cnt_hint -= min(tp->lost_cnt_hint, delta);
  		}
  	} else if (skb && rtt_update && sack_rtt_us >= 0 &&
  		   sack_rtt_us > tcp_stamp_us_delta(tp->tcp_mstamp, skb->skb_mstamp)) {
  		/* Do not re-arm RTO if the sack RTT is measured from data sent
  		 * after when the head was last (re)transmitted. Otherwise the
  		 * timeout may continue to extend in loss recovery.
  		 */
  		flag |= FLAG_SET_XMIT_TIMER;  /* set TLP or RTO timer */
  	}
  
  	if (icsk->icsk_ca_ops->pkts_acked) {
  		struct ack_sample sample = { .pkts_acked = pkts_acked,
  					     .rtt_us = sack->rate->rtt_us,
  					     .in_flight = last_in_flight };
  
  		icsk->icsk_ca_ops->pkts_acked(sk, &sample);
  	}
  
  #if FASTRETRANS_DEBUG > 0
  	WARN_ON((int)tp->sacked_out < 0);
  	WARN_ON((int)tp->lost_out < 0);
  	WARN_ON((int)tp->retrans_out < 0);
  	if (!tp->packets_out && tcp_is_sack(tp)) {
  		icsk = inet_csk(sk);
  		if (tp->lost_out) {
  			pr_debug("Leak l=%u %d\n",
  				 tp->lost_out, icsk->icsk_ca_state);
  			tp->lost_out = 0;
  		}
  		if (tp->sacked_out) {
  			pr_debug("Leak s=%u %d\n",
  				 tp->sacked_out, icsk->icsk_ca_state);
  			tp->sacked_out = 0;
  		}
  		if (tp->retrans_out) {
  			pr_debug("Leak r=%u %d\n",
  				 tp->retrans_out, icsk->icsk_ca_state);
  			tp->retrans_out = 0;
  		}
  	}
  #endif
  	return flag;
  }
  
  static void tcp_ack_probe(struct sock *sk)
  {
  	struct inet_connection_sock *icsk = inet_csk(sk);
  	struct sk_buff *head = tcp_send_head(sk);
  	const struct tcp_sock *tp = tcp_sk(sk);
  
  	/* Was it a usable window open? */
  	if (!head)
  		return;
  	if (!after(TCP_SKB_CB(head)->end_seq, tcp_wnd_end(tp))) {
  		icsk->icsk_backoff = 0;
  		inet_csk_clear_xmit_timer(sk, ICSK_TIME_PROBE0);
  		/* Socket must be waked up by subsequent tcp_data_snd_check().
  		 * This function is not for random using!
  		 */
  	} else {
  		unsigned long when = tcp_probe0_when(sk, TCP_RTO_MAX);
  
  		inet_csk_reset_xmit_timer(sk, ICSK_TIME_PROBE0,
  					  when, TCP_RTO_MAX);
  	}
  }
  
  static inline bool tcp_ack_is_dubious(const struct sock *sk, const int flag)
  {
  	return !(flag & FLAG_NOT_DUP) || (flag & FLAG_CA_ALERT) ||
  		inet_csk(sk)->icsk_ca_state != TCP_CA_Open;
  }
  
  /* Decide wheather to run the increase function of congestion control. */
  static inline bool tcp_may_raise_cwnd(const struct sock *sk, const int flag)
  {
  	/* If reordering is high then always grow cwnd whenever data is
  	 * delivered regardless of its ordering. Otherwise stay conservative
  	 * and only grow cwnd on in-order delivery (RFC5681). A stretched ACK w/
  	 * new SACK or ECE mark may first advance cwnd here and later reduce
  	 * cwnd in tcp_fastretrans_alert() based on more states.
  	 */
  	if (tcp_sk(sk)->reordering > sock_net(sk)->ipv4.sysctl_tcp_reordering)
  		return flag & FLAG_FORWARD_PROGRESS;
  
  	return flag & FLAG_DATA_ACKED;
  }
  
  /* The "ultimate" congestion control function that aims to replace the rigid
   * cwnd increase and decrease control (tcp_cong_avoid,tcp_*cwnd_reduction).
   * It's called toward the end of processing an ACK with precise rate
   * information. All transmission or retransmission are delayed afterwards.
   */
  static void tcp_cong_control(struct sock *sk, u32 ack, u32 acked_sacked,
  			     int flag, const struct rate_sample *rs)
  {
  	const struct inet_connection_sock *icsk = inet_csk(sk);
  
  	if (icsk->icsk_ca_ops->cong_control) {
  		icsk->icsk_ca_ops->cong_control(sk, rs);
  		return;
  	}
  
  	if (tcp_in_cwnd_reduction(sk)) {
  		/* Reduce cwnd if state mandates */
  		tcp_cwnd_reduction(sk, acked_sacked, flag);
  	} else if (tcp_may_raise_cwnd(sk, flag)) {
  		/* Advance cwnd if state allows */
  		tcp_cong_avoid(sk, ack, acked_sacked);
  	}
  	tcp_update_pacing_rate(sk);
  }
  
  /* Check that window update is acceptable.
   * The function assumes that snd_una<=ack<=snd_next.
   */
  static inline bool tcp_may_update_window(const struct tcp_sock *tp,
  					const u32 ack, const u32 ack_seq,
  					const u32 nwin)
  {
  	return	after(ack, tp->snd_una) ||
  		after(ack_seq, tp->snd_wl1) ||
  		(ack_seq == tp->snd_wl1 && nwin > tp->snd_wnd);
  }
  
  /* If we update tp->snd_una, also update tp->bytes_acked */
  static void tcp_snd_una_update(struct tcp_sock *tp, u32 ack)
  {
  	u32 delta = ack - tp->snd_una;
  
  	sock_owned_by_me((struct sock *)tp);
  	tp->bytes_acked += delta;
  	tp->snd_una = ack;
  }
  
  /* If we update tp->rcv_nxt, also update tp->bytes_received */
  static void tcp_rcv_nxt_update(struct tcp_sock *tp, u32 seq)
  {
  	u32 delta = seq - tp->rcv_nxt;
  
  	sock_owned_by_me((struct sock *)tp);
  	tp->bytes_received += delta;
  	tp->rcv_nxt = seq;
  }
  
  /* Update our send window.
   *
   * Window update algorithm, described in RFC793/RFC1122 (used in linux-2.2
   * and in FreeBSD. NetBSD's one is even worse.) is wrong.
   */
  static int tcp_ack_update_window(struct sock *sk, const struct sk_buff *skb, u32 ack,
  				 u32 ack_seq)
  {
  	struct tcp_sock *tp = tcp_sk(sk);
  	int flag = 0;
  	u32 nwin = ntohs(tcp_hdr(skb)->window);
  
  	if (likely(!tcp_hdr(skb)->syn))
  		nwin <<= tp->rx_opt.snd_wscale;
  
  	if (tcp_may_update_window(tp, ack, ack_seq, nwin)) {
  		flag |= FLAG_WIN_UPDATE;
  		tcp_update_wl(tp, ack_seq);
  
  		if (tp->snd_wnd != nwin) {
  			tp->snd_wnd = nwin;
  
  			/* Note, it is the only place, where
  			 * fast path is recovered for sending TCP.
  			 */
  			tp->pred_flags = 0;
  			tcp_fast_path_check(sk);
  
  			if (!tcp_write_queue_empty(sk))
  				tcp_slow_start_after_idle_check(sk);
  
  			if (nwin > tp->max_window) {
  				tp->max_window = nwin;
  				tcp_sync_mss(sk, inet_csk(sk)->icsk_pmtu_cookie);
  			}
  		}
  	}
  
  	tcp_snd_una_update(tp, ack);
  
  	return flag;
  }
  
  static bool __tcp_oow_rate_limited(struct net *net, int mib_idx,
  				   u32 *last_oow_ack_time)
  {
  	if (*last_oow_ack_time) {
  		s32 elapsed = (s32)(tcp_jiffies32 - *last_oow_ack_time);
  
  		if (0 <= elapsed && elapsed < net->ipv4.sysctl_tcp_invalid_ratelimit) {
  			NET_INC_STATS(net, mib_idx);
  			return true;	/* rate-limited: don't send yet! */
  		}
  	}
  
  	*last_oow_ack_time = tcp_jiffies32;
  
  	return false;	/* not rate-limited: go ahead, send dupack now! */
  }
  
  /* Return true if we're currently rate-limiting out-of-window ACKs and
   * thus shouldn't send a dupack right now. We rate-limit dupacks in
   * response to out-of-window SYNs or ACKs to mitigate ACK loops or DoS
   * attacks that send repeated SYNs or ACKs for the same connection. To
   * do this, we do not send a duplicate SYNACK or ACK if the remote
   * endpoint is sending out-of-window SYNs or pure ACKs at a high rate.
   */
  bool tcp_oow_rate_limited(struct net *net, const struct sk_buff *skb,
  			  int mib_idx, u32 *last_oow_ack_time)
  {
  	/* Data packets without SYNs are not likely part of an ACK loop. */
  	if ((TCP_SKB_CB(skb)->seq != TCP_SKB_CB(skb)->end_seq) &&
  	    !tcp_hdr(skb)->syn)
  		return false;
  
  	return __tcp_oow_rate_limited(net, mib_idx, last_oow_ack_time);
  }
  
  /* RFC 5961 7 [ACK Throttling] */
  static void tcp_send_challenge_ack(struct sock *sk, const struct sk_buff *skb)
  {
  	/* unprotected vars, we dont care of overwrites */
  	static u32 challenge_timestamp;
  	static unsigned int challenge_count;
  	struct tcp_sock *tp = tcp_sk(sk);
  	struct net *net = sock_net(sk);
  	u32 count, now;
  
  	/* First check our per-socket dupack rate limit. */
  	if (__tcp_oow_rate_limited(net,
  				   LINUX_MIB_TCPACKSKIPPEDCHALLENGE,
  				   &tp->last_oow_ack_time))
  		return;
  
  	/* Then check host-wide RFC 5961 rate limit. */
  	now = jiffies / HZ;
  	if (now != challenge_timestamp) {
  		u32 ack_limit = net->ipv4.sysctl_tcp_challenge_ack_limit;
  		u32 half = (ack_limit + 1) >> 1;
  
  		challenge_timestamp = now;
  		WRITE_ONCE(challenge_count, half + prandom_u32_max(ack_limit));
  	}
  	count = READ_ONCE(challenge_count);
  	if (count > 0) {
  		WRITE_ONCE(challenge_count, count - 1);
  		NET_INC_STATS(net, LINUX_MIB_TCPCHALLENGEACK);
  		tcp_send_ack(sk);
  	}
  }
  
  static void tcp_store_ts_recent(struct tcp_sock *tp)
  {
  	tp->rx_opt.ts_recent = tp->rx_opt.rcv_tsval;
  	tp->rx_opt.ts_recent_stamp = get_seconds();
  }
  
  static void tcp_replace_ts_recent(struct tcp_sock *tp, u32 seq)
  {
  	if (tp->rx_opt.saw_tstamp && !after(seq, tp->rcv_wup)) {
  		/* PAWS bug workaround wrt. ACK frames, the PAWS discard
  		 * extra check below makes sure this can only happen
  		 * for pure ACK frames.  -DaveM
  		 *
  		 * Not only, also it occurs for expired timestamps.
  		 */
  
  		if (tcp_paws_check(&tp->rx_opt, 0))
  			tcp_store_ts_recent(tp);
  	}
  }
  
  /* This routine deals with acks during a TLP episode.
   * We mark the end of a TLP episode on receiving TLP dupack or when
   * ack is after tlp_high_seq.
   * Ref: loss detection algorithm in draft-dukkipati-tcpm-tcp-loss-probe.
   */
  static void tcp_process_tlp_ack(struct sock *sk, u32 ack, int flag)
  {
  	struct tcp_sock *tp = tcp_sk(sk);
  
  	if (before(ack, tp->tlp_high_seq))
  		return;
  
  	if (flag & FLAG_DSACKING_ACK) {
  		/* This DSACK means original and TLP probe arrived; no loss */
  		tp->tlp_high_seq = 0;
  	} else if (after(ack, tp->tlp_high_seq)) {
  		/* ACK advances: there was a loss, so reduce cwnd. Reset
  		 * tlp_high_seq in tcp_init_cwnd_reduction()
  		 */
  		tcp_init_cwnd_reduction(sk);
  		tcp_set_ca_state(sk, TCP_CA_CWR);
  		tcp_end_cwnd_reduction(sk);
  		tcp_try_keep_open(sk);
  		NET_INC_STATS(sock_net(sk),
  				LINUX_MIB_TCPLOSSPROBERECOVERY);
  	} else if (!(flag & (FLAG_SND_UNA_ADVANCED |
  			     FLAG_NOT_DUP | FLAG_DATA_SACKED))) {
  		/* Pure dupack: original and TLP probe arrived; no loss */
  		tp->tlp_high_seq = 0;
  	}
  }
  
  static inline void tcp_in_ack_event(struct sock *sk, u32 flags)
  {
  	const struct inet_connection_sock *icsk = inet_csk(sk);
  
  	if (icsk->icsk_ca_ops->in_ack_event)
  		icsk->icsk_ca_ops->in_ack_event(sk, flags);
  }
  
  /* Congestion control has updated the cwnd already. So if we're in
   * loss recovery then now we do any new sends (for FRTO) or
   * retransmits (for CA_Loss or CA_recovery) that make sense.
   */
  static void tcp_xmit_recovery(struct sock *sk, int rexmit)
  {
  	struct tcp_sock *tp = tcp_sk(sk);
  
  	if (rexmit == REXMIT_NONE)
  		return;
  
  	if (unlikely(rexmit == 2)) {
  		__tcp_push_pending_frames(sk, tcp_current_mss(sk),
  					  TCP_NAGLE_OFF);
  		if (after(tp->snd_nxt, tp->high_seq))
  			return;
  		tp->frto = 0;
  	}
  	tcp_xmit_retransmit_queue(sk);
  }
  
  /* Returns the number of packets newly acked or sacked by the current ACK */
  static u32 tcp_newly_delivered(struct sock *sk, u32 prior_delivered, int flag)
  {
  	const struct net *net = sock_net(sk);
  	struct tcp_sock *tp = tcp_sk(sk);
  	u32 delivered;
  
  	delivered = tp->delivered - prior_delivered;
  	NET_ADD_STATS(net, LINUX_MIB_TCPDELIVERED, delivered);
  	if (flag & FLAG_ECE) {
  		tp->delivered_ce += delivered;
  		NET_ADD_STATS(net, LINUX_MIB_TCPDELIVEREDCE, delivered);
  	}
  	return delivered;
  }
  
  /* This routine deals with incoming acks, but not outgoing ones. */
  static int tcp_ack(struct sock *sk, const struct sk_buff *skb, int flag)
  {
  	struct inet_connection_sock *icsk = inet_csk(sk);
  	struct tcp_sock *tp = tcp_sk(sk);
  	struct tcp_sacktag_state sack_state;
  	struct rate_sample rs = { .prior_delivered = 0 };
  	u32 prior_snd_una = tp->snd_una;
  	bool is_sack_reneg = tp->is_sack_reneg;
  	u32 ack_seq = TCP_SKB_CB(skb)->seq;
  	u32 ack = TCP_SKB_CB(skb)->ack_seq;
  	bool is_dupack = false;
  	int prior_packets = tp->packets_out;
  	u32 delivered = tp->delivered;
  	u32 lost = tp->lost;
  	int rexmit = REXMIT_NONE; /* Flag to (re)transmit to recover losses */
  	u32 prior_fack;
  
  	sack_state.first_sackt = 0;
  	sack_state.rate = &rs;
  
  	/* We very likely will need to access rtx queue. */
  	prefetch(sk->tcp_rtx_queue.rb_node);
  
  	/* If the ack is older than previous acks
  	 * then we can probably ignore it.
  	 */
  	if (before(ack, prior_snd_una)) {
  		/* RFC 5961 5.2 [Blind Data Injection Attack].[Mitigation] */
  		if (before(ack, prior_snd_una - tp->max_window)) {
  			if (!(flag & FLAG_NO_CHALLENGE_ACK))
  				tcp_send_challenge_ack(sk, skb);
  			return -1;
  		}
  		goto old_ack;
  	}
  
  	/* If the ack includes data we haven't sent yet, discard
  	 * this segment (RFC793 Section 3.9).
  	 */
  	if (after(ack, tp->snd_nxt))
  		goto invalid_ack;
  
  	if (after(ack, prior_snd_una)) {
  		flag |= FLAG_SND_UNA_ADVANCED;
  		icsk->icsk_retransmits = 0;
  
  #if IS_ENABLED(CONFIG_TLS_DEVICE)
  		if (static_branch_unlikely(&clean_acked_data_enabled))
  			if (icsk->icsk_clean_acked)
  				icsk->icsk_clean_acked(sk, ack);
  #endif
  	}
  
  	prior_fack = tcp_is_sack(tp) ? tcp_highest_sack_seq(tp) : tp->snd_una;
  	rs.prior_in_flight = tcp_packets_in_flight(tp);
  
  	/* ts_recent update must be made after we are sure that the packet
  	 * is in window.
  	 */
  	if (flag & FLAG_UPDATE_TS_RECENT)
  		tcp_replace_ts_recent(tp, TCP_SKB_CB(skb)->seq);
  
  	if (!(flag & FLAG_SLOWPATH) && after(ack, prior_snd_una)) {
  		/* Window is constant, pure forward advance.
  		 * No more checks are required.
  		 * Note, we use the fact that SND.UNA>=SND.WL2.
  		 */
  		tcp_update_wl(tp, ack_seq);
  		tcp_snd_una_update(tp, ack);
  		flag |= FLAG_WIN_UPDATE;
  
  		tcp_in_ack_event(sk, CA_ACK_WIN_UPDATE);
  
  		NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPHPACKS);
  	} else {
  		u32 ack_ev_flags = CA_ACK_SLOWPATH;
  
  		if (ack_seq != TCP_SKB_CB(skb)->end_seq)
  			flag |= FLAG_DATA;
  		else
  			NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPPUREACKS);
  
  		flag |= tcp_ack_update_window(sk, skb, ack, ack_seq);
  
  		if (TCP_SKB_CB(skb)->sacked)
  			flag |= tcp_sacktag_write_queue(sk, skb, prior_snd_una,
  							&sack_state);
  
  		if (tcp_ecn_rcv_ecn_echo(tp, tcp_hdr(skb))) {
  			flag |= FLAG_ECE;
  			ack_ev_flags |= CA_ACK_ECE;
  		}
  
  		if (flag & FLAG_WIN_UPDATE)
  			ack_ev_flags |= CA_ACK_WIN_UPDATE;
  
  		tcp_in_ack_event(sk, ack_ev_flags);
  	}
  
  	/* We passed data and got it acked, remove any soft error
  	 * log. Something worked...
  	 */
  	sk->sk_err_soft = 0;
  	icsk->icsk_probes_out = 0;
  	tp->rcv_tstamp = tcp_jiffies32;
  	if (!prior_packets)
  		goto no_queue;
  
  	/* See if we can take anything off of the retransmit queue. */
  	flag |= tcp_clean_rtx_queue(sk, prior_fack, prior_snd_una, &sack_state);
  
  	tcp_rack_update_reo_wnd(sk, &rs);
  
  	if (tp->tlp_high_seq)
  		tcp_process_tlp_ack(sk, ack, flag);
  	/* If needed, reset TLP/RTO timer; RACK may later override this. */
  	if (flag & FLAG_SET_XMIT_TIMER)
  		tcp_set_xmit_timer(sk);
  
  	if (tcp_ack_is_dubious(sk, flag)) {
  		is_dupack = !(flag & (FLAG_SND_UNA_ADVANCED | FLAG_NOT_DUP));
  		tcp_fastretrans_alert(sk, prior_snd_una, is_dupack, &flag,
  				      &rexmit);
  	}
  
  	if ((flag & FLAG_FORWARD_PROGRESS) || !(flag & FLAG_NOT_DUP))
  		sk_dst_confirm(sk);
  
  	delivered = tcp_newly_delivered(sk, delivered, flag);
  	lost = tp->lost - lost;			/* freshly marked lost */
  	rs.is_ack_delayed = !!(flag & FLAG_ACK_MAYBE_DELAYED);
  	tcp_rate_gen(sk, delivered, lost, is_sack_reneg, sack_state.rate);
  	tcp_cong_control(sk, ack, delivered, flag, sack_state.rate);
  	tcp_xmit_recovery(sk, rexmit);
  	return 1;
  
  no_queue:
  	/* If data was DSACKed, see if we can undo a cwnd reduction. */
  	if (flag & FLAG_DSACKING_ACK) {
  		tcp_fastretrans_alert(sk, prior_snd_una, is_dupack, &flag,
  				      &rexmit);
  		tcp_newly_delivered(sk, delivered, flag);
  	}
  	/* If this ack opens up a zero window, clear backoff.  It was
  	 * being used to time the probes, and is probably far higher than
  	 * it needs to be for normal retransmission.
  	 */
  	tcp_ack_probe(sk);
  
  	if (tp->tlp_high_seq)
  		tcp_process_tlp_ack(sk, ack, flag);
  	return 1;
  
  invalid_ack:
  	SOCK_DEBUG(sk, "Ack %u after %u:%u\n", ack, tp->snd_una, tp->snd_nxt);
  	return -1;
  
  old_ack:
  	/* If data was SACKed, tag it and see if we should send more data.
  	 * If data was DSACKed, see if we can undo a cwnd reduction.
  	 */
  	if (TCP_SKB_CB(skb)->sacked) {
  		flag |= tcp_sacktag_write_queue(sk, skb, prior_snd_una,
  						&sack_state);
  		tcp_fastretrans_alert(sk, prior_snd_una, is_dupack, &flag,
  				      &rexmit);
  		tcp_newly_delivered(sk, delivered, flag);
  		tcp_xmit_recovery(sk, rexmit);
  	}
  
  	SOCK_DEBUG(sk, "Ack %u before %u:%u\n", ack, tp->snd_una, tp->snd_nxt);
  	return 0;
  }
  
  static void tcp_parse_fastopen_option(int len, const unsigned char *cookie,
  				      bool syn, struct tcp_fastopen_cookie *foc,
  				      bool exp_opt)
  {
  	/* Valid only in SYN or SYN-ACK with an even length.  */
  	if (!foc || !syn || len < 0 || (len & 1))
  		return;
  
  	if (len >= TCP_FASTOPEN_COOKIE_MIN &&
  	    len <= TCP_FASTOPEN_COOKIE_MAX)
  		memcpy(foc->val, cookie, len);
  	else if (len != 0)
  		len = -1;
  	foc->len = len;
  	foc->exp = exp_opt;
  }
  
  static void smc_parse_options(const struct tcphdr *th,
  			      struct tcp_options_received *opt_rx,
  			      const unsigned char *ptr,
  			      int opsize)
  {
  #if IS_ENABLED(CONFIG_SMC)
  	if (static_branch_unlikely(&tcp_have_smc)) {
  		if (th->syn && !(opsize & 1) &&
  		    opsize >= TCPOLEN_EXP_SMC_BASE &&
  		    get_unaligned_be32(ptr) == TCPOPT_SMC_MAGIC)
  			opt_rx->smc_ok = 1;
  	}
  #endif
  }
  
  /* Look for tcp options. Normally only called on SYN and SYNACK packets.
   * But, this can also be called on packets in the established flow when
   * the fast version below fails.
   */
  void tcp_parse_options(const struct net *net,
  		       const struct sk_buff *skb,
  		       struct tcp_options_received *opt_rx, int estab,
  		       struct tcp_fastopen_cookie *foc)
  {
  	const unsigned char *ptr;
  	const struct tcphdr *th = tcp_hdr(skb);
  	int length = (th->doff * 4) - sizeof(struct tcphdr);
  
  	ptr = (const unsigned char *)(th + 1);
  	opt_rx->saw_tstamp = 0;
  
  	while (length > 0) {
  		int opcode = *ptr++;
  		int opsize;
  
  		switch (opcode) {
  		case TCPOPT_EOL:
  			return;
  		case TCPOPT_NOP:	/* Ref: RFC 793 section 3.1 */
  			length--;
  			continue;
  		default:
  			opsize = *ptr++;
  			if (opsize < 2) /* "silly options" */
  				return;
  			if (opsize > length)
  				return;	/* don't parse partial options */
  			switch (opcode) {
  			case TCPOPT_MSS:
  				if (opsize == TCPOLEN_MSS && th->syn && !estab) {
  					u16 in_mss = get_unaligned_be16(ptr);
  					if (in_mss) {
  						if (opt_rx->user_mss &&
  						    opt_rx->user_mss < in_mss)
  							in_mss = opt_rx->user_mss;
  						opt_rx->mss_clamp = in_mss;
  					}
  				}
  				break;
  			case TCPOPT_WINDOW:
  				if (opsize == TCPOLEN_WINDOW && th->syn &&
  				    !estab && net->ipv4.sysctl_tcp_window_scaling) {
  					__u8 snd_wscale = *(__u8 *)ptr;
  					opt_rx->wscale_ok = 1;
  					if (snd_wscale > TCP_MAX_WSCALE) {
  						net_info_ratelimited("%s: Illegal window scaling value %d > %u received\n",
  								     __func__,
  								     snd_wscale,
  								     TCP_MAX_WSCALE);
  						snd_wscale = TCP_MAX_WSCALE;
  					}
  					opt_rx->snd_wscale = snd_wscale;
  				}
  				break;
  			case TCPOPT_TIMESTAMP:
  				if ((opsize == TCPOLEN_TIMESTAMP) &&
  				    ((estab && opt_rx->tstamp_ok) ||
  				     (!estab && net->ipv4.sysctl_tcp_timestamps))) {
  					opt_rx->saw_tstamp = 1;
  					opt_rx->rcv_tsval = get_unaligned_be32(ptr);
  					opt_rx->rcv_tsecr = get_unaligned_be32(ptr + 4);
  				}
  				break;
  			case TCPOPT_SACK_PERM:
  				if (opsize == TCPOLEN_SACK_PERM && th->syn &&
  				    !estab && net->ipv4.sysctl_tcp_sack) {
  					opt_rx->sack_ok = TCP_SACK_SEEN;
  					tcp_sack_reset(opt_rx);
  				}
  				break;
  
  			case TCPOPT_SACK:
  				if ((opsize >= (TCPOLEN_SACK_BASE + TCPOLEN_SACK_PERBLOCK)) &&
  				   !((opsize - TCPOLEN_SACK_BASE) % TCPOLEN_SACK_PERBLOCK) &&
  				   opt_rx->sack_ok) {
  					TCP_SKB_CB(skb)->sacked = (ptr - 2) - (unsigned char *)th;
  				}
  				break;
  #ifdef CONFIG_TCP_MD5SIG
  			case TCPOPT_MD5SIG:
  				/*
  				 * The MD5 Hash has already been
  				 * checked (see tcp_v{4,6}_do_rcv()).
  				 */
  				break;
  #endif
  			case TCPOPT_FASTOPEN:
  				tcp_parse_fastopen_option(
  					opsize - TCPOLEN_FASTOPEN_BASE,
  					ptr, th->syn, foc, false);
  				break;
  
  			case TCPOPT_EXP:
  				/* Fast Open option shares code 254 using a
  				 * 16 bits magic number.
  				 */
  				if (opsize >= TCPOLEN_EXP_FASTOPEN_BASE &&
  				    get_unaligned_be16(ptr) ==
  				    TCPOPT_FASTOPEN_MAGIC)
  					tcp_parse_fastopen_option(opsize -
  						TCPOLEN_EXP_FASTOPEN_BASE,
  						ptr + 2, th->syn, foc, true);
  				else
  					smc_parse_options(th, opt_rx, ptr,
  							  opsize);
  				break;
  
  			}
  			ptr += opsize-2;
  			length -= opsize;
  		}
  	}
  }
  EXPORT_SYMBOL(tcp_parse_options);
  
  static bool tcp_parse_aligned_timestamp(struct tcp_sock *tp, const struct tcphdr *th)
  {
  	const __be32 *ptr = (const __be32 *)(th + 1);
  
  	if (*ptr == htonl((TCPOPT_NOP << 24) | (TCPOPT_NOP << 16)
  			  | (TCPOPT_TIMESTAMP << 8) | TCPOLEN_TIMESTAMP)) {
  		tp->rx_opt.saw_tstamp = 1;
  		++ptr;
  		tp->rx_opt.rcv_tsval = ntohl(*ptr);
  		++ptr;
  		if (*ptr)
  			tp->rx_opt.rcv_tsecr = ntohl(*ptr) - tp->tsoffset;
  		else
  			tp->rx_opt.rcv_tsecr = 0;
  		return true;
  	}
  	return false;
  }
  
  /* Fast parse options. This hopes to only see timestamps.
   * If it is wrong it falls back on tcp_parse_options().
   */
  static bool tcp_fast_parse_options(const struct net *net,
  				   const struct sk_buff *skb,
  				   const struct tcphdr *th, struct tcp_sock *tp)
  {
  	/* In the spirit of fast parsing, compare doff directly to constant
  	 * values.  Because equality is used, short doff can be ignored here.
  	 */
  	if (th->doff == (sizeof(*th) / 4)) {
  		tp->rx_opt.saw_tstamp = 0;
  		return false;
  	} else if (tp->rx_opt.tstamp_ok &&
  		   th->doff == ((sizeof(*th) + TCPOLEN_TSTAMP_ALIGNED) / 4)) {
  		if (tcp_parse_aligned_timestamp(tp, th))
  			return true;
  	}
  
  	tcp_parse_options(net, skb, &tp->rx_opt, 1, NULL);
  	if (tp->rx_opt.saw_tstamp && tp->rx_opt.rcv_tsecr)
  		tp->rx_opt.rcv_tsecr -= tp->tsoffset;
  
  	return true;
  }
  
  #ifdef CONFIG_TCP_MD5SIG
  /*
   * Parse MD5 Signature option
   */
  const u8 *tcp_parse_md5sig_option(const struct tcphdr *th)
  {
  	int length = (th->doff << 2) - sizeof(*th);
  	const u8 *ptr = (const u8 *)(th + 1);
  
  	/* If not enough data remaining, we can short cut */
  	while (length >= TCPOLEN_MD5SIG) {
  		int opcode = *ptr++;
  		int opsize;
  
  		switch (opcode) {
  		case TCPOPT_EOL:
  			return NULL;
  		case TCPOPT_NOP:
  			length--;
  			continue;
  		default:
  			opsize = *ptr++;
  			if (opsize < 2 || opsize > length)
  				return NULL;
  			if (opcode == TCPOPT_MD5SIG)
  				return opsize == TCPOLEN_MD5SIG ? ptr : NULL;
  		}
  		ptr += opsize - 2;
  		length -= opsize;
  	}
  	return NULL;
  }
  EXPORT_SYMBOL(tcp_parse_md5sig_option);
  #endif
  
  /* Sorry, PAWS as specified is broken wrt. pure-ACKs -DaveM
   *
   * It is not fatal. If this ACK does _not_ change critical state (seqs, window)
   * it can pass through stack. So, the following predicate verifies that
   * this segment is not used for anything but congestion avoidance or
   * fast retransmit. Moreover, we even are able to eliminate most of such
   * second order effects, if we apply some small "replay" window (~RTO)
   * to timestamp space.
   *
   * All these measures still do not guarantee that we reject wrapped ACKs
   * on networks with high bandwidth, when sequence space is recycled fastly,
   * but it guarantees that such events will be very rare and do not affect
   * connection seriously. This doesn't look nice, but alas, PAWS is really
   * buggy extension.
   *
   * [ Later note. Even worse! It is buggy for segments _with_ data. RFC
   * states that events when retransmit arrives after original data are rare.
   * It is a blatant lie. VJ forgot about fast retransmit! 8)8) It is
   * the biggest problem on large power networks even with minor reordering.
   * OK, let's give it small replay window. If peer clock is even 1hz, it is safe
   * up to bandwidth of 18Gigabit/sec. 8) ]
   */
  
  static int tcp_disordered_ack(const struct sock *sk, const struct sk_buff *skb)
  {
  	const struct tcp_sock *tp = tcp_sk(sk);
  	const struct tcphdr *th = tcp_hdr(skb);
  	u32 seq = TCP_SKB_CB(skb)->seq;
  	u32 ack = TCP_SKB_CB(skb)->ack_seq;
  
  	return (/* 1. Pure ACK with correct sequence number. */
  		(th->ack && seq == TCP_SKB_CB(skb)->end_seq && seq == tp->rcv_nxt) &&
  
  		/* 2. ... and duplicate ACK. */
  		ack == tp->snd_una &&
  
  		/* 3. ... and does not update window. */
  		!tcp_may_update_window(tp, ack, seq, ntohs(th->window) << tp->rx_opt.snd_wscale) &&
  
  		/* 4. ... and sits in replay window. */
  		(s32)(tp->rx_opt.ts_recent - tp->rx_opt.rcv_tsval) <= (inet_csk(sk)->icsk_rto * 1024) / HZ);
  }
  
  static inline bool tcp_paws_discard(const struct sock *sk,
  				   const struct sk_buff *skb)
  {
  	const struct tcp_sock *tp = tcp_sk(sk);
  
  	return !tcp_paws_check(&tp->rx_opt, TCP_PAWS_WINDOW) &&
  	       !tcp_disordered_ack(sk, skb);
  }
  
  /* Check segment sequence number for validity.
   *
   * Segment controls are considered valid, if the segment
   * fits to the window after truncation to the window. Acceptability
   * of data (and SYN, FIN, of course) is checked separately.
   * See tcp_data_queue(), for example.
   *
   * Also, controls (RST is main one) are accepted using RCV.WUP instead
   * of RCV.NXT. Peer still did not advance his SND.UNA when we
   * delayed ACK, so that hisSND.UNA<=ourRCV.WUP.
   * (borrowed from freebsd)
   */
  
  static inline bool tcp_sequence(const struct tcp_sock *tp, u32 seq, u32 end_seq)
  {
  	return	!before(end_seq, tp->rcv_wup) &&
  		!after(seq, tp->rcv_nxt + tcp_receive_window(tp));
  }
  
  /* When we get a reset we do this. */
  void tcp_reset(struct sock *sk)
  {
  	trace_tcp_receive_reset(sk);
  
  	/* We want the right error as BSD sees it (and indeed as we do). */
  	switch (sk->sk_state) {
  	case TCP_SYN_SENT:
  		sk->sk_err = ECONNREFUSED;
  		break;
  	case TCP_CLOSE_WAIT:
  		sk->sk_err = EPIPE;
  		break;
  	case TCP_CLOSE:
  		return;
  	default:
  		sk->sk_err = ECONNRESET;
  	}
  	/* This barrier is coupled with smp_rmb() in tcp_poll() */
  	smp_wmb();
  
  	tcp_write_queue_purge(sk);
  	tcp_done(sk);
  
  	if (!sock_flag(sk, SOCK_DEAD))
  		sk->sk_error_report(sk);
  }
  
  /*
   * 	Process the FIN bit. This now behaves as it is supposed to work
   *	and the FIN takes effect when it is validly part of sequence
   *	space. Not before when we get holes.
   *
   *	If we are ESTABLISHED, a received fin moves us to CLOSE-WAIT
   *	(and thence onto LAST-ACK and finally, CLOSE, we never enter
   *	TIME-WAIT)
   *
   *	If we are in FINWAIT-1, a received FIN indicates simultaneous
   *	close and we go into CLOSING (and later onto TIME-WAIT)
   *
   *	If we are in FINWAIT-2, a received FIN moves us to TIME-WAIT.
   */
  void tcp_fin(struct sock *sk)
  {
  	struct tcp_sock *tp = tcp_sk(sk);
  
  	inet_csk_schedule_ack(sk);
  
  	sk->sk_shutdown |= RCV_SHUTDOWN;
  	sock_set_flag(sk, SOCK_DONE);
  
  	switch (sk->sk_state) {
  	case TCP_SYN_RECV:
  	case TCP_ESTABLISHED:
  		/* Move to CLOSE_WAIT */
  		tcp_set_state(sk, TCP_CLOSE_WAIT);
  		inet_csk(sk)->icsk_ack.pingpong = 1;
  		break;
  
  	case TCP_CLOSE_WAIT:
  	case TCP_CLOSING:
  		/* Received a retransmission of the FIN, do
  		 * nothing.
  		 */
  		break;
  	case TCP_LAST_ACK:
  		/* RFC793: Remain in the LAST-ACK state. */
  		break;
  
  	case TCP_FIN_WAIT1:
  		/* This case occurs when a simultaneous close
  		 * happens, we must ack the received FIN and
  		 * enter the CLOSING state.
  		 */
  		tcp_send_ack(sk);
  		tcp_set_state(sk, TCP_CLOSING);
  		break;
  	case TCP_FIN_WAIT2:
  		/* Received a FIN -- send ACK and enter TIME_WAIT. */
  		tcp_send_ack(sk);
  		tcp_time_wait(sk, TCP_TIME_WAIT, 0);
  		break;
  	default:
  		/* Only TCP_LISTEN and TCP_CLOSE are left, in these
  		 * cases we should never reach this piece of code.
  		 */
  		pr_err("%s: Impossible, sk->sk_state=%d\n",
  		       __func__, sk->sk_state);
  		break;
  	}
  
  	/* It _is_ possible, that we have something out-of-order _after_ FIN.
  	 * Probably, we should reset in this case. For now drop them.
  	 */
  	skb_rbtree_purge(&tp->out_of_order_queue);
  	if (tcp_is_sack(tp))
  		tcp_sack_reset(&tp->rx_opt);
  	sk_mem_reclaim(sk);
  
  	if (!sock_flag(sk, SOCK_DEAD)) {
  		sk->sk_state_change(sk);
  
  		/* Do not send POLL_HUP for half duplex close. */
  		if (sk->sk_shutdown == SHUTDOWN_MASK ||
  		    sk->sk_state == TCP_CLOSE)
  			sk_wake_async(sk, SOCK_WAKE_WAITD, POLL_HUP);
  		else
  			sk_wake_async(sk, SOCK_WAKE_WAITD, POLL_IN);
  	}
  }
  
  static inline bool tcp_sack_extend(struct tcp_sack_block *sp, u32 seq,
  				  u32 end_seq)
  {
  	if (!after(seq, sp->end_seq) && !after(sp->start_seq, end_seq)) {
  		if (before(seq, sp->start_seq))
  			sp->start_seq = seq;
  		if (after(end_seq, sp->end_seq))
  			sp->end_seq = end_seq;
  		return true;
  	}
  	return false;
  }
  
  static void tcp_dsack_set(struct sock *sk, u32 seq, u32 end_seq)
  {
  	struct tcp_sock *tp = tcp_sk(sk);
  
  	if (tcp_is_sack(tp) && sock_net(sk)->ipv4.sysctl_tcp_dsack) {
  		int mib_idx;
  
  		if (before(seq, tp->rcv_nxt))
  			mib_idx = LINUX_MIB_TCPDSACKOLDSENT;
  		else
  			mib_idx = LINUX_MIB_TCPDSACKOFOSENT;
  
  		NET_INC_STATS(sock_net(sk), mib_idx);
  
  		tp->rx_opt.dsack = 1;
  		tp->duplicate_sack[0].start_seq = seq;
  		tp->duplicate_sack[0].end_seq = end_seq;
  	}
  }
  
  static void tcp_dsack_extend(struct sock *sk, u32 seq, u32 end_seq)
  {
  	struct tcp_sock *tp = tcp_sk(sk);
  
  	if (!tp->rx_opt.dsack)
  		tcp_dsack_set(sk, seq, end_seq);
  	else
  		tcp_sack_extend(tp->duplicate_sack, seq, end_seq);
  }
  
  static void tcp_send_dupack(struct sock *sk, const struct sk_buff *skb)
  {
  	struct tcp_sock *tp = tcp_sk(sk);
  
  	if (TCP_SKB_CB(skb)->end_seq != TCP_SKB_CB(skb)->seq &&
  	    before(TCP_SKB_CB(skb)->seq, tp->rcv_nxt)) {
  		NET_INC_STATS(sock_net(sk), LINUX_MIB_DELAYEDACKLOST);
  		tcp_enter_quickack_mode(sk, TCP_MAX_QUICKACKS);
  
  		if (tcp_is_sack(tp) && sock_net(sk)->ipv4.sysctl_tcp_dsack) {
  			u32 end_seq = TCP_SKB_CB(skb)->end_seq;
  
  			if (after(TCP_SKB_CB(skb)->end_seq, tp->rcv_nxt))
  				end_seq = tp->rcv_nxt;
  			tcp_dsack_set(sk, TCP_SKB_CB(skb)->seq, end_seq);
  		}
  	}
  
  	tcp_send_ack(sk);
  }
  
  /* These routines update the SACK block as out-of-order packets arrive or
   * in-order packets close up the sequence space.
   */
  static void tcp_sack_maybe_coalesce(struct tcp_sock *tp)
  {
  	int this_sack;
  	struct tcp_sack_block *sp = &tp->selective_acks[0];
  	struct tcp_sack_block *swalk = sp + 1;
  
  	/* See if the recent change to the first SACK eats into
  	 * or hits the sequence space of other SACK blocks, if so coalesce.
  	 */
  	for (this_sack = 1; this_sack < tp->rx_opt.num_sacks;) {
  		if (tcp_sack_extend(sp, swalk->start_seq, swalk->end_seq)) {
  			int i;
  
  			/* Zap SWALK, by moving every further SACK up by one slot.
  			 * Decrease num_sacks.
  			 */
  			tp->rx_opt.num_sacks--;
  			for (i = this_sack; i < tp->rx_opt.num_sacks; i++)
  				sp[i] = sp[i + 1];
  			continue;
  		}
  		this_sack++, swalk++;
  	}
  }
  
  static void tcp_sack_new_ofo_skb(struct sock *sk, u32 seq, u32 end_seq)
  {
  	struct tcp_sock *tp = tcp_sk(sk);
  	struct tcp_sack_block *sp = &tp->selective_acks[0];
  	int cur_sacks = tp->rx_opt.num_sacks;
  	int this_sack;
  
  	if (!cur_sacks)
  		goto new_sack;
  
  	for (this_sack = 0; this_sack < cur_sacks; this_sack++, sp++) {
  		if (tcp_sack_extend(sp, seq, end_seq)) {
  			/* Rotate this_sack to the first one. */
  			for (; this_sack > 0; this_sack--, sp--)
  				swap(*sp, *(sp - 1));
  			if (cur_sacks > 1)
  				tcp_sack_maybe_coalesce(tp);
  			return;
  		}
  	}
  
  	/* Could not find an adjacent existing SACK, build a new one,
  	 * put it at the front, and shift everyone else down.  We
  	 * always know there is at least one SACK present already here.
  	 *
  	 * If the sack array is full, forget about the last one.
  	 */
  	if (this_sack >= TCP_NUM_SACKS) {
  		if (tp->compressed_ack)
  			tcp_send_ack(sk);
  		this_sack--;
  		tp->rx_opt.num_sacks--;
  		sp--;
  	}
  	for (; this_sack > 0; this_sack--, sp--)
  		*sp = *(sp - 1);
  
  new_sack:
  	/* Build the new head SACK, and we're done. */
  	sp->start_seq = seq;
  	sp->end_seq = end_seq;
  	tp->rx_opt.num_sacks++;
  }
  
  /* RCV.NXT advances, some SACKs should be eaten. */
  
  static void tcp_sack_remove(struct tcp_sock *tp)
  {
  	struct tcp_sack_block *sp = &tp->selective_acks[0];
  	int num_sacks = tp->rx_opt.num_sacks;
  	int this_sack;
  
  	/* Empty ofo queue, hence, all the SACKs are eaten. Clear. */
  	if (RB_EMPTY_ROOT(&tp->out_of_order_queue)) {
  		tp->rx_opt.num_sacks = 0;
  		return;
  	}
  
  	for (this_sack = 0; this_sack < num_sacks;) {
  		/* Check if the start of the sack is covered by RCV.NXT. */
  		if (!before(tp->rcv_nxt, sp->start_seq)) {
  			int i;
  
  			/* RCV.NXT must cover all the block! */
  			WARN_ON(before(tp->rcv_nxt, sp->end_seq));
  
  			/* Zap this SACK, by moving forward any other SACKS. */
  			for (i = this_sack+1; i < num_sacks; i++)
  				tp->selective_acks[i-1] = tp->selective_acks[i];
  			num_sacks--;
  			continue;
  		}
  		this_sack++;
  		sp++;
  	}
  	tp->rx_opt.num_sacks = num_sacks;
  }
  
  /**
   * tcp_try_coalesce - try to merge skb to prior one
   * @sk: socket
   * @dest: destination queue
   * @to: prior buffer
   * @from: buffer to add in queue
   * @fragstolen: pointer to boolean
   *
   * Before queueing skb @from after @to, try to merge them
   * to reduce overall memory use and queue lengths, if cost is small.
   * Packets in ofo or receive queues can stay a long time.
   * Better try to coalesce them right now to avoid future collapses.
   * Returns true if caller should free @from instead of queueing it
   */
  static bool tcp_try_coalesce(struct sock *sk,
  			     struct sk_buff *to,
  			     struct sk_buff *from,
  			     bool *fragstolen)
  {
  	int delta;
  
  	*fragstolen = false;
  
  	/* Its possible this segment overlaps with prior segment in queue */
  	if (TCP_SKB_CB(from)->seq != TCP_SKB_CB(to)->end_seq)
  		return false;
  
  	if (!skb_try_coalesce(to, from, fragstolen, &delta))
  		return false;
  
  	atomic_add(delta, &sk->sk_rmem_alloc);
  	sk_mem_charge(sk, delta);
  	NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPRCVCOALESCE);
  	TCP_SKB_CB(to)->end_seq = TCP_SKB_CB(from)->end_seq;
  	TCP_SKB_CB(to)->ack_seq = TCP_SKB_CB(from)->ack_seq;
  	TCP_SKB_CB(to)->tcp_flags |= TCP_SKB_CB(from)->tcp_flags;
  
  	if (TCP_SKB_CB(from)->has_rxtstamp) {
  		TCP_SKB_CB(to)->has_rxtstamp = true;
  		to->tstamp = from->tstamp;
  	}
  
  	return true;
  }
  
  static void tcp_drop(struct sock *sk, struct sk_buff *skb)
  {
  	sk_drops_add(sk, skb);
  	__kfree_skb(skb);
  }
  
  /* This one checks to see if we can put data from the
   * out_of_order queue into the receive_queue.
   */
  static void tcp_ofo_queue(struct sock *sk)
  {
  	struct tcp_sock *tp = tcp_sk(sk);
  	__u32 dsack_high = tp->rcv_nxt;
  	bool fin, fragstolen, eaten;
  	struct sk_buff *skb, *tail;
  	struct rb_node *p;
  
  	p = rb_first(&tp->out_of_order_queue);
  	while (p) {
  		skb = rb_to_skb(p);
  		if (after(TCP_SKB_CB(skb)->seq, tp->rcv_nxt))
  			break;
  
  		if (before(TCP_SKB_CB(skb)->seq, dsack_high)) {
  			__u32 dsack = dsack_high;
  			if (before(TCP_SKB_CB(skb)->end_seq, dsack_high))
  				dsack_high = TCP_SKB_CB(skb)->end_seq;
  			tcp_dsack_extend(sk, TCP_SKB_CB(skb)->seq, dsack);
  		}
  		p = rb_next(p);
  		rb_erase(&skb->rbnode, &tp->out_of_order_queue);
  
  		if (unlikely(!after(TCP_SKB_CB(skb)->end_seq, tp->rcv_nxt))) {
  			SOCK_DEBUG(sk, "ofo packet was already received\n");
  			tcp_drop(sk, skb);
  			continue;
  		}
  		SOCK_DEBUG(sk, "ofo requeuing : rcv_next %X seq %X - %X\n",
  			   tp->rcv_nxt, TCP_SKB_CB(skb)->seq,
  			   TCP_SKB_CB(skb)->end_seq);
  
  		tail = skb_peek_tail(&sk->sk_receive_queue);
  		eaten = tail && tcp_try_coalesce(sk, tail, skb, &fragstolen);
  		tcp_rcv_nxt_update(tp, TCP_SKB_CB(skb)->end_seq);
  		fin = TCP_SKB_CB(skb)->tcp_flags & TCPHDR_FIN;
  		if (!eaten)
  			__skb_queue_tail(&sk->sk_receive_queue, skb);
  		else
  			kfree_skb_partial(skb, fragstolen);
  
  		if (unlikely(fin)) {
  			tcp_fin(sk);
  			/* tcp_fin() purges tp->out_of_order_queue,
  			 * so we must end this loop right now.
  			 */
  			break;
  		}
  	}
  }
  
  static bool tcp_prune_ofo_queue(struct sock *sk);
  static int tcp_prune_queue(struct sock *sk);
  
  static int tcp_try_rmem_schedule(struct sock *sk, struct sk_buff *skb,
  				 unsigned int size)
  {
  	if (atomic_read(&sk->sk_rmem_alloc) > sk->sk_rcvbuf ||
  	    !sk_rmem_schedule(sk, skb, size)) {
  
  		if (tcp_prune_queue(sk) < 0)
  			return -1;
  
  		while (!sk_rmem_schedule(sk, skb, size)) {
  			if (!tcp_prune_ofo_queue(sk))
  				return -1;
  		}
  	}
  	return 0;
  }
  
  static void tcp_data_queue_ofo(struct sock *sk, struct sk_buff *skb)
  {
  	struct tcp_sock *tp = tcp_sk(sk);
  	struct rb_node **p, *parent;
  	struct sk_buff *skb1;
  	u32 seq, end_seq;
  	bool fragstolen;
  
  	tcp_ecn_check_ce(sk, skb);
  
  	if (unlikely(tcp_try_rmem_schedule(sk, skb, skb->truesize))) {
  		NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPOFODROP);
  		tcp_drop(sk, skb);
  		return;
  	}
  
  	/* Disable header prediction. */
  	tp->pred_flags = 0;
  	inet_csk_schedule_ack(sk);
  
  	NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPOFOQUEUE);
  	seq = TCP_SKB_CB(skb)->seq;
  	end_seq = TCP_SKB_CB(skb)->end_seq;
  	SOCK_DEBUG(sk, "out of order segment: rcv_next %X seq %X - %X\n",
  		   tp->rcv_nxt, seq, end_seq);
  
  	p = &tp->out_of_order_queue.rb_node;
  	if (RB_EMPTY_ROOT(&tp->out_of_order_queue)) {
  		/* Initial out of order segment, build 1 SACK. */
  		if (tcp_is_sack(tp)) {
  			tp->rx_opt.num_sacks = 1;
  			tp->selective_acks[0].start_seq = seq;
  			tp->selective_acks[0].end_seq = end_seq;
  		}
  		rb_link_node(&skb->rbnode, NULL, p);
  		rb_insert_color(&skb->rbnode, &tp->out_of_order_queue);
  		tp->ooo_last_skb = skb;
  		goto end;
  	}
  
  	/* In the typical case, we are adding an skb to the end of the list.
  	 * Use of ooo_last_skb avoids the O(Log(N)) rbtree lookup.
  	 */
  	if (tcp_try_coalesce(sk, tp->ooo_last_skb,
  			     skb, &fragstolen)) {
  coalesce_done:
  		tcp_grow_window(sk, skb);
  		kfree_skb_partial(skb, fragstolen);
  		skb = NULL;
  		goto add_sack;
  	}
  	/* Can avoid an rbtree lookup if we are adding skb after ooo_last_skb */
  	if (!before(seq, TCP_SKB_CB(tp->ooo_last_skb)->end_seq)) {
  		parent = &tp->ooo_last_skb->rbnode;
  		p = &parent->rb_right;
  		goto insert;
  	}
  
  	/* Find place to insert this segment. Handle overlaps on the way. */
  	parent = NULL;
  	while (*p) {
  		parent = *p;
  		skb1 = rb_to_skb(parent);
  		if (before(seq, TCP_SKB_CB(skb1)->seq)) {
  			p = &parent->rb_left;
  			continue;
  		}
  		if (before(seq, TCP_SKB_CB(skb1)->end_seq)) {
  			if (!after(end_seq, TCP_SKB_CB(skb1)->end_seq)) {
  				/* All the bits are present. Drop. */
  				NET_INC_STATS(sock_net(sk),
  					      LINUX_MIB_TCPOFOMERGE);
  				__kfree_skb(skb);
  				skb = NULL;
  				tcp_dsack_set(sk, seq, end_seq);
  				goto add_sack;
  			}
  			if (after(seq, TCP_SKB_CB(skb1)->seq)) {
  				/* Partial overlap. */
  				tcp_dsack_set(sk, seq, TCP_SKB_CB(skb1)->end_seq);
  			} else {
  				/* skb's seq == skb1's seq and skb covers skb1.
  				 * Replace skb1 with skb.
  				 */
  				rb_replace_node(&skb1->rbnode, &skb->rbnode,
  						&tp->out_of_order_queue);
  				tcp_dsack_extend(sk,
  						 TCP_SKB_CB(skb1)->seq,
  						 TCP_SKB_CB(skb1)->end_seq);
  				NET_INC_STATS(sock_net(sk),
  					      LINUX_MIB_TCPOFOMERGE);
  				__kfree_skb(skb1);
  				goto merge_right;
  			}
  		} else if (tcp_try_coalesce(sk, skb1,
  					    skb, &fragstolen)) {
  			goto coalesce_done;
  		}
  		p = &parent->rb_right;
  	}
  insert:
  	/* Insert segment into RB tree. */
  	rb_link_node(&skb->rbnode, parent, p);
  	rb_insert_color(&skb->rbnode, &tp->out_of_order_queue);
  
  merge_right:
  	/* Remove other segments covered by skb. */
  	while ((skb1 = skb_rb_next(skb)) != NULL) {
  		if (!after(end_seq, TCP_SKB_CB(skb1)->seq))
  			break;
  		if (before(end_seq, TCP_SKB_CB(skb1)->end_seq)) {
  			tcp_dsack_extend(sk, TCP_SKB_CB(skb1)->seq,
  					 end_seq);
  			break;
  		}
  		rb_erase(&skb1->rbnode, &tp->out_of_order_queue);
  		tcp_dsack_extend(sk, TCP_SKB_CB(skb1)->seq,
  				 TCP_SKB_CB(skb1)->end_seq);
  		NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPOFOMERGE);
  		tcp_drop(sk, skb1);
  	}
  	/* If there is no skb after us, we are the last_skb ! */
  	if (!skb1)
  		tp->ooo_last_skb = skb;
  
  add_sack:
  	if (tcp_is_sack(tp))
  		tcp_sack_new_ofo_skb(sk, seq, end_seq);
  end:
  	if (skb) {
  		tcp_grow_window(sk, skb);
  		skb_condense(skb);
  		skb_set_owner_r(skb, sk);
  	}
  }
  
  static int __must_check tcp_queue_rcv(struct sock *sk, struct sk_buff *skb, int hdrlen,
  		  bool *fragstolen)
  {
  	int eaten;
  	struct sk_buff *tail = skb_peek_tail(&sk->sk_receive_queue);
  
  	__skb_pull(skb, hdrlen);
  	eaten = (tail &&
  		 tcp_try_coalesce(sk, tail,
  				  skb, fragstolen)) ? 1 : 0;
  	tcp_rcv_nxt_update(tcp_sk(sk), TCP_SKB_CB(skb)->end_seq);
  	if (!eaten) {
  		__skb_queue_tail(&sk->sk_receive_queue, skb);
  		skb_set_owner_r(skb, sk);
  	}
  	return eaten;
  }
  
  int tcp_send_rcvq(struct sock *sk, struct msghdr *msg, size_t size)
  {
  	struct sk_buff *skb;
  	int err = -ENOMEM;
  	int data_len = 0;
  	bool fragstolen;
  
  	if (size == 0)
  		return 0;
  
  	if (size > PAGE_SIZE) {
  		int npages = min_t(size_t, size >> PAGE_SHIFT, MAX_SKB_FRAGS);
  
  		data_len = npages << PAGE_SHIFT;
  		size = data_len + (size & ~PAGE_MASK);
  	}
  	skb = alloc_skb_with_frags(size - data_len, data_len,
  				   PAGE_ALLOC_COSTLY_ORDER,
  				   &err, sk->sk_allocation);
  	if (!skb)
  		goto err;
  
  	skb_put(skb, size - data_len);
  	skb->data_len = data_len;
  	skb->len = size;
  
  	if (tcp_try_rmem_schedule(sk, skb, skb->truesize))
  		goto err_free;
  
  	err = skb_copy_datagram_from_iter(skb, 0, &msg->msg_iter, size);
  	if (err)
  		goto err_free;
  
  	TCP_SKB_CB(skb)->seq = tcp_sk(sk)->rcv_nxt;
  	TCP_SKB_CB(skb)->end_seq = TCP_SKB_CB(skb)->seq + size;
  	TCP_SKB_CB(skb)->ack_seq = tcp_sk(sk)->snd_una - 1;
  
  	if (tcp_queue_rcv(sk, skb, 0, &fragstolen)) {
  		WARN_ON_ONCE(fragstolen); /* should not happen */
  		__kfree_skb(skb);
  	}
  	return size;
  
  err_free:
  	kfree_skb(skb);
  err:
  	return err;
  
  }
  
  void tcp_data_ready(struct sock *sk)
  {
  	const struct tcp_sock *tp = tcp_sk(sk);
  	int avail = tp->rcv_nxt - tp->copied_seq;
  
  	if (avail < sk->sk_rcvlowat && !sock_flag(sk, SOCK_DONE))
  		return;
  
  	sk->sk_data_ready(sk);
  }
  
  static void tcp_data_queue(struct sock *sk, struct sk_buff *skb)
  {
  	struct tcp_sock *tp = tcp_sk(sk);
  	bool fragstolen;
  	int eaten;
  
  	if (TCP_SKB_CB(skb)->seq == TCP_SKB_CB(skb)->end_seq) {
  		__kfree_skb(skb);
  		return;
  	}
  	skb_dst_drop(skb);
  	__skb_pull(skb, tcp_hdr(skb)->doff * 4);
  
  	tcp_ecn_accept_cwr(tp, skb);
  
  	tp->rx_opt.dsack = 0;
  
  	/*  Queue data for delivery to the user.
  	 *  Packets in sequence go to the receive queue.
  	 *  Out of sequence packets to the out_of_order_queue.
  	 */
  	if (TCP_SKB_CB(skb)->seq == tp->rcv_nxt) {
  		if (tcp_receive_window(tp) == 0)
  			goto out_of_window;
  
  		/* Ok. In sequence. In window. */
  queue_and_out:
  		if (skb_queue_len(&sk->sk_receive_queue) == 0)
  			sk_forced_mem_schedule(sk, skb->truesize);
  		else if (tcp_try_rmem_schedule(sk, skb, skb->truesize))
  			goto drop;
  
  		eaten = tcp_queue_rcv(sk, skb, 0, &fragstolen);
  		tcp_rcv_nxt_update(tp, TCP_SKB_CB(skb)->end_seq);
  		if (skb->len)
  			tcp_event_data_recv(sk, skb);
  		if (TCP_SKB_CB(skb)->tcp_flags & TCPHDR_FIN)
  			tcp_fin(sk);
  
  		if (!RB_EMPTY_ROOT(&tp->out_of_order_queue)) {
  			tcp_ofo_queue(sk);
  
  			/* RFC2581. 4.2. SHOULD send immediate ACK, when
  			 * gap in queue is filled.
  			 */
  			if (RB_EMPTY_ROOT(&tp->out_of_order_queue))
  				inet_csk(sk)->icsk_ack.pingpong = 0;
  		}
  
  		if (tp->rx_opt.num_sacks)
  			tcp_sack_remove(tp);
  
  		tcp_fast_path_check(sk);
  
  		if (eaten > 0)
  			kfree_skb_partial(skb, fragstolen);
  		if (!sock_flag(sk, SOCK_DEAD))
  			tcp_data_ready(sk);
  		return;
  	}
  
  	if (!after(TCP_SKB_CB(skb)->end_seq, tp->rcv_nxt)) {
  		/* A retransmit, 2nd most common case.  Force an immediate ack. */
  		NET_INC_STATS(sock_net(sk), LINUX_MIB_DELAYEDACKLOST);
  		tcp_dsack_set(sk, TCP_SKB_CB(skb)->seq, TCP_SKB_CB(skb)->end_seq);
  
  out_of_window:
  		tcp_enter_quickack_mode(sk, TCP_MAX_QUICKACKS);
  		inet_csk_schedule_ack(sk);
  drop:
  		tcp_drop(sk, skb);
  		return;
  	}
  
  	/* Out of window. F.e. zero window probe. */
  	if (!before(TCP_SKB_CB(skb)->seq, tp->rcv_nxt + tcp_receive_window(tp)))
  		goto out_of_window;
  
  	if (before(TCP_SKB_CB(skb)->seq, tp->rcv_nxt)) {
  		/* Partial packet, seq < rcv_next < end_seq */
  		SOCK_DEBUG(sk, "partial packet: rcv_next %X seq %X - %X\n",
  			   tp->rcv_nxt, TCP_SKB_CB(skb)->seq,
  			   TCP_SKB_CB(skb)->end_seq);
  
  		tcp_dsack_set(sk, TCP_SKB_CB(skb)->seq, tp->rcv_nxt);
  
  		/* If window is closed, drop tail of packet. But after
  		 * remembering D-SACK for its head made in previous line.
  		 */
  		if (!tcp_receive_window(tp))
  			goto out_of_window;
  		goto queue_and_out;
  	}
  
  	tcp_data_queue_ofo(sk, skb);
  }
  
  static struct sk_buff *tcp_skb_next(struct sk_buff *skb, struct sk_buff_head *list)
  {
  	if (list)
  		return !skb_queue_is_last(list, skb) ? skb->next : NULL;
  
  	return skb_rb_next(skb);
  }
  
  static struct sk_buff *tcp_collapse_one(struct sock *sk, struct sk_buff *skb,
  					struct sk_buff_head *list,
  					struct rb_root *root)
  {
  	struct sk_buff *next = tcp_skb_next(skb, list);
  
  	if (list)
  		__skb_unlink(skb, list);
  	else
  		rb_erase(&skb->rbnode, root);
  
  	__kfree_skb(skb);
  	NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPRCVCOLLAPSED);
  
  	return next;
  }
  
  /* Insert skb into rb tree, ordered by TCP_SKB_CB(skb)->seq */
  void tcp_rbtree_insert(struct rb_root *root, struct sk_buff *skb)
  {
  	struct rb_node **p = &root->rb_node;
  	struct rb_node *parent = NULL;
  	struct sk_buff *skb1;
  
  	while (*p) {
  		parent = *p;
  		skb1 = rb_to_skb(parent);
  		if (before(TCP_SKB_CB(skb)->seq, TCP_SKB_CB(skb1)->seq))
  			p = &parent->rb_left;
  		else
  			p = &parent->rb_right;
  	}
  	rb_link_node(&skb->rbnode, parent, p);
  	rb_insert_color(&skb->rbnode, root);
  }
  
  /* Collapse contiguous sequence of skbs head..tail with
   * sequence numbers start..end.
   *
   * If tail is NULL, this means until the end of the queue.
   *
   * Segments with FIN/SYN are not collapsed (only because this
   * simplifies code)
   */
  static void
  tcp_collapse(struct sock *sk, struct sk_buff_head *list, struct rb_root *root,
  	     struct sk_buff *head, struct sk_buff *tail, u32 start, u32 end)
  {
  	struct sk_buff *skb = head, *n;
  	struct sk_buff_head tmp;
  	bool end_of_skbs;
  
  	/* First, check that queue is collapsible and find
  	 * the point where collapsing can be useful.
  	 */
  restart:
  	for (end_of_skbs = true; skb != NULL && skb != tail; skb = n) {
  		n = tcp_skb_next(skb, list);
  
  		/* No new bits? It is possible on ofo queue. */
  		if (!before(start, TCP_SKB_CB(skb)->end_seq)) {
  			skb = tcp_collapse_one(sk, skb, list, root);
  			if (!skb)
  				break;
  			goto restart;
  		}
  
  		/* The first skb to collapse is:
  		 * - not SYN/FIN and
  		 * - bloated or contains data before "start" or
  		 *   overlaps to the next one.
  		 */
  		if (!(TCP_SKB_CB(skb)->tcp_flags & (TCPHDR_SYN | TCPHDR_FIN)) &&
  		    (tcp_win_from_space(sk, skb->truesize) > skb->len ||
  		     before(TCP_SKB_CB(skb)->seq, start))) {
  			end_of_skbs = false;
  			break;
  		}
  
  		if (n && n != tail &&
  		    TCP_SKB_CB(skb)->end_seq != TCP_SKB_CB(n)->seq) {
  			end_of_skbs = false;
  			break;
  		}
  
  		/* Decided to skip this, advance start seq. */
  		start = TCP_SKB_CB(skb)->end_seq;
  	}
  	if (end_of_skbs ||
  	    (TCP_SKB_CB(skb)->tcp_flags & (TCPHDR_SYN | TCPHDR_FIN)))
  		return;
  
  	__skb_queue_head_init(&tmp);
  
  	while (before(start, end)) {
  		int copy = min_t(int, SKB_MAX_ORDER(0, 0), end - start);
  		struct sk_buff *nskb;
  
  		nskb = alloc_skb(copy, GFP_ATOMIC);
  		if (!nskb)
  			break;
  
  		memcpy(nskb->cb, skb->cb, sizeof(skb->cb));
  		TCP_SKB_CB(nskb)->seq = TCP_SKB_CB(nskb)->end_seq = start;
  		if (list)
  			__skb_queue_before(list, skb, nskb);
  		else
  			__skb_queue_tail(&tmp, nskb); /* defer rbtree insertion */
  		skb_set_owner_r(nskb, sk);
  
  		/* Copy data, releasing collapsed skbs. */
  		while (copy > 0) {
  			int offset = start - TCP_SKB_CB(skb)->seq;
  			int size = TCP_SKB_CB(skb)->end_seq - start;
  
  			BUG_ON(offset < 0);
  			if (size > 0) {
  				size = min(copy, size);
  				if (skb_copy_bits(skb, offset, skb_put(nskb, size), size))
  					BUG();
  				TCP_SKB_CB(nskb)->end_seq += size;
  				copy -= size;
  				start += size;
  			}
  			if (!before(start, TCP_SKB_CB(skb)->end_seq)) {
  				skb = tcp_collapse_one(sk, skb, list, root);
  				if (!skb ||
  				    skb == tail ||
  				    (TCP_SKB_CB(skb)->tcp_flags & (TCPHDR_SYN | TCPHDR_FIN)))
  					goto end;
  			}
  		}
  	}
  end:
  	skb_queue_walk_safe(&tmp, skb, n)
  		tcp_rbtree_insert(root, skb);
  }
  
  /* Collapse ofo queue. Algorithm: select contiguous sequence of skbs
   * and tcp_collapse() them until all the queue is collapsed.
   */
  static void tcp_collapse_ofo_queue(struct sock *sk)
  {
  	struct tcp_sock *tp = tcp_sk(sk);
  	struct sk_buff *skb, *head;
  	u32 start, end;
  
  	skb = skb_rb_first(&tp->out_of_order_queue);
  new_range:
  	if (!skb) {
  		tp->ooo_last_skb = skb_rb_last(&tp->out_of_order_queue);
  		return;
  	}
  	start = TCP_SKB_CB(skb)->seq;
  	end = TCP_SKB_CB(skb)->end_seq;
  
  	for (head = skb;;) {
  		skb = skb_rb_next(skb);
  
  		/* Range is terminated when we see a gap or when
  		 * we are at the queue end.
  		 */
  		if (!skb ||
  		    after(TCP_SKB_CB(skb)->seq, end) ||
  		    before(TCP_SKB_CB(skb)->end_seq, start)) {
  			tcp_collapse(sk, NULL, &tp->out_of_order_queue,
  				     head, skb, start, end);
  			goto new_range;
  		}
  
  		if (unlikely(before(TCP_SKB_CB(skb)->seq, start)))
  			start = TCP_SKB_CB(skb)->seq;
  		if (after(TCP_SKB_CB(skb)->end_seq, end))
  			end = TCP_SKB_CB(skb)->end_seq;
  	}
  }
  
  /*
   * Clean the out-of-order queue to make room.
   * We drop high sequences packets to :
   * 1) Let a chance for holes to be filled.
   * 2) not add too big latencies if thousands of packets sit there.
   *    (But if application shrinks SO_RCVBUF, we could still end up
   *     freeing whole queue here)
   *
   * Return true if queue has shrunk.
   */
  static bool tcp_prune_ofo_queue(struct sock *sk)
  {
  	struct tcp_sock *tp = tcp_sk(sk);
  	struct rb_node *node, *prev;
  
  	if (RB_EMPTY_ROOT(&tp->out_of_order_queue))
  		return false;
  
  	NET_INC_STATS(sock_net(sk), LINUX_MIB_OFOPRUNED);
  	node = &tp->ooo_last_skb->rbnode;
  	do {
  		prev = rb_prev(node);
  		rb_erase(node, &tp->out_of_order_queue);
  		tcp_drop(sk, rb_to_skb(node));
  		sk_mem_reclaim(sk);
  		if (atomic_read(&sk->sk_rmem_alloc) <= sk->sk_rcvbuf &&
  		    !tcp_under_memory_pressure(sk))
  			break;
  		node = prev;
  	} while (node);
  	tp->ooo_last_skb = rb_to_skb(prev);
  
  	/* Reset SACK state.  A conforming SACK implementation will
  	 * do the same at a timeout based retransmit.  When a connection
  	 * is in a sad state like this, we care only about integrity
  	 * of the connection not performance.
  	 */
  	if (tp->rx_opt.sack_ok)
  		tcp_sack_reset(&tp->rx_opt);
  	return true;
  }
  
  /* Reduce allocated memory if we can, trying to get
   * the socket within its memory limits again.
   *
   * Return less than zero if we should start dropping frames
   * until the socket owning process reads some of the data
   * to stabilize the situation.
   */
  static int tcp_prune_queue(struct sock *sk)
  {
  	struct tcp_sock *tp = tcp_sk(sk);
  
  	SOCK_DEBUG(sk, "prune_queue: c=%x\n", tp->copied_seq);
  
  	NET_INC_STATS(sock_net(sk), LINUX_MIB_PRUNECALLED);
  
  	if (atomic_read(&sk->sk_rmem_alloc) >= sk->sk_rcvbuf)
  		tcp_clamp_window(sk);
  	else if (tcp_under_memory_pressure(sk))
  		tp->rcv_ssthresh = min(tp->rcv_ssthresh, 4U * tp->advmss);
  
  	tcp_collapse_ofo_queue(sk);
  	if (!skb_queue_empty(&sk->sk_receive_queue))
  		tcp_collapse(sk, &sk->sk_receive_queue, NULL,
  			     skb_peek(&sk->sk_receive_queue),
  			     NULL,
  			     tp->copied_seq, tp->rcv_nxt);
  	sk_mem_reclaim(sk);
  
  	if (atomic_read(&sk->sk_rmem_alloc) <= sk->sk_rcvbuf)
  		return 0;
  
  	/* Collapsing did not help, destructive actions follow.
  	 * This must not ever occur. */
  
  	tcp_prune_ofo_queue(sk);
  
  	if (atomic_read(&sk->sk_rmem_alloc) <= sk->sk_rcvbuf)
  		return 0;
  
  	/* If we are really being abused, tell the caller to silently
  	 * drop receive data on the floor.  It will get retransmitted
  	 * and hopefully then we'll have sufficient space.
  	 */
  	NET_INC_STATS(sock_net(sk), LINUX_MIB_RCVPRUNED);
  
  	/* Massive buffer overcommit. */
  	tp->pred_flags = 0;
  	return -1;
  }
  
  static bool tcp_should_expand_sndbuf(const struct sock *sk)
  {
  	const struct tcp_sock *tp = tcp_sk(sk);
  
  	/* If the user specified a specific send buffer setting, do
  	 * not modify it.
  	 */
  	if (sk->sk_userlocks & SOCK_SNDBUF_LOCK)
  		return false;
  
  	/* If we are under global TCP memory pressure, do not expand.  */
  	if (tcp_under_memory_pressure(sk))
  		return false;
  
  	/* If we are under soft global TCP memory pressure, do not expand.  */
  	if (sk_memory_allocated(sk) >= sk_prot_mem_limits(sk, 0))
  		return false;
  
  	/* If we filled the congestion window, do not expand.  */
  	if (tcp_packets_in_flight(tp) >= tp->snd_cwnd)
  		return false;
  
  	return true;
  }
  
  /* When incoming ACK allowed to free some skb from write_queue,
   * we remember this event in flag SOCK_QUEUE_SHRUNK and wake up socket
   * on the exit from tcp input handler.
   *
   * PROBLEM: sndbuf expansion does not work well with largesend.
   */
  static void tcp_new_space(struct sock *sk)
  {
  	struct tcp_sock *tp = tcp_sk(sk);
  
  	if (tcp_should_expand_sndbuf(sk)) {
  		tcp_sndbuf_expand(sk);
  		tp->snd_cwnd_stamp = tcp_jiffies32;
  	}
  
  	sk->sk_write_space(sk);
  }
  
  static void tcp_check_space(struct sock *sk)
  {
  	if (sock_flag(sk, SOCK_QUEUE_SHRUNK)) {
  		sock_reset_flag(sk, SOCK_QUEUE_SHRUNK);
  		/* pairs with tcp_poll() */
  		smp_mb();
  		if (sk->sk_socket &&
  		    test_bit(SOCK_NOSPACE, &sk->sk_socket->flags)) {
  			tcp_new_space(sk);
  			if (!test_bit(SOCK_NOSPACE, &sk->sk_socket->flags))
  				tcp_chrono_stop(sk, TCP_CHRONO_SNDBUF_LIMITED);
  		}
  	}
  }
  
  static inline void tcp_data_snd_check(struct sock *sk)
  {
  	tcp_push_pending_frames(sk);
  	tcp_check_space(sk);
  }
  
  /*
   * Check if sending an ack is needed.
   */
  static void __tcp_ack_snd_check(struct sock *sk, int ofo_possible)
  {
  	struct tcp_sock *tp = tcp_sk(sk);
  	unsigned long rtt, delay;
  
  	    /* More than one full frame received... */
  	if (((tp->rcv_nxt - tp->rcv_wup) > inet_csk(sk)->icsk_ack.rcv_mss &&
  	     /* ... and right edge of window advances far enough.
  	      * (tcp_recvmsg() will send ACK otherwise).
  	      * If application uses SO_RCVLOWAT, we want send ack now if
  	      * we have not received enough bytes to satisfy the condition.
  	      */
  	    (tp->rcv_nxt - tp->copied_seq < sk->sk_rcvlowat ||
  	     __tcp_select_window(sk) >= tp->rcv_wnd)) ||
  	    /* We ACK each frame or... */
  	    tcp_in_quickack_mode(sk)) {
  send_now:
  		tcp_send_ack(sk);
  		return;
  	}
  
  	if (!ofo_possible || RB_EMPTY_ROOT(&tp->out_of_order_queue)) {
  		tcp_send_delayed_ack(sk);
  		return;
  	}
  
  	if (!tcp_is_sack(tp) ||
  	    tp->compressed_ack >= sock_net(sk)->ipv4.sysctl_tcp_comp_sack_nr)
  		goto send_now;
  	tp->compressed_ack++;
  
  	if (hrtimer_is_queued(&tp->compressed_ack_timer))
  		return;
  
  	/* compress ack timer : 5 % of rtt, but no more than tcp_comp_sack_delay_ns */
  
  	rtt = tp->rcv_rtt_est.rtt_us;
  	if (tp->srtt_us && tp->srtt_us < rtt)
  		rtt = tp->srtt_us;
  
  	delay = min_t(unsigned long, sock_net(sk)->ipv4.sysctl_tcp_comp_sack_delay_ns,
  		      rtt * (NSEC_PER_USEC >> 3)/20);
  	sock_hold(sk);
  	hrtimer_start(&tp->compressed_ack_timer, ns_to_ktime(delay),
  		      HRTIMER_MODE_REL_PINNED_SOFT);
  }
  
  static inline void tcp_ack_snd_check(struct sock *sk)
  {
  	if (!inet_csk_ack_scheduled(sk)) {
  		/* We sent a data segment already. */
  		return;
  	}
  	__tcp_ack_snd_check(sk, 1);
  }
  
  /*
   *	This routine is only called when we have urgent data
   *	signaled. Its the 'slow' part of tcp_urg. It could be
   *	moved inline now as tcp_urg is only called from one
   *	place. We handle URGent data wrong. We have to - as
   *	BSD still doesn't use the correction from RFC961.
   *	For 1003.1g we should support a new option TCP_STDURG to permit
   *	either form (or just set the sysctl tcp_stdurg).
   */
  
  static void tcp_check_urg(struct sock *sk, const struct tcphdr *th)
  {
  	struct tcp_sock *tp = tcp_sk(sk);
  	u32 ptr = ntohs(th->urg_ptr);
  
  	if (ptr && !sock_net(sk)->ipv4.sysctl_tcp_stdurg)
  		ptr--;
  	ptr += ntohl(th->seq);
  
  	/* Ignore urgent data that we've already seen and read. */
  	if (after(tp->copied_seq, ptr))
  		return;
  
  	/* Do not replay urg ptr.
  	 *
  	 * NOTE: interesting situation not covered by specs.
  	 * Misbehaving sender may send urg ptr, pointing to segment,
  	 * which we already have in ofo queue. We are not able to fetch
  	 * such data and will stay in TCP_URG_NOTYET until will be eaten
  	 * by recvmsg(). Seems, we are not obliged to handle such wicked
  	 * situations. But it is worth to think about possibility of some
  	 * DoSes using some hypothetical application level deadlock.
  	 */
  	if (before(ptr, tp->rcv_nxt))
  		return;
  
  	/* Do we already have a newer (or duplicate) urgent pointer? */
  	if (tp->urg_data && !after(ptr, tp->urg_seq))
  		return;
  
  	/* Tell the world about our new urgent pointer. */
  	sk_send_sigurg(sk);
  
  	/* We may be adding urgent data when the last byte read was
  	 * urgent. To do this requires some care. We cannot just ignore
  	 * tp->copied_seq since we would read the last urgent byte again
  	 * as data, nor can we alter copied_seq until this data arrives
  	 * or we break the semantics of SIOCATMARK (and thus sockatmark())
  	 *
  	 * NOTE. Double Dutch. Rendering to plain English: author of comment
  	 * above did something sort of 	send("A", MSG_OOB); send("B", MSG_OOB);
  	 * and expect that both A and B disappear from stream. This is _wrong_.
  	 * Though this happens in BSD with high probability, this is occasional.
  	 * Any application relying on this is buggy. Note also, that fix "works"
  	 * only in this artificial test. Insert some normal data between A and B and we will
  	 * decline of BSD again. Verdict: it is better to remove to trap
  	 * buggy users.
  	 */
  	if (tp->urg_seq == tp->copied_seq && tp->urg_data &&
  	    !sock_flag(sk, SOCK_URGINLINE) && tp->copied_seq != tp->rcv_nxt) {
  		struct sk_buff *skb = skb_peek(&sk->sk_receive_queue);
  		tp->copied_seq++;
  		if (skb && !before(tp->copied_seq, TCP_SKB_CB(skb)->end_seq)) {
  			__skb_unlink(skb, &sk->sk_receive_queue);
  			__kfree_skb(skb);
  		}
  	}
  
  	tp->urg_data = TCP_URG_NOTYET;
  	tp->urg_seq = ptr;
  
  	/* Disable header prediction. */
  	tp->pred_flags = 0;
  }
  
  /* This is the 'fast' part of urgent handling. */
  static void tcp_urg(struct sock *sk, struct sk_buff *skb, const struct tcphdr *th)
  {
  	struct tcp_sock *tp = tcp_sk(sk);
  
  	/* Check if we get a new urgent pointer - normally not. */
  	if (th->urg)
  		tcp_check_urg(sk, th);
  
  	/* Do we wait for any urgent data? - normally not... */
  	if (tp->urg_data == TCP_URG_NOTYET) {
  		u32 ptr = tp->urg_seq - ntohl(th->seq) + (th->doff * 4) -
  			  th->syn;
  
  		/* Is the urgent pointer pointing into this packet? */
  		if (ptr < skb->len) {
  			u8 tmp;
  			if (skb_copy_bits(skb, ptr, &tmp, 1))
  				BUG();
  			tp->urg_data = TCP_URG_VALID | tmp;
  			if (!sock_flag(sk, SOCK_DEAD))
  				sk->sk_data_ready(sk);
  		}
  	}
  }
  
  /* Accept RST for rcv_nxt - 1 after a FIN.
   * When tcp connections are abruptly terminated from Mac OSX (via ^C), a
   * FIN is sent followed by a RST packet. The RST is sent with the same
   * sequence number as the FIN, and thus according to RFC 5961 a challenge
   * ACK should be sent. However, Mac OSX rate limits replies to challenge
   * ACKs on the closed socket. In addition middleboxes can drop either the
   * challenge ACK or a subsequent RST.
   */
  static bool tcp_reset_check(const struct sock *sk, const struct sk_buff *skb)
  {
  	struct tcp_sock *tp = tcp_sk(sk);
  
  	return unlikely(TCP_SKB_CB(skb)->seq == (tp->rcv_nxt - 1) &&
  			(1 << sk->sk_state) & (TCPF_CLOSE_WAIT | TCPF_LAST_ACK |
  					       TCPF_CLOSING));
  }
  
  /* Does PAWS and seqno based validation of an incoming segment, flags will
   * play significant role here.
   */
  static bool tcp_validate_incoming(struct sock *sk, struct sk_buff *skb,
  				  const struct tcphdr *th, int syn_inerr)
  {
  	struct tcp_sock *tp = tcp_sk(sk);
  	bool rst_seq_match = false;
  
  	/* RFC1323: H1. Apply PAWS check first. */
  	if (tcp_fast_parse_options(sock_net(sk), skb, th, tp) &&
  	    tp->rx_opt.saw_tstamp &&
  	    tcp_paws_discard(sk, skb)) {
  		if (!th->rst) {
  			NET_INC_STATS(sock_net(sk), LINUX_MIB_PAWSESTABREJECTED);
  			if (!tcp_oow_rate_limited(sock_net(sk), skb,
  						  LINUX_MIB_TCPACKSKIPPEDPAWS,
  						  &tp->last_oow_ack_time))
  				tcp_send_dupack(sk, skb);
  			goto discard;
  		}
  		/* Reset is accepted even if it did not pass PAWS. */
  	}
  
  	/* Step 1: check sequence number */
  	if (!tcp_sequence(tp, TCP_SKB_CB(skb)->seq, TCP_SKB_CB(skb)->end_seq)) {
  		/* RFC793, page 37: "In all states except SYN-SENT, all reset
  		 * (RST) segments are validated by checking their SEQ-fields."
  		 * And page 69: "If an incoming segment is not acceptable,
  		 * an acknowledgment should be sent in reply (unless the RST
  		 * bit is set, if so drop the segment and return)".
  		 */
  		if (!th->rst) {
  			if (th->syn)
  				goto syn_challenge;
  			if (!tcp_oow_rate_limited(sock_net(sk), skb,
  						  LINUX_MIB_TCPACKSKIPPEDSEQ,
  						  &tp->last_oow_ack_time))
  				tcp_send_dupack(sk, skb);
  		} else if (tcp_reset_check(sk, skb)) {
  			tcp_reset(sk);
  		}
  		goto discard;
  	}
  
  	/* Step 2: check RST bit */
  	if (th->rst) {
  		/* RFC 5961 3.2 (extend to match against (RCV.NXT - 1) after a
  		 * FIN and SACK too if available):
  		 * If seq num matches RCV.NXT or (RCV.NXT - 1) after a FIN, or
  		 * the right-most SACK block,
  		 * then
  		 *     RESET the connection
  		 * else
  		 *     Send a challenge ACK
  		 */
  		if (TCP_SKB_CB(skb)->seq == tp->rcv_nxt ||
  		    tcp_reset_check(sk, skb)) {
  			rst_seq_match = true;
  		} else if (tcp_is_sack(tp) && tp->rx_opt.num_sacks > 0) {
  			struct tcp_sack_block *sp = &tp->selective_acks[0];
  			int max_sack = sp[0].end_seq;
  			int this_sack;
  
  			for (this_sack = 1; this_sack < tp->rx_opt.num_sacks;
  			     ++this_sack) {
  				max_sack = after(sp[this_sack].end_seq,
  						 max_sack) ?
  					sp[this_sack].end_seq : max_sack;
  			}
  
  			if (TCP_SKB_CB(skb)->seq == max_sack)
  				rst_seq_match = true;
  		}
  
  		if (rst_seq_match)
  			tcp_reset(sk);
  		else {
  			/* Disable TFO if RST is out-of-order
  			 * and no data has been received
  			 * for current active TFO socket
  			 */
  			if (tp->syn_fastopen && !tp->data_segs_in &&
  			    sk->sk_state == TCP_ESTABLISHED)
  				tcp_fastopen_active_disable(sk);
  			tcp_send_challenge_ack(sk, skb);
  		}
  		goto discard;
  	}
  
  	/* step 3: check security and precedence [ignored] */
  
  	/* step 4: Check for a SYN
  	 * RFC 5961 4.2 : Send a challenge ack
  	 */
  	if (th->syn) {
  syn_challenge:
  		if (syn_inerr)
  			TCP_INC_STATS(sock_net(sk), TCP_MIB_INERRS);
  		NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPSYNCHALLENGE);
  		tcp_send_challenge_ack(sk, skb);
  		goto discard;
  	}
  
  	return true;
  
  discard:
  	tcp_drop(sk, skb);
  	return false;
  }
  
  /*
   *	TCP receive function for the ESTABLISHED state.
   *
   *	It is split into a fast path and a slow path. The fast path is
   * 	disabled when:
   *	- A zero window was announced from us - zero window probing
   *        is only handled properly in the slow path.
   *	- Out of order segments arrived.
   *	- Urgent data is expected.
   *	- There is no buffer space left
   *	- Unexpected TCP flags/window values/header lengths are received
   *	  (detected by checking the TCP header against pred_flags)
   *	- Data is sent in both directions. Fast path only supports pure senders
   *	  or pure receivers (this means either the sequence number or the ack
   *	  value must stay constant)
   *	- Unexpected TCP option.
   *
   *	When these conditions are not satisfied it drops into a standard
   *	receive procedure patterned after RFC793 to handle all cases.
   *	The first three cases are guaranteed by proper pred_flags setting,
   *	the rest is checked inline. Fast processing is turned on in
   *	tcp_data_queue when everything is OK.
   */
  void tcp_rcv_established(struct sock *sk, struct sk_buff *skb)
  {
  	const struct tcphdr *th = (const struct tcphdr *)skb->data;
  	struct tcp_sock *tp = tcp_sk(sk);
  	unsigned int len = skb->len;
  
  	/* TCP congestion window tracking */
  	trace_tcp_probe(sk, skb);
  
  	tcp_mstamp_refresh(tp);
  	if (unlikely(!sk->sk_rx_dst))
  		inet_csk(sk)->icsk_af_ops->sk_rx_dst_set(sk, skb);
  	/*
  	 *	Header prediction.
  	 *	The code loosely follows the one in the famous
  	 *	"30 instruction TCP receive" Van Jacobson mail.
  	 *
  	 *	Van's trick is to deposit buffers into socket queue
  	 *	on a device interrupt, to call tcp_recv function
  	 *	on the receive process context and checksum and copy
  	 *	the buffer to user space. smart...
  	 *
  	 *	Our current scheme is not silly either but we take the
  	 *	extra cost of the net_bh soft interrupt processing...
  	 *	We do checksum and copy also but from device to kernel.
  	 */
  
  	tp->rx_opt.saw_tstamp = 0;
  
  	/*	pred_flags is 0xS?10 << 16 + snd_wnd
  	 *	if header_prediction is to be made
  	 *	'S' will always be tp->tcp_header_len >> 2
  	 *	'?' will be 0 for the fast path, otherwise pred_flags is 0 to
  	 *  turn it off	(when there are holes in the receive
  	 *	 space for instance)
  	 *	PSH flag is ignored.
  	 */
  
  	if ((tcp_flag_word(th) & TCP_HP_BITS) == tp->pred_flags &&
  	    TCP_SKB_CB(skb)->seq == tp->rcv_nxt &&
  	    !after(TCP_SKB_CB(skb)->ack_seq, tp->snd_nxt)) {
  		int tcp_header_len = tp->tcp_header_len;
  
  		/* Timestamp header prediction: tcp_header_len
  		 * is automatically equal to th->doff*4 due to pred_flags
  		 * match.
  		 */
  
  		/* Check timestamp */
  		if (tcp_header_len == sizeof(struct tcphdr) + TCPOLEN_TSTAMP_ALIGNED) {
  			/* No? Slow path! */
  			if (!tcp_parse_aligned_timestamp(tp, th))
  				goto slow_path;
  
  			/* If PAWS failed, check it more carefully in slow path */
  			if ((s32)(tp->rx_opt.rcv_tsval - tp->rx_opt.ts_recent) < 0)
  				goto slow_path;
  
  			/* DO NOT update ts_recent here, if checksum fails
  			 * and timestamp was corrupted part, it will result
  			 * in a hung connection since we will drop all
  			 * future packets due to the PAWS test.
  			 */
  		}
  
  		if (len <= tcp_header_len) {
  			/* Bulk data transfer: sender */
  			if (len == tcp_header_len) {
  				/* Predicted packet is in window by definition.
  				 * seq == rcv_nxt and rcv_wup <= rcv_nxt.
  				 * Hence, check seq<=rcv_wup reduces to:
  				 */
  				if (tcp_header_len ==
  				    (sizeof(struct tcphdr) + TCPOLEN_TSTAMP_ALIGNED) &&
  				    tp->rcv_nxt == tp->rcv_wup)
  					tcp_store_ts_recent(tp);
  
  				/* We know that such packets are checksummed
  				 * on entry.
  				 */
  				tcp_ack(sk, skb, 0);
  				__kfree_skb(skb);
  				tcp_data_snd_check(sk);
  				return;
  			} else { /* Header too small */
  				TCP_INC_STATS(sock_net(sk), TCP_MIB_INERRS);
  				goto discard;
  			}
  		} else {
  			int eaten = 0;
  			bool fragstolen = false;
  
  			if (tcp_checksum_complete(skb))
  				goto csum_error;
  
  			if ((int)skb->truesize > sk->sk_forward_alloc)
  				goto step5;
  
  			/* Predicted packet is in window by definition.
  			 * seq == rcv_nxt and rcv_wup <= rcv_nxt.
  			 * Hence, check seq<=rcv_wup reduces to:
  			 */
  			if (tcp_header_len ==
  			    (sizeof(struct tcphdr) + TCPOLEN_TSTAMP_ALIGNED) &&
  			    tp->rcv_nxt == tp->rcv_wup)
  				tcp_store_ts_recent(tp);
  
  			tcp_rcv_rtt_measure_ts(sk, skb);
  
  			NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPHPHITS);
  
  			/* Bulk data transfer: receiver */
  			eaten = tcp_queue_rcv(sk, skb, tcp_header_len,
  					      &fragstolen);
  
  			tcp_event_data_recv(sk, skb);
  
  			if (TCP_SKB_CB(skb)->ack_seq != tp->snd_una) {
  				/* Well, only one small jumplet in fast path... */
  				tcp_ack(sk, skb, FLAG_DATA);
  				tcp_data_snd_check(sk);
  				if (!inet_csk_ack_scheduled(sk))
  					goto no_ack;
  			}
  
  			__tcp_ack_snd_check(sk, 0);
  no_ack:
  			if (eaten)
  				kfree_skb_partial(skb, fragstolen);
  			tcp_data_ready(sk);
  			return;
  		}
  	}
  
  slow_path:
  	if (len < (th->doff << 2) || tcp_checksum_complete(skb))
  		goto csum_error;
  
  	if (!th->ack && !th->rst && !th->syn)
  		goto discard;
  
  	/*
  	 *	Standard slow path.
  	 */
  
  	if (!tcp_validate_incoming(sk, skb, th, 1))
  		return;
  
  step5:
  	if (tcp_ack(sk, skb, FLAG_SLOWPATH | FLAG_UPDATE_TS_RECENT) < 0)
  		goto discard;
  
  	tcp_rcv_rtt_measure_ts(sk, skb);
  
  	/* Process urgent data. */
  	tcp_urg(sk, skb, th);
  
  	/* step 7: process the segment text */
  	tcp_data_queue(sk, skb);
  
  	tcp_data_snd_check(sk);
  	tcp_ack_snd_check(sk);
  	return;
  
  csum_error:
  	TCP_INC_STATS(sock_net(sk), TCP_MIB_CSUMERRORS);
  	TCP_INC_STATS(sock_net(sk), TCP_MIB_INERRS);
  
  discard:
  	tcp_drop(sk, skb);
  }
  EXPORT_SYMBOL(tcp_rcv_established);
  
  void tcp_finish_connect(struct sock *sk, struct sk_buff *skb)
  {
  	struct tcp_sock *tp = tcp_sk(sk);
  	struct inet_connection_sock *icsk = inet_csk(sk);
  
  	tcp_set_state(sk, TCP_ESTABLISHED);
  	icsk->icsk_ack.lrcvtime = tcp_jiffies32;
  
  	if (skb) {
  		icsk->icsk_af_ops->sk_rx_dst_set(sk, skb);
  		security_inet_conn_established(sk, skb);
  	}
  
  	tcp_init_transfer(sk, BPF_SOCK_OPS_ACTIVE_ESTABLISHED_CB);
  
  	/* Prevent spurious tcp_cwnd_restart() on first data
  	 * packet.
  	 */
  	tp->lsndtime = tcp_jiffies32;
  
  	if (sock_flag(sk, SOCK_KEEPOPEN))
  		inet_csk_reset_keepalive_timer(sk, keepalive_time_when(tp));
  
  	if (!tp->rx_opt.snd_wscale)
  		__tcp_fast_path_on(tp, tp->snd_wnd);
  	else
  		tp->pred_flags = 0;
  }
  
  static bool tcp_rcv_fastopen_synack(struct sock *sk, struct sk_buff *synack,
  				    struct tcp_fastopen_cookie *cookie)
  {
  	struct tcp_sock *tp = tcp_sk(sk);
  	struct sk_buff *data = tp->syn_data ? tcp_rtx_queue_head(sk) : NULL;
  	u16 mss = tp->rx_opt.mss_clamp, try_exp = 0;
  	bool syn_drop = false;
  
  	if (mss == tp->rx_opt.user_mss) {
  		struct tcp_options_received opt;
  
  		/* Get original SYNACK MSS value if user MSS sets mss_clamp */
  		tcp_clear_options(&opt);
  		opt.user_mss = opt.mss_clamp = 0;
  		tcp_parse_options(sock_net(sk), synack, &opt, 0, NULL);
  		mss = opt.mss_clamp;
  	}
  
  	if (!tp->syn_fastopen) {
  		/* Ignore an unsolicited cookie */
  		cookie->len = -1;
  	} else if (tp->total_retrans) {
  		/* SYN timed out and the SYN-ACK neither has a cookie nor
  		 * acknowledges data. Presumably the remote received only
  		 * the retransmitted (regular) SYNs: either the original
  		 * SYN-data or the corresponding SYN-ACK was dropped.
  		 */
  		syn_drop = (cookie->len < 0 && data);
  	} else if (cookie->len < 0 && !tp->syn_data) {
  		/* We requested a cookie but didn't get it. If we did not use
  		 * the (old) exp opt format then try so next time (try_exp=1).
  		 * Otherwise we go back to use the RFC7413 opt (try_exp=2).
  		 */
  		try_exp = tp->syn_fastopen_exp ? 2 : 1;
  	}
  
  	tcp_fastopen_cache_set(sk, mss, cookie, syn_drop, try_exp);
  
  	if (data) { /* Retransmit unacked data in SYN */
  		skb_rbtree_walk_from(data) {
  			if (__tcp_retransmit_skb(sk, data, 1))
  				break;
  		}
  		tcp_rearm_rto(sk);
  		NET_INC_STATS(sock_net(sk),
  				LINUX_MIB_TCPFASTOPENACTIVEFAIL);
  		return true;
  	}
  	tp->syn_data_acked = tp->syn_data;
  	if (tp->syn_data_acked) {
  		NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPFASTOPENACTIVE);
  		/* SYN-data is counted as two separate packets in tcp_ack() */
  		if (tp->delivered > 1)
  			--tp->delivered;
  	}
  
  	tcp_fastopen_add_skb(sk, synack);
  
  	return false;
  }
  
  static void smc_check_reset_syn(struct tcp_sock *tp)
  {
  #if IS_ENABLED(CONFIG_SMC)
  	if (static_branch_unlikely(&tcp_have_smc)) {
  		if (tp->syn_smc && !tp->rx_opt.smc_ok)
  			tp->syn_smc = 0;
  	}
  #endif
  }
  
  static int tcp_rcv_synsent_state_process(struct sock *sk, struct sk_buff *skb,
  					 const struct tcphdr *th)
  {
  	struct inet_connection_sock *icsk = inet_csk(sk);
  	struct tcp_sock *tp = tcp_sk(sk);
  	struct tcp_fastopen_cookie foc = { .len = -1 };
  	int saved_clamp = tp->rx_opt.mss_clamp;
  	bool fastopen_fail;
  
  	tcp_parse_options(sock_net(sk), skb, &tp->rx_opt, 0, &foc);
  	if (tp->rx_opt.saw_tstamp && tp->rx_opt.rcv_tsecr)
  		tp->rx_opt.rcv_tsecr -= tp->tsoffset;
  
  	if (th->ack) {
  		/* rfc793:
  		 * "If the state is SYN-SENT then
  		 *    first check the ACK bit
  		 *      If the ACK bit is set
  		 *	  If SEG.ACK =< ISS, or SEG.ACK > SND.NXT, send
  		 *        a reset (unless the RST bit is set, if so drop
  		 *        the segment and return)"
  		 */
  		if (!after(TCP_SKB_CB(skb)->ack_seq, tp->snd_una) ||
  		    after(TCP_SKB_CB(skb)->ack_seq, tp->snd_nxt))
  			goto reset_and_undo;
  
  		if (tp->rx_opt.saw_tstamp && tp->rx_opt.rcv_tsecr &&
  		    !between(tp->rx_opt.rcv_tsecr, tp->retrans_stamp,
  			     tcp_time_stamp(tp))) {
  			NET_INC_STATS(sock_net(sk),
  					LINUX_MIB_PAWSACTIVEREJECTED);
  			goto reset_and_undo;
  		}
  
  		/* Now ACK is acceptable.
  		 *
  		 * "If the RST bit is set
  		 *    If the ACK was acceptable then signal the user "error:
  		 *    connection reset", drop the segment, enter CLOSED state,
  		 *    delete TCB, and return."
  		 */
  
  		if (th->rst) {
  			tcp_reset(sk);
  			goto discard;
  		}
  
  		/* rfc793:
  		 *   "fifth, if neither of the SYN or RST bits is set then
  		 *    drop the segment and return."
  		 *
  		 *    See note below!
  		 *                                        --ANK(990513)
  		 */
  		if (!th->syn)
  			goto discard_and_undo;
  
  		/* rfc793:
  		 *   "If the SYN bit is on ...
  		 *    are acceptable then ...
  		 *    (our SYN has been ACKed), change the connection
  		 *    state to ESTABLISHED..."
  		 */
  
  		tcp_ecn_rcv_synack(tp, th);
  
  		tcp_init_wl(tp, TCP_SKB_CB(skb)->seq);
  		tcp_ack(sk, skb, FLAG_SLOWPATH);
  
  		/* Ok.. it's good. Set up sequence numbers and
  		 * move to established.
  		 */
  		tp->rcv_nxt = TCP_SKB_CB(skb)->seq + 1;
  		tp->rcv_wup = TCP_SKB_CB(skb)->seq + 1;
  
  		/* RFC1323: The window in SYN & SYN/ACK segments is
  		 * never scaled.
  		 */
  		tp->snd_wnd = ntohs(th->window);
  
  		if (!tp->rx_opt.wscale_ok) {
  			tp->rx_opt.snd_wscale = tp->rx_opt.rcv_wscale = 0;
  			tp->window_clamp = min(tp->window_clamp, 65535U);
  		}
  
  		if (tp->rx_opt.saw_tstamp) {
  			tp->rx_opt.tstamp_ok	   = 1;
  			tp->tcp_header_len =
  				sizeof(struct tcphdr) + TCPOLEN_TSTAMP_ALIGNED;
  			tp->advmss	    -= TCPOLEN_TSTAMP_ALIGNED;
  			tcp_store_ts_recent(tp);
  		} else {
  			tp->tcp_header_len = sizeof(struct tcphdr);
  		}
  
  		tcp_sync_mss(sk, icsk->icsk_pmtu_cookie);
  		tcp_initialize_rcv_mss(sk);
  
  		/* Remember, tcp_poll() does not lock socket!
  		 * Change state from SYN-SENT only after copied_seq
  		 * is initialized. */
  		tp->copied_seq = tp->rcv_nxt;
  
  		smc_check_reset_syn(tp);
  
  		smp_mb();
  
  		tcp_finish_connect(sk, skb);
  
  		fastopen_fail = (tp->syn_fastopen || tp->syn_data) &&
  				tcp_rcv_fastopen_synack(sk, skb, &foc);
  
  		if (!sock_flag(sk, SOCK_DEAD)) {
  			sk->sk_state_change(sk);
  			sk_wake_async(sk, SOCK_WAKE_IO, POLL_OUT);
  		}
  		if (fastopen_fail)
  			return -1;
  		if (sk->sk_write_pending ||
  		    icsk->icsk_accept_queue.rskq_defer_accept ||
  		    icsk->icsk_ack.pingpong) {
  			/* Save one ACK. Data will be ready after
  			 * several ticks, if write_pending is set.
  			 *
  			 * It may be deleted, but with this feature tcpdumps
  			 * look so _wonderfully_ clever, that I was not able
  			 * to stand against the temptation 8)     --ANK
  			 */
  			inet_csk_schedule_ack(sk);
  			tcp_enter_quickack_mode(sk, TCP_MAX_QUICKACKS);
  			inet_csk_reset_xmit_timer(sk, ICSK_TIME_DACK,
  						  TCP_DELACK_MAX, TCP_RTO_MAX);
  
  discard:
  			tcp_drop(sk, skb);
  			return 0;
  		} else {
  			tcp_send_ack(sk);
  		}
  		return -1;
  	}
  
  	/* No ACK in the segment */
  
  	if (th->rst) {
  		/* rfc793:
  		 * "If the RST bit is set
  		 *
  		 *      Otherwise (no ACK) drop the segment and return."
  		 */
  
  		goto discard_and_undo;
  	}
  
  	/* PAWS check. */
  	if (tp->rx_opt.ts_recent_stamp && tp->rx_opt.saw_tstamp &&
  	    tcp_paws_reject(&tp->rx_opt, 0))
  		goto discard_and_undo;
  
  	if (th->syn) {
  		/* We see SYN without ACK. It is attempt of
  		 * simultaneous connect with crossed SYNs.
  		 * Particularly, it can be connect to self.
  		 */
  		tcp_set_state(sk, TCP_SYN_RECV);
  
  		if (tp->rx_opt.saw_tstamp) {
  			tp->rx_opt.tstamp_ok = 1;
  			tcp_store_ts_recent(tp);
  			tp->tcp_header_len =
  				sizeof(struct tcphdr) + TCPOLEN_TSTAMP_ALIGNED;
  		} else {
  			tp->tcp_header_len = sizeof(struct tcphdr);
  		}
  
  		tp->rcv_nxt = TCP_SKB_CB(skb)->seq + 1;
  		tp->copied_seq = tp->rcv_nxt;
  		tp->rcv_wup = TCP_SKB_CB(skb)->seq + 1;
  
  		/* RFC1323: The window in SYN & SYN/ACK segments is
  		 * never scaled.
  		 */
  		tp->snd_wnd    = ntohs(th->window);
  		tp->snd_wl1    = TCP_SKB_CB(skb)->seq;
  		tp->max_window = tp->snd_wnd;
  
  		tcp_ecn_rcv_syn(tp, th);
  
  		tcp_mtup_init(sk);
  		tcp_sync_mss(sk, icsk->icsk_pmtu_cookie);
  		tcp_initialize_rcv_mss(sk);
  
  		tcp_send_synack(sk);
  #if 0
  		/* Note, we could accept data and URG from this segment.
  		 * There are no obstacles to make this (except that we must
  		 * either change tcp_recvmsg() to prevent it from returning data
  		 * before 3WHS completes per RFC793, or employ TCP Fast Open).
  		 *
  		 * However, if we ignore data in ACKless segments sometimes,
  		 * we have no reasons to accept it sometimes.
  		 * Also, seems the code doing it in step6 of tcp_rcv_state_process
  		 * is not flawless. So, discard packet for sanity.
  		 * Uncomment this return to process the data.
  		 */
  		return -1;
  #else
  		goto discard;
  #endif
  	}
  	/* "fifth, if neither of the SYN or RST bits is set then
  	 * drop the segment and return."
  	 */
  
  discard_and_undo:
  	tcp_clear_options(&tp->rx_opt);
  	tp->rx_opt.mss_clamp = saved_clamp;
  	goto discard;
  
  reset_and_undo:
  	tcp_clear_options(&tp->rx_opt);
  	tp->rx_opt.mss_clamp = saved_clamp;
  	return 1;
  }
  
  /*
   *	This function implements the receiving procedure of RFC 793 for
   *	all states except ESTABLISHED and TIME_WAIT.
   *	It's called from both tcp_v4_rcv and tcp_v6_rcv and should be
   *	address independent.
   */
  
  int tcp_rcv_state_process(struct sock *sk, struct sk_buff *skb)
  {
  	struct tcp_sock *tp = tcp_sk(sk);
  	struct inet_connection_sock *icsk = inet_csk(sk);
  	const struct tcphdr *th = tcp_hdr(skb);
  	struct request_sock *req;
  	int queued = 0;
  	bool acceptable;
  
  	switch (sk->sk_state) {
  	case TCP_CLOSE:
  		goto discard;
  
  	case TCP_LISTEN:
  		if (th->ack)
  			return 1;
  
  		if (th->rst)
  			goto discard;
  
  		if (th->syn) {
  			if (th->fin)
  				goto discard;
  			/* It is possible that we process SYN packets from backlog,
  			 * so we need to make sure to disable BH right there.
  			 */
  			local_bh_disable();
  			acceptable = icsk->icsk_af_ops->conn_request(sk, skb) >= 0;
  			local_bh_enable();
  
  			if (!acceptable)
  				return 1;
  			consume_skb(skb);
  			return 0;
  		}
  		goto discard;
  
  	case TCP_SYN_SENT:
  		tp->rx_opt.saw_tstamp = 0;
  		tcp_mstamp_refresh(tp);
  		queued = tcp_rcv_synsent_state_process(sk, skb, th);
  		if (queued >= 0)
  			return queued;
  
  		/* Do step6 onward by hand. */
  		tcp_urg(sk, skb, th);
  		__kfree_skb(skb);
  		tcp_data_snd_check(sk);
  		return 0;
  	}
  
  	tcp_mstamp_refresh(tp);
  	tp->rx_opt.saw_tstamp = 0;
  	req = tp->fastopen_rsk;
  	if (req) {
  		bool req_stolen;
  
  		WARN_ON_ONCE(sk->sk_state != TCP_SYN_RECV &&
  		    sk->sk_state != TCP_FIN_WAIT1);
  
  		if (!tcp_check_req(sk, skb, req, true, &req_stolen))
  			goto discard;
  	}
  
  	if (!th->ack && !th->rst && !th->syn)
  		goto discard;
  
  	if (!tcp_validate_incoming(sk, skb, th, 0))
  		return 0;
  
  	/* step 5: check the ACK field */
  	acceptable = tcp_ack(sk, skb, FLAG_SLOWPATH |
  				      FLAG_UPDATE_TS_RECENT |
  				      FLAG_NO_CHALLENGE_ACK) > 0;
  
  	if (!acceptable) {
  		if (sk->sk_state == TCP_SYN_RECV)
  			return 1;	/* send one RST */
  		tcp_send_challenge_ack(sk, skb);
  		goto discard;
  	}
  	switch (sk->sk_state) {
  	case TCP_SYN_RECV:
  		tp->delivered++; /* SYN-ACK delivery isn't tracked in tcp_ack */
  		if (!tp->srtt_us)
  			tcp_synack_rtt_meas(sk, req);
  
  		/* Once we leave TCP_SYN_RECV, we no longer need req
  		 * so release it.
  		 */
  		if (req) {
  			inet_csk(sk)->icsk_retransmits = 0;
  			reqsk_fastopen_remove(sk, req, false);
  			/* Re-arm the timer because data may have been sent out.
  			 * This is similar to the regular data transmission case
  			 * when new data has just been ack'ed.
  			 *
  			 * (TFO) - we could try to be more aggressive and
  			 * retransmitting any data sooner based on when they
  			 * are sent out.
  			 */
  			tcp_rearm_rto(sk);
  		} else {
  			tcp_init_transfer(sk, BPF_SOCK_OPS_PASSIVE_ESTABLISHED_CB);
  			tp->copied_seq = tp->rcv_nxt;
  		}
  		smp_mb();
  		tcp_set_state(sk, TCP_ESTABLISHED);
  		sk->sk_state_change(sk);
  
  		/* Note, that this wakeup is only for marginal crossed SYN case.
  		 * Passively open sockets are not waked up, because
  		 * sk->sk_sleep == NULL and sk->sk_socket == NULL.
  		 */
  		if (sk->sk_socket)
  			sk_wake_async(sk, SOCK_WAKE_IO, POLL_OUT);
  
  		tp->snd_una = TCP_SKB_CB(skb)->ack_seq;
  		tp->snd_wnd = ntohs(th->window) << tp->rx_opt.snd_wscale;
  		tcp_init_wl(tp, TCP_SKB_CB(skb)->seq);
  
  		if (tp->rx_opt.tstamp_ok)
  			tp->advmss -= TCPOLEN_TSTAMP_ALIGNED;
  
  		if (!inet_csk(sk)->icsk_ca_ops->cong_control)
  			tcp_update_pacing_rate(sk);
  
  		/* Prevent spurious tcp_cwnd_restart() on first data packet */
  		tp->lsndtime = tcp_jiffies32;
  
  		tcp_initialize_rcv_mss(sk);
  		tcp_fast_path_on(tp);
  		break;
  
  	case TCP_FIN_WAIT1: {
  		int tmo;
  
  		/* If we enter the TCP_FIN_WAIT1 state and we are a
  		 * Fast Open socket and this is the first acceptable
  		 * ACK we have received, this would have acknowledged
  		 * our SYNACK so stop the SYNACK timer.
  		 */
  		if (req) {
  			/* We no longer need the request sock. */
  			reqsk_fastopen_remove(sk, req, false);
  			tcp_rearm_rto(sk);
  		}
  		if (tp->snd_una != tp->write_seq)
  			break;
  
  		tcp_set_state(sk, TCP_FIN_WAIT2);
  		sk->sk_shutdown |= SEND_SHUTDOWN;
  
  		sk_dst_confirm(sk);
  
  		if (!sock_flag(sk, SOCK_DEAD)) {
  			/* Wake up lingering close() */
  			sk->sk_state_change(sk);
  			break;
  		}
  
  		if (tp->linger2 < 0) {
  			tcp_done(sk);
  			NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPABORTONDATA);
  			return 1;
  		}
  		if (TCP_SKB_CB(skb)->end_seq != TCP_SKB_CB(skb)->seq &&
  		    after(TCP_SKB_CB(skb)->end_seq - th->fin, tp->rcv_nxt)) {
  			/* Receive out of order FIN after close() */
  			if (tp->syn_fastopen && th->fin)
  				tcp_fastopen_active_disable(sk);
  			tcp_done(sk);
  			NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPABORTONDATA);
  			return 1;
  		}
  
  		tmo = tcp_fin_time(sk);
  		if (tmo > TCP_TIMEWAIT_LEN) {
  			inet_csk_reset_keepalive_timer(sk, tmo - TCP_TIMEWAIT_LEN);
  		} else if (th->fin || sock_owned_by_user(sk)) {
  			/* Bad case. We could lose such FIN otherwise.
  			 * It is not a big problem, but it looks confusing
  			 * and not so rare event. We still can lose it now,
  			 * if it spins in bh_lock_sock(), but it is really
  			 * marginal case.
  			 */
  			inet_csk_reset_keepalive_timer(sk, tmo);
  		} else {
  			tcp_time_wait(sk, TCP_FIN_WAIT2, tmo);
  			goto discard;
  		}
  		break;
  	}
  
  	case TCP_CLOSING:
  		if (tp->snd_una == tp->write_seq) {
  			tcp_time_wait(sk, TCP_TIME_WAIT, 0);
  			goto discard;
  		}
  		break;
  
  	case TCP_LAST_ACK:
  		if (tp->snd_una == tp->write_seq) {
  			tcp_update_metrics(sk);
  			tcp_done(sk);
  			goto discard;
  		}
  		break;
  	}
  
  	/* step 6: check the URG bit */
  	tcp_urg(sk, skb, th);
  
  	/* step 7: process the segment text */
  	switch (sk->sk_state) {
  	case TCP_CLOSE_WAIT:
  	case TCP_CLOSING:
  	case TCP_LAST_ACK:
  		if (!before(TCP_SKB_CB(skb)->seq, tp->rcv_nxt))
  			break;
  		/* fall through */
  	case TCP_FIN_WAIT1:
  	case TCP_FIN_WAIT2:
  		/* RFC 793 says to queue data in these states,
  		 * RFC 1122 says we MUST send a reset.
  		 * BSD 4.4 also does reset.
  		 */
  		if (sk->sk_shutdown & RCV_SHUTDOWN) {
  			if (TCP_SKB_CB(skb)->end_seq != TCP_SKB_CB(skb)->seq &&
  			    after(TCP_SKB_CB(skb)->end_seq - th->fin, tp->rcv_nxt)) {
  				NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPABORTONDATA);
  				tcp_reset(sk);
  				return 1;
  			}
  		}
  		/* Fall through */
  	case TCP_ESTABLISHED:
  		tcp_data_queue(sk, skb);
  		queued = 1;
  		break;
  	}
  
  	/* tcp_data could move socket to TIME-WAIT */
  	if (sk->sk_state != TCP_CLOSE) {
  		tcp_data_snd_check(sk);
  		tcp_ack_snd_check(sk);
  	}
  
  	if (!queued) {
  discard:
  		tcp_drop(sk, skb);
  	}
  	return 0;
  }
  EXPORT_SYMBOL(tcp_rcv_state_process);
  
  static inline void pr_drop_req(struct request_sock *req, __u16 port, int family)
  {
  	struct inet_request_sock *ireq = inet_rsk(req);
  
  	if (family == AF_INET)
  		net_dbg_ratelimited("drop open request from %pI4/%u\n",
  				    &ireq->ir_rmt_addr, port);
  #if IS_ENABLED(CONFIG_IPV6)
  	else if (family == AF_INET6)
  		net_dbg_ratelimited("drop open request from %pI6/%u\n",
  				    &ireq->ir_v6_rmt_addr, port);
  #endif
  }
  
  /* RFC3168 : 6.1.1 SYN packets must not have ECT/ECN bits set
   *
   * If we receive a SYN packet with these bits set, it means a
   * network is playing bad games with TOS bits. In order to
   * avoid possible false congestion notifications, we disable
   * TCP ECN negotiation.
   *
   * Exception: tcp_ca wants ECN. This is required for DCTCP
   * congestion control: Linux DCTCP asserts ECT on all packets,
   * including SYN, which is most optimal solution; however,
   * others, such as FreeBSD do not.
   */
  static void tcp_ecn_create_request(struct request_sock *req,
  				   const struct sk_buff *skb,
  				   const struct sock *listen_sk,
  				   const struct dst_entry *dst)
  {
  	const struct tcphdr *th = tcp_hdr(skb);
  	const struct net *net = sock_net(listen_sk);
  	bool th_ecn = th->ece && th->cwr;
  	bool ect, ecn_ok;
  	u32 ecn_ok_dst;
  
  	if (!th_ecn)
  		return;
  
  	ect = !INET_ECN_is_not_ect(TCP_SKB_CB(skb)->ip_dsfield);
  	ecn_ok_dst = dst_feature(dst, DST_FEATURE_ECN_MASK);
  	ecn_ok = net->ipv4.sysctl_tcp_ecn || ecn_ok_dst;
  
  	if ((!ect && ecn_ok) || tcp_ca_needs_ecn(listen_sk) ||
  	    (ecn_ok_dst & DST_FEATURE_ECN_CA) ||
  	    tcp_bpf_ca_needs_ecn((struct sock *)req))
  		inet_rsk(req)->ecn_ok = 1;
  }
  
  static void tcp_openreq_init(struct request_sock *req,
  			     const struct tcp_options_received *rx_opt,
  			     struct sk_buff *skb, const struct sock *sk)
  {
  	struct inet_request_sock *ireq = inet_rsk(req);
  
  	req->rsk_rcv_wnd = 0;		/* So that tcp_send_synack() knows! */
  	req->cookie_ts = 0;
  	tcp_rsk(req)->rcv_isn = TCP_SKB_CB(skb)->seq;
  	tcp_rsk(req)->rcv_nxt = TCP_SKB_CB(skb)->seq + 1;
  	tcp_rsk(req)->snt_synack = tcp_clock_us();
  	tcp_rsk(req)->last_oow_ack_time = 0;
  	req->mss = rx_opt->mss_clamp;
  	req->ts_recent = rx_opt->saw_tstamp ? rx_opt->rcv_tsval : 0;
  	ireq->tstamp_ok = rx_opt->tstamp_ok;
  	ireq->sack_ok = rx_opt->sack_ok;
  	ireq->snd_wscale = rx_opt->snd_wscale;
  	ireq->wscale_ok = rx_opt->wscale_ok;
  	ireq->acked = 0;
  	ireq->ecn_ok = 0;
  	ireq->ir_rmt_port = tcp_hdr(skb)->source;
  	ireq->ir_num = ntohs(tcp_hdr(skb)->dest);
  	ireq->ir_mark = inet_request_mark(sk, skb);
  #if IS_ENABLED(CONFIG_SMC)
  	ireq->smc_ok = rx_opt->smc_ok;
  #endif
  }
  
  struct request_sock *inet_reqsk_alloc(const struct request_sock_ops *ops,
  				      struct sock *sk_listener,
  				      bool attach_listener)
  {
  	struct request_sock *req = reqsk_alloc(ops, sk_listener,
  					       attach_listener);
  
  	if (req) {
  		struct inet_request_sock *ireq = inet_rsk(req);
  
  		ireq->ireq_opt = NULL;
  #if IS_ENABLED(CONFIG_IPV6)
  		ireq->pktopts = NULL;
  #endif
  		atomic64_set(&ireq->ir_cookie, 0);
  		ireq->ireq_state = TCP_NEW_SYN_RECV;
  		write_pnet(&ireq->ireq_net, sock_net(sk_listener));
  		ireq->ireq_family = sk_listener->sk_family;
  	}
  
  	return req;
  }
  EXPORT_SYMBOL(inet_reqsk_alloc);
  
  /*
   * Return true if a syncookie should be sent
   */
  static bool tcp_syn_flood_action(const struct sock *sk,
  				 const struct sk_buff *skb,
  				 const char *proto)
  {
  	struct request_sock_queue *queue = &inet_csk(sk)->icsk_accept_queue;
  	const char *msg = "Dropping request";
  	bool want_cookie = false;
  	struct net *net = sock_net(sk);
  
  #ifdef CONFIG_SYN_COOKIES
  	if (net->ipv4.sysctl_tcp_syncookies) {
  		msg = "Sending cookies";
  		want_cookie = true;
  		__NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPREQQFULLDOCOOKIES);
  	} else
  #endif
  		__NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPREQQFULLDROP);
  
  	if (!queue->synflood_warned &&
  	    net->ipv4.sysctl_tcp_syncookies != 2 &&
  	    xchg(&queue->synflood_warned, 1) == 0)
  		pr_info("%s: Possible SYN flooding on port %d. %s.  Check SNMP counters.\n",
  			proto, ntohs(tcp_hdr(skb)->dest), msg);
  
  	return want_cookie;
  }
  
  static void tcp_reqsk_record_syn(const struct sock *sk,
  				 struct request_sock *req,
  				 const struct sk_buff *skb)
  {
  	if (tcp_sk(sk)->save_syn) {
  		u32 len = skb_network_header_len(skb) + tcp_hdrlen(skb);
  		u32 *copy;
  
  		copy = kmalloc(len + sizeof(u32), GFP_ATOMIC);
  		if (copy) {
  			copy[0] = len;
  			memcpy(&copy[1], skb_network_header(skb), len);
  			req->saved_syn = copy;
  		}
  	}
  }
  
  int tcp_conn_request(struct request_sock_ops *rsk_ops,
  		     const struct tcp_request_sock_ops *af_ops,
  		     struct sock *sk, struct sk_buff *skb)
  {
  	struct tcp_fastopen_cookie foc = { .len = -1 };
  	__u32 isn = TCP_SKB_CB(skb)->tcp_tw_isn;
  	struct tcp_options_received tmp_opt;
  	struct tcp_sock *tp = tcp_sk(sk);
  	struct net *net = sock_net(sk);
  	struct sock *fastopen_sk = NULL;
  	struct request_sock *req;
  	bool want_cookie = false;
  	struct dst_entry *dst;
  	struct flowi fl;
  
  	/* TW buckets are converted to open requests without
  	 * limitations, they conserve resources and peer is
  	 * evidently real one.
  	 */
  	if ((net->ipv4.sysctl_tcp_syncookies == 2 ||
  	     inet_csk_reqsk_queue_is_full(sk)) && !isn) {
  		want_cookie = tcp_syn_flood_action(sk, skb, rsk_ops->slab_name);
  		if (!want_cookie)
  			goto drop;
  	}
  
  	if (sk_acceptq_is_full(sk)) {
  		NET_INC_STATS(sock_net(sk), LINUX_MIB_LISTENOVERFLOWS);
  		goto drop;
  	}
  
  	req = inet_reqsk_alloc(rsk_ops, sk, !want_cookie);
  	if (!req)
  		goto drop;
  
  	tcp_rsk(req)->af_specific = af_ops;
  	tcp_rsk(req)->ts_off = 0;
  
  	tcp_clear_options(&tmp_opt);
  	tmp_opt.mss_clamp = af_ops->mss_clamp;
  	tmp_opt.user_mss  = tp->rx_opt.user_mss;
  	tcp_parse_options(sock_net(sk), skb, &tmp_opt, 0,
  			  want_cookie ? NULL : &foc);
  
  	if (want_cookie && !tmp_opt.saw_tstamp)
  		tcp_clear_options(&tmp_opt);
  
  	if (IS_ENABLED(CONFIG_SMC) && want_cookie)
  		tmp_opt.smc_ok = 0;
  
  	tmp_opt.tstamp_ok = tmp_opt.saw_tstamp;
  	tcp_openreq_init(req, &tmp_opt, skb, sk);
  	inet_rsk(req)->no_srccheck = inet_sk(sk)->transparent;
  
  	/* Note: tcp_v6_init_req() might override ir_iif for link locals */
  	inet_rsk(req)->ir_iif = inet_request_bound_dev_if(sk, skb);
  
  	af_ops->init_req(req, sk, skb);
  
  	if (security_inet_conn_request(sk, skb, req))
  		goto drop_and_free;
  
  	if (tmp_opt.tstamp_ok)
  		tcp_rsk(req)->ts_off = af_ops->init_ts_off(net, skb);
  
  	dst = af_ops->route_req(sk, &fl, req);
  	if (!dst)
  		goto drop_and_free;
  
  	if (!want_cookie && !isn) {
  		/* Kill the following clause, if you dislike this way. */
  		if (!net->ipv4.sysctl_tcp_syncookies &&
  		    (net->ipv4.sysctl_max_syn_backlog - inet_csk_reqsk_queue_len(sk) <
  		     (net->ipv4.sysctl_max_syn_backlog >> 2)) &&
  		    !tcp_peer_is_proven(req, dst)) {
  			/* Without syncookies last quarter of
  			 * backlog is filled with destinations,
  			 * proven to be alive.
  			 * It means that we continue to communicate
  			 * to destinations, already remembered
  			 * to the moment of synflood.
  			 */
  			pr_drop_req(req, ntohs(tcp_hdr(skb)->source),
  				    rsk_ops->family);
  			goto drop_and_release;
  		}
  
  		isn = af_ops->init_seq(skb);
  	}
  
  	tcp_ecn_create_request(req, skb, sk, dst);
  
  	if (want_cookie) {
  		isn = cookie_init_sequence(af_ops, sk, skb, &req->mss);
  		req->cookie_ts = tmp_opt.tstamp_ok;
  		if (!tmp_opt.tstamp_ok)
  			inet_rsk(req)->ecn_ok = 0;
  	}
  
  	tcp_rsk(req)->snt_isn = isn;
  	tcp_rsk(req)->txhash = net_tx_rndhash();
  	tcp_openreq_init_rwin(req, sk, dst);
  	if (!want_cookie) {
  		tcp_reqsk_record_syn(sk, req, skb);
  		fastopen_sk = tcp_try_fastopen(sk, skb, req, &foc, dst);
  	}
  	if (fastopen_sk) {
  		af_ops->send_synack(fastopen_sk, dst, &fl, req,
  				    &foc, TCP_SYNACK_FASTOPEN);
  		/* Add the child socket directly into the accept queue */
  		inet_csk_reqsk_queue_add(sk, req, fastopen_sk);
  		sk->sk_data_ready(sk);
  		bh_unlock_sock(fastopen_sk);
  		sock_put(fastopen_sk);
  	} else {
  		tcp_rsk(req)->tfo_listener = false;
  		if (!want_cookie)
  			inet_csk_reqsk_queue_hash_add(sk, req,
  				tcp_timeout_init((struct sock *)req));
  		af_ops->send_synack(sk, dst, &fl, req, &foc,
  				    !want_cookie ? TCP_SYNACK_NORMAL :
  						   TCP_SYNACK_COOKIE);
  		if (want_cookie) {
  			reqsk_free(req);
  			return 0;
  		}
  	}
  	reqsk_put(req);
  	return 0;
  
  drop_and_release:
  	dst_release(dst);
  drop_and_free:
  	reqsk_free(req);
  drop:
  	tcp_listendrop(sk);
  	return 0;
  }
  EXPORT_SYMBOL(tcp_conn_request);
  