text: /*
   * 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>
  
  int sysctl_tcp_timestamps __read_mostly = 1;
  int sysctl_tcp_window_scaling __read_mostly = 1;
  int sysctl_tcp_sack __read_mostly = 1;
  int sysctl_tcp_fack __read_mostly = 1;
  int sysctl_tcp_reordering __read_mostly = TCP_FASTRETRANS_THRESH;
  EXPORT_SYMBOL(sysctl_tcp_reordering);
  int sysctl_tcp_dsack __read_mostly = 1;
  int sysctl_tcp_app_win __read_mostly = 31;
  int sysctl_tcp_adv_win_scale __read_mostly = 1;
  EXPORT_SYMBOL(sysctl_tcp_adv_win_scale);
  
  /* rfc5961 challenge ack rate limiting */
  int sysctl_tcp_challenge_ack_limit = 100;
  
  int sysctl_tcp_stdurg __read_mostly;
  int sysctl_tcp_rfc1337 __read_mostly;
  int sysctl_tcp_max_orphans __read_mostly = NR_FILE;
  int sysctl_tcp_frto __read_mostly = 2;
  
  int sysctl_tcp_thin_dupack __read_mostly;
  
  int sysctl_tcp_moderate_rcvbuf __read_mostly = 1;
  int sysctl_tcp_early_retrans __read_mostly = 3;
  int sysctl_tcp_default_init_rwnd __read_mostly = TCP_INIT_CWND * 2;
  
  #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_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_SACK_RENEGING	0x2000 /* snd_una advanced to a sacked seq */
  #define FLAG_UPDATE_TS_RECENT	0x4000 /* tcp_replace_ts_recent() */
  
  #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)
  #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))
  
  /* 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 = 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)
  {
  	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;
  	if (quickacks > icsk->icsk_ack.quick)
  		icsk->icsk_ack.quick = min(quickacks, TCP_MAX_QUICKACKS);
  }
  
  static void tcp_enter_quickack_mode(struct sock *sk)
  {
  	struct inet_connection_sock *icsk = inet_csk(sk);
  	tcp_incr_quickack(sk);
  	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 inline bool tcp_in_quickack_mode(const struct sock *sk)
  {
  	const struct inet_connection_sock *icsk = inet_csk(sk);
  
  	return 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 tcp_sock *tp, const struct sk_buff *skb)
  {
  	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((struct sock *)tp);
  		break;
  	case INET_ECN_CE:
  		if (tcp_ca_needs_ecn((struct sock *)tp))
  			tcp_ca_event((struct sock *)tp, 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((struct sock *)tp);
  			tp->ecn_flags |= TCP_ECN_DEMAND_CWR;
  		}
  		tp->ecn_flags |= TCP_ECN_SEEN;
  		break;
  	default:
  		if (tcp_ca_needs_ecn((struct sock *)tp))
  			tcp_ca_event((struct sock *)tp, CA_EVENT_ECN_NO_CE);
  		tp->ecn_flags |= TCP_ECN_SEEN;
  		break;
  	}
  }
  
  static void tcp_ecn_check_ce(struct tcp_sock *tp, const struct sk_buff *skb)
  {
  	if (tp->ecn_flags & TCP_ECN_OK)
  		__tcp_ecn_check_ce(tp, 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);
  	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 POLLOUT)
  	 */
  	sndmem = 2 * nr_segs * per_mss;
  
  	if (sk->sk_sndbuf < sndmem)
  		sk->sk_sndbuf = min(sndmem, 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(skb->truesize) >> 1;
  	int window = tcp_win_from_space(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) &&
  	    !sk_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(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 (sysctl_tcp_moderate_rcvbuf)
  		rcvmem <<= 2;
  
  	if (sk->sk_rcvbuf < rcvmem)
  		sk->sk_rcvbuf = min(rcvmem, 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)
  {
  	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;
  	tp->rcvq_space.time = tcp_time_stamp;
  	tp->rcvq_space.seq = tp->copied_seq;
  
  	maxwin = tcp_full_space(sk);
  
  	if (tp->window_clamp >= maxwin) {
  		tp->window_clamp = maxwin;
  
  		if (sysctl_tcp_app_win && maxwin > 4 * tp->advmss)
  			tp->window_clamp = max(maxwin -
  					       (maxwin >> sysctl_tcp_app_win),
  					       4 * tp->advmss);
  	}
  
  	/* Force reservation of one segment. */
  	if (sysctl_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_time_stamp;
  }
  
  /* 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);
  
  	icsk->icsk_ack.quick = 0;
  
  	if (sk->sk_rcvbuf < sysctl_tcp_rmem[2] &&
  	    !(sk->sk_userlocks & SOCK_RCVBUF_LOCK) &&
  	    !sk_under_memory_pressure(sk) &&
  	    sk_memory_allocated(sk) < sk_prot_mem_limits(sk, 0)) {
  		sk->sk_rcvbuf = min(atomic_read(&sk->sk_rmem_alloc),
  				    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;
  	long m = sample;
  
  	if (m == 0)
  		m = 1;
  
  	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;
  	}
  
  	if (tp->rcv_rtt_est.rtt != new_sample)
  		tp->rcv_rtt_est.rtt = new_sample;
  }
  
  static inline void tcp_rcv_rtt_measure(struct tcp_sock *tp)
  {
  	if (tp->rcv_rtt_est.time == 0)
  		goto new_measure;
  	if (before(tp->rcv_nxt, tp->rcv_rtt_est.seq))
  		return;
  	tcp_rcv_rtt_update(tp, tcp_time_stamp - tp->rcv_rtt_est.time, 1);
  
  new_measure:
  	tp->rcv_rtt_est.seq = tp->rcv_nxt + tp->rcv_wnd;
  	tp->rcv_rtt_est.time = tcp_time_stamp;
  }
  
  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))
  		tcp_rcv_rtt_update(tp, tcp_time_stamp - tp->rx_opt.rcv_tsecr, 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);
  	int time;
  	int copied;
  
  	time = tcp_time_stamp - tp->rcvq_space.time;
  	if (time < (tp->rcv_rtt_est.rtt >> 3) || tp->rcv_rtt_est.rtt == 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 (sysctl_tcp_moderate_rcvbuf &&
  	    !(sk->sk_userlocks & SOCK_RCVBUF_LOCK)) {
  		int rcvwin, rcvmem, rcvbuf;
  
  		/* minimal window to cope with packet losses, assuming
  		 * steady state. Add some cushion because of small variations.
  		 */
  		rcvwin = (copied << 1) + 16 * tp->advmss;
  
  		/* If rate increased by 25%,
  		 *	assume slow start, rcvwin = 3 * copied
  		 * If rate increased by 50%,
  		 *	assume sender can use 2x growth, rcvwin = 4 * copied
  		 */
  		if (copied >=
  		    tp->rcvq_space.space + (tp->rcvq_space.space >> 2)) {
  			if (copied >=
  			    tp->rcvq_space.space + (tp->rcvq_space.space >> 1))
  				rcvwin <<= 1;
  			else
  				rcvwin += (rcvwin >> 1);
  		}
  
  		rcvmem = SKB_TRUESIZE(tp->advmss + MAX_TCP_HEADER);
  		while (tcp_win_from_space(rcvmem) < tp->advmss)
  			rcvmem += 128;
  
  		rcvbuf = min(rcvwin / tp->advmss * rcvmem, sysctl_tcp_rmem[2]);
  		if (rcvbuf > sk->sk_rcvbuf) {
  			sk->sk_rcvbuf = rcvbuf;
  
  			/* Make the window clamp follow along.  */
  			tp->window_clamp = rcvwin;
  		}
  	}
  	tp->rcvq_space.space = copied;
  
  new_measure:
  	tp->rcvq_space.seq = tp->copied_seq;
  	tp->rcvq_space.time = tcp_time_stamp;
  }
  
  /* 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_time_stamp;
  
  	if (!icsk->icsk_ack.ato) {
  		/* The _first_ data packet received, initialize
  		 * delayed ACK engine.
  		 */
  		tcp_incr_quickack(sk);
  		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);
  			sk_mem_reclaim(sk);
  		}
  	}
  	icsk->icsk_ack.lrcvtime = now;
  
  	tcp_ecn_check_ce(tp, 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);
  }
  
  /* Set the sk_pacing_rate to allow proper sizing of TSO packets.
   * Note: TCP stack does not yet implement pacing.
   * FQ packet scheduler can be used to implement cheap but effective
   * TCP pacing, to smooth the burst on large writes when packets
   * in flight is significantly lower than cwnd (or rwin)
   */
  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 * 2 * (USEC_PER_SEC << 3);
  
  	rate *= max(tp->snd_cwnd, tp->packets_out);
  
  	if (likely(tp->srtt_us))
  		do_div(rate, tp->srtt_us);
  
  	/* ACCESS_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.
  	 */
  	ACCESS_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);
  }
  
  /*
   * Packet counting of FACK is based on in-order assumptions, therefore TCP
   * disables it when reordering is detected
   */
  void tcp_disable_fack(struct tcp_sock *tp)
  {
  	/* RFC3517 uses different metric in lost marker => reset on change */
  	if (tcp_is_fack(tp))
  		tp->lost_skb_hint = NULL;
  	tp->rx_opt.sack_ok &= ~TCP_FACK_ENABLED;
  }
  
  /* 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;
  }
  
  static void tcp_update_reordering(struct sock *sk, const int metric,
  				  const int ts)
  {
  	struct tcp_sock *tp = tcp_sk(sk);
  	if (metric > tp->reordering) {
  		int mib_idx;
  
  		tp->reordering = min(TCP_MAX_REORDERING, metric);
  
  		/* This exciting event is worth to be remembered. 8) */
  		if (ts)
  			mib_idx = LINUX_MIB_TCPTSREORDER;
  		else if (tcp_is_reno(tp))
  			mib_idx = LINUX_MIB_TCPRENOREORDER;
  		else if (tcp_is_fack(tp))
  			mib_idx = LINUX_MIB_TCPFACKREORDER;
  		else
  			mib_idx = LINUX_MIB_TCPSACKREORDER;
  
  		NET_INC_STATS_BH(sock_net(sk), mib_idx);
  #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,
  			 tp->fackets_out,
  			 tp->sacked_out,
  			 tp->undo_marker ? tp->undo_retrans : 0);
  #endif
  		tcp_disable_fack(tp);
  	}
  
  	if (metric > 0)
  		tcp_disable_early_retrans(tp);
  }
  
  /* 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 == NULL) ||
  	    before(TCP_SKB_CB(skb)->seq,
  		   TCP_SKB_CB(tp->retransmit_skb_hint)->seq))
  		tp->retransmit_skb_hint = skb;
  
  	if (!tp->lost_out ||
  	    after(TCP_SKB_CB(skb)->end_seq, tp->retransmit_high))
  		tp->retransmit_high = TCP_SKB_CB(skb)->end_seq;
  }
  
  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_SKB_CB(skb)->sacked |= TCPCB_LOST;
  	}
  }
  
  static void tcp_skb_mark_lost_uncond_verify(struct tcp_sock *tp,
  					    struct sk_buff *skb)
  {
  	tcp_verify_retransmit_hint(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.
   *	   A''. Its FACK modification, head until snd.fack is 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);
  }
  
  /* Check for lost retransmit. This superb idea is borrowed from "ratehalving".
   * Event "B". Later note: FACK people cheated me again 8), we have to account
   * for reordering! Ugly, but should help.
   *
   * Search retransmitted skbs from write_queue that were sent when snd_nxt was
   * less than what is now known to be received by the other end (derived from
   * highest SACK block). Also calculate the lowest snd_nxt among the remaining
   * retransmitted skbs to avoid some costly processing per ACKs.
   */
  static void tcp_mark_lost_retrans(struct sock *sk)
  {
  	const struct inet_connection_sock *icsk = inet_csk(sk);
  	struct tcp_sock *tp = tcp_sk(sk);
  	struct sk_buff *skb;
  	int cnt = 0;
  	u32 new_low_seq = tp->snd_nxt;
  	u32 received_upto = tcp_highest_sack_seq(tp);
  
  	if (!tcp_is_fack(tp) || !tp->retrans_out ||
  	    !after(received_upto, tp->lost_retrans_low) ||
  	    icsk->icsk_ca_state != TCP_CA_Recovery)
  		return;
  
  	tcp_for_write_queue(skb, sk) {
  		u32 ack_seq = TCP_SKB_CB(skb)->ack_seq;
  
  		if (skb == tcp_send_head(sk))
  			break;
  		if (cnt == tp->retrans_out)
  			break;
  		if (!after(TCP_SKB_CB(skb)->end_seq, tp->snd_una))
  			continue;
  
  		if (!(TCP_SKB_CB(skb)->sacked & TCPCB_SACKED_RETRANS))
  			continue;
  
  		/* TODO: We would like to get rid of tcp_is_fack(tp) only
  		 * constraint here (see above) but figuring out that at
  		 * least tp->reordering SACK blocks reside between ack_seq
  		 * and received_upto is not easy task to do cheaply with
  		 * the available datastructures.
  		 *
  		 * Whether FACK should check here for tp->reordering segs
  		 * in-between one could argue for either way (it would be
  		 * rather simple to implement as we could count fack_count
  		 * during the walk and do tp->fackets_out - fack_count).
  		 */
  		if (after(received_upto, ack_seq)) {
  			TCP_SKB_CB(skb)->sacked &= ~TCPCB_SACKED_RETRANS;
  			tp->retrans_out -= tcp_skb_pcount(skb);
  
  			tcp_skb_mark_lost_uncond_verify(tp, skb);
  			NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_TCPLOSTRETRANSMIT);
  		} else {
  			if (before(ack_seq, new_low_seq))
  				new_low_seq = ack_seq;
  			cnt += tcp_skb_pcount(skb);
  		}
  	}
  
  	if (tp->retrans_out)
  		tp->lost_retrans_low = new_low_seq;
  }
  
  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_BH(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_BH(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 {
  	int	reord;
  	int	fack_count;
  	long	rtt_us; /* RTT measured by SACKing never-retransmitted data */
  	int	flag;
  };
  
  /* 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;
  				if (new_len >= skb->len)
  					return 0;
  			}
  			pkt_len = new_len;
  		}
  		err = tcp_fragment(sk, 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,
  			  const struct skb_mstamp *xmit_time)
  {
  	struct tcp_sock *tp = tcp_sk(sk);
  	int fack_count = state->fack_count;
  
  	/* 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)
  			state->reord = min(fack_count, state->reord);
  	}
  
  	/* 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)) {
  		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)))
  					state->reord = min(fack_count,
  							   state->reord);
  				if (!after(end_seq, tp->high_seq))
  					state->flag |= FLAG_ORIG_SACK_ACKED;
  				/* Pick the earliest sequence sacked for RTT */
  				if (state->rtt_us < 0) {
  					struct skb_mstamp now;
  
  					skb_mstamp_get(&now);
  					state->rtt_us = skb_mstamp_us_delta(&now,
  								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;
  
  		fack_count += pcount;
  
  		/* Lost marker hint past SACKed? Tweak RFC3517 cnt */
  		if (!tcp_is_fack(tp) && (tp->lost_skb_hint != NULL) &&
  		    before(start_seq, TCP_SKB_CB(tp->lost_skb_hint)->seq))
  			tp->lost_cnt_hint += pcount;
  
  		if (fack_count > tp->fackets_out)
  			tp->fackets_out = fack_count;
  	}
  
  	/* 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 *skb,
  			    struct tcp_sacktag_state *state,
  			    unsigned int pcount, int shifted, int mss,
  			    bool dup_sack)
  {
  	struct tcp_sock *tp = tcp_sk(sk);
  	struct sk_buff *prev = tcp_write_queue_prev(sk, skb);
  	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);
  
  	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 (!skb_shinfo(prev)->gso_size) {
  		skb_shinfo(prev)->gso_size = mss;
  		skb_shinfo(prev)->gso_type = sk->sk_gso_type;
  	}
  
  	/* CHECKME: To clear or not to clear? Mimics normal skb currently */
  	if (tcp_skb_pcount(skb) <= 1) {
  		skb_shinfo(skb)->gso_size = 0;
  		skb_shinfo(skb)->gso_type = 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_BH(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;
  	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_unlink_write_queue(skb, sk);
  	sk_wmem_free_skb(sk, skb);
  
  	NET_INC_STATS_BH(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;
  
  	if (!sk_can_gso(sk))
  		goto fallback;
  
  	/* 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 */
  	if (unlikely(skb == tcp_write_queue_head(sk)))
  		goto fallback;
  	prev = tcp_write_queue_prev(sk, skb);
  
  	if ((TCP_SKB_CB(prev)->sacked & TCPCB_TAGBITS) != TCPCB_SACKED_ACKED)
  		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, 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
  	 */
  	if (prev == tcp_write_queue_tail(sk))
  		goto out;
  	skb = tcp_write_queue_next(sk, prev);
  
  	if (!skb_can_shift(skb) ||
  	    (skb == tcp_send_head(sk)) ||
  	    ((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, skb, state, tcp_skb_pcount(skb), len, mss, 0);
  	}
  
  out:
  	state->fack_count += pcount;
  	return prev;
  
  noop:
  	return skb;
  
  fallback:
  	NET_INC_STATS_BH(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;
  
  	tcp_for_write_queue_from(skb, sk) {
  		int in_sack = 0;
  		bool dup_sack = dup_sack_in;
  
  		if (skb == tcp_send_head(sk))
  			break;
  
  		/* queue is in-order => we can short-circuit the walk early */
  		if (!before(TCP_SKB_CB(skb)->seq, end_seq))
  			break;
  
  		if ((next_dup != NULL) &&
  		    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 != NULL) {
  				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);
  
  			if (!before(TCP_SKB_CB(skb)->seq,
  				    tcp_highest_sack_seq(tp)))
  				tcp_advance_highest_sack(sk, skb);
  		}
  
  		state->fack_count += tcp_skb_pcount(skb);
  	}
  	return skb;
  }
  
  /* Avoid all extra work that is being done by sacktag while walking in
   * a normal way
   */
  static struct sk_buff *tcp_sacktag_skip(struct sk_buff *skb, struct sock *sk,
  					struct tcp_sacktag_state *state,
  					u32 skip_to_seq)
  {
  	tcp_for_write_queue_from(skb, sk) {
  		if (skb == tcp_send_head(sk))
  			break;
  
  		if (after(TCP_SKB_CB(skb)->end_seq, skip_to_seq))
  			break;
  
  		state->fack_count += tcp_skb_pcount(skb);
  	}
  	return skb;
  }
  
  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 == NULL)
  		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, long *sack_rtt_us)
  {
  	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 tcp_sacktag_state state;
  	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->packets_out;
  	state.rtt_us = -1L;
  
  	if (!tp->sacked_out) {
  		if (WARN_ON(tp->fackets_out))
  			tp->fackets_out = 0;
  		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;
  
  	/* 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_BH(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;
  			}
  		}
  	}
  
  	skb = tcp_write_queue_head(sk);
  	state.fack_count = 0;
  	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 == NULL)
  					break;
  				state.fack_count = tp->fackets_out;
  				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 == NULL)
  				break;
  			state.fack_count = tp->fackets_out;
  		}
  		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];
  
  	tcp_mark_lost_retrans(sk);
  
  	tcp_verify_left_out(tp);
  
  	if ((state.reord < tp->fackets_out) &&
  	    ((inet_csk(sk)->icsk_ca_state != TCP_CA_Loss) || tp->undo_marker))
  		tcp_update_reordering(sk, tp->fackets_out - state.reord, 0);
  
  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
  	*sack_rtt_us = state.rtt_us;
  	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))
  		tcp_update_reordering(sk, tp->packets_out + addend, 0);
  }
  
  /* 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);
  	tp->sacked_out++;
  	tcp_check_reno_reordering(sk, 0);
  	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. */
  		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->fackets_out = 0;
  	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;
  }
  
  /* Enter Loss state. 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.
   */
  void tcp_enter_loss(struct sock *sk)
  {
  	const struct inet_connection_sock *icsk = inet_csk(sk);
  	struct tcp_sock *tp = tcp_sk(sk);
  	struct sk_buff *skb;
  	bool new_recovery = false;
  	bool is_reneg;			/* is receiver reneging on SACKs? */
  
  	/* 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)) {
  		new_recovery = true;
  		tp->prior_ssthresh = tcp_current_ssthresh(sk);
  		tp->snd_ssthresh = icsk->icsk_ca_ops->ssthresh(sk);
  		tcp_ca_event(sk, CA_EVENT_LOSS);
  		tcp_init_undo(tp);
  	}
  	tp->snd_cwnd	   = 1;
  	tp->snd_cwnd_cnt   = 0;
  	tp->snd_cwnd_stamp = tcp_time_stamp;
  
  	tp->retrans_out = 0;
  	tp->lost_out = 0;
  
  	if (tcp_is_reno(tp))
  		tcp_reset_reno_sack(tp);
  
  	skb = tcp_write_queue_head(sk);
  	is_reneg = skb && (TCP_SKB_CB(skb)->sacked & TCPCB_SACKED_ACKED);
  	if (is_reneg) {
  		NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_TCPSACKRENEGING);
  		tp->sacked_out = 0;
  		tp->fackets_out = 0;
  	}
  	tcp_clear_all_retrans_hints(tp);
  
  	tcp_for_write_queue(skb, sk) {
  		if (skb == tcp_send_head(sk))
  			break;
  
  		TCP_SKB_CB(skb)->sacked &= (~TCPCB_TAGBITS)|TCPCB_SACKED_ACKED;
  		if (!(TCP_SKB_CB(skb)->sacked&TCPCB_SACKED_ACKED) || is_reneg) {
  			TCP_SKB_CB(skb)->sacked &= ~TCPCB_SACKED_ACKED;
  			TCP_SKB_CB(skb)->sacked |= TCPCB_LOST;
  			tp->lost_out += tcp_skb_pcount(skb);
  			tp->retransmit_high = TCP_SKB_CB(skb)->end_seq;
  		}
  	}
  	tcp_verify_left_out(tp);
  
  	/* 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 >= sysctl_tcp_reordering)
  		tp->reordering = min_t(unsigned int, tp->reordering,
  				       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 = 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, sysctl_tcp_rto_max);
  		return true;
  	}
  	return false;
  }
  
  static inline int tcp_fackets_out(const struct tcp_sock *tp)
  {
  	return tcp_is_reno(tp) ? tp->sacked_out + 1 : tp->fackets_out;
  }
  
  /* 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).
   *
   * Instead, with FACK TCP uses fackets_out that includes both SACKed
   * segments up to the highest received SACK block so far and holes in
   * between them.
   *
   * 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 tcp_is_fack(tp) ? tp->fackets_out : tp->sacked_out + 1;
  }
  
  static bool tcp_pause_early_retransmit(struct sock *sk, int flag)
  {
  	struct tcp_sock *tp = tcp_sk(sk);
  	unsigned long delay;
  
  	/* Delay early retransmit and entering fast recovery for
  	 * max(RTT/4, 2msec) unless ack has ECE mark, no RTT samples
  	 * available, or RTO is scheduled to fire first.
  	 */
  	if (sysctl_tcp_early_retrans < 2 || sysctl_tcp_early_retrans > 3 ||
  	    (flag & FLAG_ECE) || !tp->srtt_us)
  		return false;
  
  	delay = max(usecs_to_jiffies(tp->srtt_us >> 5),
  		    msecs_to_jiffies(2));
  
  	if (!time_after(inet_csk(sk)->icsk_timeout, (jiffies + delay)))
  		return false;
  
  	inet_csk_reset_xmit_timer(sk, ICSK_TIME_EARLY_RETRANS, delay,
  				  sysctl_tcp_rto_max);
  	return true;
  }
  
  /* Linux NewReno/SACK/FACK/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 two algorithms counting
   *		lost packets.
   *
   *		FACK: It is the simplest heuristics. As soon as we decided
   *		that something is lost, we decide that _all_ not SACKed
   *		packets until the most forward SACK are lost. I.e.
   *		lost_out = fackets_out - sacked_out and left_out = fackets_out.
   *		It is absolutely correct estimate, if network does not reorder
   *		packets. And it loses any connection to reality when reordering
   *		takes place. We use FACK by default until reordering
   *		is suspected on the path to this destination.
   *
   *		NewReno: when Recovery is entered, 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.
   *
   *  Imagine, that's all! Forget about all this shamanism about CWND inflation
   *  deflation etc. CWND is real congestion window, never inflated, changes
   *  only according to classic VJ rules.
   *
   * 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);
  	__u32 packets_out;
  
  	/* Trick#1: The loss is proven. */
  	if (tp->lost_out)
  		return true;
  
  	/* Not-A-Trick#2 : Classic rule... */
  	if (tcp_dupack_heuristics(tp) > tp->reordering)
  		return true;
  
  	/* Trick#4: It is still not OK... But will it be useful to delay
  	 * recovery more?
  	 */
  	packets_out = tp->packets_out;
  	if (packets_out <= tp->reordering &&
  	    tp->sacked_out >= max_t(__u32, packets_out/2, sysctl_tcp_reordering) &&
  	    !tcp_may_send_now(sk)) {
  		/* We have nothing to send. This connection is limited
  		 * either by receiver window or by application.
  		 */
  		return true;
  	}
  
  	/* If a thin stream is detected, retransmit after first
  	 * received dupack. Employ only if SACK is supported in order
  	 * to avoid possible corner-case series of spurious retransmissions
  	 * Use only if there are no unsent data.
  	 */
  	if ((tp->thin_dupack || sysctl_tcp_thin_dupack) &&
  	    tcp_stream_is_thin(tp) && tcp_dupack_heuristics(tp) > 1 &&
  	    tcp_is_sack(tp) && !tcp_send_head(sk))
  		return true;
  
  	/* Trick#6: TCP early retransmit, per RFC5827.  To avoid spurious
  	 * retransmissions due to small network reorderings, we implement
  	 * Mitigation A.3 in the RFC and delay the retransmission for a short
  	 * interval if appropriate.
  	 */
  	if (tp->do_early_retrans && !tp->retrans_out && tp->sacked_out &&
  	    (tp->packets_out >= (tp->sacked_out + 1) && tp->packets_out < 4) &&
  	    !tcp_may_send_now(sk))
  		return !tcp_pause_early_retransmit(sk, flag);
  
  	return false;
  }
  
  /* Detect loss in event "A" above by marking head of queue up as lost.
   * For FACK or 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;
  	int err;
  	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);
  	if (tp->lost_skb_hint) {
  		skb = tp->lost_skb_hint;
  		cnt = tp->lost_cnt_hint;
  		/* Head already handled? */
  		if (mark_head && skb != tcp_write_queue_head(sk))
  			return;
  	} else {
  		skb = tcp_write_queue_head(sk);
  		cnt = 0;
  	}
  
  	tcp_for_write_queue_from(skb, sk) {
  		if (skb == tcp_send_head(sk))
  			break;
  		/* 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_fack(tp) || 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_is_fack(tp)) ||
  			    (TCP_SKB_CB(skb)->sacked & TCPCB_SACKED_ACKED) ||
  			    (oldcnt >= packets))
  				break;
  
  			mss = skb_shinfo(skb)->gso_size;
  			err = tcp_fragment(sk, skb, (packets - oldcnt) * mss,
  					   mss, GFP_ATOMIC);
  			if (err < 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_reno(tp)) {
  		tcp_mark_head_lost(sk, 1, 1);
  	} else if (tcp_is_fack(tp)) {
  		int lost = tp->fackets_out - tp->reordering;
  		if (lost <= 0)
  			lost = 1;
  		tcp_mark_head_lost(sk, lost, 0);
  	} else {
  		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);
  	}
  }
  
  /* CWND moderation, preventing bursts due to too big ACKs
   * in dubious situations.
   */
  static inline void tcp_moderate_cwnd(struct tcp_sock *tp)
  {
  	tp->snd_cwnd = min(tp->snd_cwnd,
  			   tcp_packets_in_flight(tp) + tcp_max_burst(tp));
  	tp->snd_cwnd_stamp = tcp_time_stamp;
  }
  
  /* 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 ||
  		(tp->rx_opt.saw_tstamp && tp->rx_opt.rcv_tsecr &&
  		 before(tp->rx_opt.rcv_tsecr, 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_write_queue_head(sk);
  	if (unlikely(skb && TCP_SKB_CB(skb)->sacked & TCPCB_EVER_RETRANS))
  		return true;
  
  	return false;
  }
  
  #if FASTRETRANS_DEBUG > 1
  static void DBGUNDO(struct sock *sk, const char *msg)
  {
  	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) {
  		struct ipv6_pinfo *np = inet6_sk(sk);
  		pr_debug("Undo %s %pI6/%u c%u l%u ss%u/%u p%u\n",
  			 msg,
  			 &np->daddr, ntohs(inet->inet_dport),
  			 tp->snd_cwnd, tcp_left_out(tp),
  			 tp->snd_ssthresh, tp->prior_ssthresh,
  			 tp->packets_out);
  	}
  #endif
  }
  #else
  #define DBGUNDO(x...) do { } while (0)
  #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;
  
  		tcp_for_write_queue(skb, sk) {
  			if (skb == tcp_send_head(sk))
  				break;
  			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);
  
  		if (icsk->icsk_ca_ops->undo_cwnd)
  			tp->snd_cwnd = icsk->icsk_ca_ops->undo_cwnd(sk);
  		else
  			tp->snd_cwnd = max(tp->snd_cwnd, tp->snd_ssthresh << 1);
  
  		if (tp->prior_ssthresh > tp->snd_ssthresh) {
  			tp->snd_ssthresh = tp->prior_ssthresh;
  			tcp_ecn_withdraw_cwr(tp);
  		}
  	} else {
  		tp->snd_cwnd = max(tp->snd_cwnd, tp->snd_ssthresh);
  	}
  	tp->snd_cwnd_stamp = tcp_time_stamp;
  	tp->undo_marker = 0;
  }
  
  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_BH(sock_net(sk), mib_idx);
  	}
  	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. */
  		tcp_moderate_cwnd(tp);
  		if (!tcp_any_retrans_done(sk))
  			tp->retrans_stamp = 0;
  		return true;
  	}
  	tcp_set_ca_state(sk, TCP_CA_Open);
  	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) {
  		DBGUNDO(sk, "D-SACK");
  		tcp_undo_cwnd_reduction(sk, false);
  		NET_INC_STATS_BH(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_BH(sock_net(sk), LINUX_MIB_TCPLOSSUNDO);
  		if (frto_undo)
  			NET_INC_STATS_BH(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);
  		return true;
  	}
  	return false;
  }
  
  /* The cwnd reduction in CWR and Recovery use the PRR algorithm
   * https://datatracker.ietf.org/doc/draft-ietf-tcpm-proportional-rate-reduction/
   * 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) If packets in flight is lower than ssthresh (such as due to excess
   *	losses and/or application stalls), do not perform any further cwnd
   *	reductions, but instead slow start up to ssthresh.
   */
  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);
  }
  
  static void tcp_cwnd_reduction(struct sock *sk, const int prior_unsacked,
  			       int fast_rexmit)
  {
  	struct tcp_sock *tp = tcp_sk(sk);
  	int sndcnt = 0;
  	int delta = tp->snd_ssthresh - tcp_packets_in_flight(tp);
  	int newly_acked_sacked = prior_unsacked -
  				 (tp->packets_out - tp->sacked_out);
  
  	tp->prr_delivered += newly_acked_sacked;
  	if (tcp_packets_in_flight(tp) > tp->snd_ssthresh) {
  		u64 dividend = (u64)tp->snd_ssthresh * tp->prr_delivered +
  			       tp->prior_cwnd - 1;
  		sndcnt = div_u64(dividend, tp->prior_cwnd) - tp->prr_out;
  	} else {
  		sndcnt = min_t(int, delta,
  			       max_t(int, tp->prr_delivered - tp->prr_out,
  				     newly_acked_sacked) + 1);
  	}
  
  	sndcnt = max(sndcnt, (fast_rexmit ? 1 : 0));
  	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);
  
  	/* Reset cwnd to ssthresh in CWR or Recovery (unless it's undone) */
  	if (inet_csk(sk)->icsk_ca_state == TCP_CA_CWR ||
  	    (tp->undo_marker && tp->snd_ssthresh < TCP_INFINITE_SSTHRESH)) {
  		tp->snd_cwnd = tp->snd_ssthresh;
  		tp->snd_cwnd_stamp = tcp_time_stamp;
  	}
  	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);
  	}
  }
  
  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, const int prior_unsacked)
  {
  	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);
  	} else {
  		tcp_cwnd_reduction(sk, prior_unsacked, 0);
  	}
  }
  
  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;
  }
  
  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_time_stamp;
  	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);
  }
  
  /* 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);
  	u32 prior_lost = tp->lost_out;
  
  	tcp_for_write_queue(skb, sk) {
  		if (skb == tcp_send_head(sk))
  			break;
  		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 (prior_lost == 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);
  
  static 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_BH(sock_net(sk), mib_idx);
  
  	tp->prior_ssthresh = 0;
  	tcp_init_undo(tp);
  
  	if (inet_csk(sk)->icsk_ca_state < TCP_CA_CWR) {
  		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)
  {
  	struct tcp_sock *tp = tcp_sk(sk);
  	bool recovered = !before(tp->snd_una, tp->high_seq);
  
  	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 (tcp_try_undo_loss(sk, flag & FLAG_ORIG_SACK_ACKED))
  			return;
  
  		if (after(tp->snd_nxt, tp->high_seq) &&
  		    (flag & FLAG_DATA_SACKED || is_dupack)) {
  			tp->frto = 0; /* Loss was real: 2nd part of step 3.a */
  		} else if (flag & FLAG_SND_UNA_ADVANCED && !recovered) {
  			tp->high_seq = tp->snd_nxt;
  			__tcp_push_pending_frames(sk, tcp_current_mss(sk),
  						  TCP_NAGLE_OFF);
  			if (after(tp->snd_nxt, tp->high_seq))
  				return; /* Step 2.b */
  			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);
  	}
  	if (tcp_try_undo_loss(sk, false))
  		return;
  	tcp_xmit_retransmit_queue(sk);
  }
  
  /* Undo during fast recovery after partial ACK. */
  static bool tcp_try_undo_partial(struct sock *sk, const int acked,
  				 const int prior_unsacked)
  {
  	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.
  		 */
  		tcp_update_reordering(sk, tcp_fackets_out(tp) + acked, 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) {
  			tcp_cwnd_reduction(sk, prior_unsacked, 0);
  			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_BH(sock_net(sk), LINUX_MIB_TCPPARTIALUNDO);
  		tcp_try_keep_open(sk);
  		return true;
  	}
  	return false;
  }
  
  /* 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 does CWND reduction, when packet loss is detected
   * and changes state of machine.
   *
   * 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 int acked,
  				  const int prior_unsacked,
  				  bool is_dupack, int flag)
  {
  	struct inet_connection_sock *icsk = inet_csk(sk);
  	struct tcp_sock *tp = tcp_sk(sk);
  	bool do_lost = is_dupack || ((flag & FLAG_DATA_SACKED) &&
  				    (tcp_fackets_out(tp) > tp->reordering));
  	int fast_rexmit = 0;
  
  	if (WARN_ON(!tp->packets_out && tp->sacked_out))
  		tp->sacked_out = 0;
  	if (WARN_ON(!tp->sacked_out && tp->fackets_out))
  		tp->fackets_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, acked, prior_unsacked))
  				return;
  			/* Partial ACK arrived. Force fast retransmit. */
  			do_lost = tcp_is_reno(tp) ||
  				  tcp_fackets_out(tp) > tp->reordering;
  		}
  		if (tcp_try_undo_dsack(sk)) {
  			tcp_try_keep_open(sk);
  			return;
  		}
  		break;
  	case TCP_CA_Loss:
  		tcp_process_loss(sk, flag, is_dupack);
  		if (icsk->icsk_ca_state != TCP_CA_Open)
  			return;
  		/* Fall through to processing in Open state. */
  	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);
  
  		if (!tcp_time_to_recover(sk, flag)) {
  			tcp_try_to_open(sk, flag, prior_unsacked);
  			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 (do_lost)
  		tcp_update_scoreboard(sk, fast_rexmit);
  	tcp_cwnd_reduction(sk, prior_unsacked, fast_rexmit);
  	tcp_xmit_retransmit_queue(sk);
  }
  
  static inline bool tcp_ack_update_rtt(struct sock *sk, const int flag,
  				      long seq_rtt_us, long sack_rtt_us)
  {
  	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 (flag & FLAG_RETRANS_DATA_ACKED)
  		seq_rtt_us = -1L;
  
  	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)
  		seq_rtt_us = jiffies_to_usecs(tcp_time_stamp - tp->rx_opt.rcv_tsecr);
  
  	if (seq_rtt_us < 0)
  		return false;
  
  	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. */
  static void tcp_synack_rtt_meas(struct sock *sk, const u32 synack_stamp)
  {
  	struct tcp_sock *tp = tcp_sk(sk);
  	long seq_rtt_us = -1L;
  
  	if (synack_stamp && !tp->total_retrans)
  		seq_rtt_us = jiffies_to_usecs(tcp_time_stamp - synack_stamp);
  
  	/* If the ACK acks both the SYNACK and the (Fast Open'd) data packets
  	 * sent in SYN_RECV, SYNACK RTT is the smooth RTT computed in tcp_ack()
  	 */
  	if (!tp->srtt_us)
  		tcp_ack_update_rtt(sk, FLAG_SYN_ACKED, seq_rtt_us, -1L);
  }
  
  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_time_stamp;
  }
  
  /* 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_EARLY_RETRANS ||
  		    icsk->icsk_pending == ICSK_TIME_LOSS_PROBE) {
  			struct sk_buff *skb = tcp_write_queue_head(sk);
  			const u32 rto_time_stamp =
  				tcp_skb_timestamp(skb) + rto;
  			s32 delta = (s32)(rto_time_stamp - tcp_time_stamp);
  			/* delta may not be positive if the socket is locked
  			 * when the retrans timer fires and is rescheduled.
  			 */
  			if (delta > 0)
  				rto = delta;
  		}
  		inet_csk_reset_xmit_timer(sk, ICSK_TIME_RETRANS, rto,
  					  sysctl_tcp_rto_max);
  	}
  }
  
  /* This function is called when the delayed ER timer fires. TCP enters
   * fast recovery and performs fast-retransmit.
   */
  void tcp_resume_early_retransmit(struct sock *sk)
  {
  	struct tcp_sock *tp = tcp_sk(sk);
  
  	tcp_rearm_rto(sk);
  
  	/* Stop if ER is disabled after the delayed ER timer is scheduled */
  	if (!tp->do_early_retrans)
  		return;
  
  	tcp_enter_recovery(sk, false);
  	tcp_update_scoreboard(sk, 1);
  	tcp_xmit_retransmit_queue(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(!(sk->sk_tsflags & SOF_TIMESTAMPING_TX_ACK)))
  		return;
  
  	shinfo = skb_shinfo(skb);
  	if ((shinfo->tx_flags & SKBTX_ACK_TSTAMP) &&
  	    between(shinfo->tskey, prior_snd_una, tcp_sk(sk)->snd_una - 1))
  		__skb_tstamp_tx(skb, NULL, sk, SCM_TSTAMP_ACK);
  }
  
  /* 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, int prior_fackets,
  			       u32 prior_snd_una, long sack_rtt_us)
  {
  	const struct inet_connection_sock *icsk = inet_csk(sk);
  	struct skb_mstamp first_ackt, last_ackt, now;
  	struct tcp_sock *tp = tcp_sk(sk);
  	u32 prior_sacked = tp->sacked_out;
  	u32 reord = tp->packets_out;
  	bool fully_acked = true;
  	long ca_seq_rtt_us = -1L;
  	long seq_rtt_us = -1L;
  	struct sk_buff *skb;
  	u32 pkts_acked = 0;
  	bool rtt_update;
  	int flag = 0;
  
  	first_ackt.v64 = 0;
  
  	while ((skb = tcp_write_queue_head(sk)) && skb != tcp_send_head(sk)) {
  		struct tcp_skb_cb *scb = TCP_SKB_CB(skb);
  		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 {
  			/* Speedup tcp_unlink_write_queue() and next loop */
  			prefetchw(skb->next);
  			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 {
  			last_ackt = skb->skb_mstamp;
  			WARN_ON_ONCE(last_ackt.v64 == 0);
  			if (!first_ackt.v64)
  				first_ackt = last_ackt;
  
  			if (!(sacked & TCPCB_SACKED_ACKED)) {
  				reord = min(pkts_acked, reord);
  				if (!after(scb->end_seq, tp->high_seq))
  					flag |= FLAG_ORIG_SACK_ACKED;
  			}
  		}
  
  		if (sacked & TCPCB_SACKED_ACKED)
  			tp->sacked_out -= acked_pcount;
  		if (sacked & TCPCB_LOST)
  			tp->lost_out -= acked_pcount;
  
  		tp->packets_out -= acked_pcount;
  		pkts_acked += acked_pcount;
  
  		/* 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;
  
  		tcp_unlink_write_queue(skb, sk);
  		sk_wmem_free_skb(sk, 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;
  	}
  
  	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;
  
  	skb_mstamp_get(&now);
  	if (likely(first_ackt.v64)) {
  		seq_rtt_us = skb_mstamp_us_delta(&now, &first_ackt);
  		ca_seq_rtt_us = skb_mstamp_us_delta(&now, &last_ackt);
  	}
  
  	rtt_update = tcp_ack_update_rtt(sk, flag, seq_rtt_us, sack_rtt_us);
  
  	if (flag & FLAG_ACKED) {
  		const struct tcp_congestion_ops *ca_ops
  			= inet_csk(sk)->icsk_ca_ops;
  
  		tcp_rearm_rto(sk);
  		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 (reord < prior_fackets)
  				tcp_update_reordering(sk, tp->fackets_out - reord, 0);
  
  			delta = tcp_is_fack(tp) ? pkts_acked :
  						  prior_sacked - tp->sacked_out;
  			tp->lost_cnt_hint -= min(tp->lost_cnt_hint, delta);
  		}
  
  		tp->fackets_out -= min(pkts_acked, tp->fackets_out);
  
  		if (ca_ops->pkts_acked)
  			ca_ops->pkts_acked(sk, pkts_acked, ca_seq_rtt_us);
  
  	} else if (skb && rtt_update && sack_rtt_us >= 0 &&
  		   sack_rtt_us > skb_mstamp_us_delta(&now, &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.
  		 */
  		tcp_rearm_rto(sk);
  	}
  
  #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)
  {
  	const struct tcp_sock *tp = tcp_sk(sk);
  	struct inet_connection_sock *icsk = inet_csk(sk);
  
  	/* Was it a usable window open? */
  
  	if (!after(TCP_SKB_CB(tcp_send_head(sk))->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 = inet_csk_rto_backoff(icsk, sysctl_tcp_rto_max);
  
  		inet_csk_reset_xmit_timer(sk, ICSK_TIME_PROBE0,
  					  when, sysctl_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 (tcp_in_cwnd_reduction(sk))
  		return false;
  
  	/* 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 > sysctl_tcp_reordering)
  		return flag & FLAG_FORWARD_PROGRESS;
  
  	return flag & FLAG_DATA_ACKED;
  }
  
  /* 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);
  }
  
  /* 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 (nwin > tp->max_window) {
  				tp->max_window = nwin;
  				tcp_sync_mss(sk, inet_csk(sk)->icsk_pmtu_cookie);
  			}
  		}
  	}
  
  	tp->snd_una = ack;
  
  	return flag;
  }
  
  /* RFC 5961 7 [ACK Throttling] */
  static void tcp_send_challenge_ack(struct sock *sk)
  {
  	/* unprotected vars, we dont care of overwrites */
  	static u32 challenge_timestamp;
  	static unsigned int challenge_count;
  	u32 now = jiffies / HZ;
  
  	if (now != challenge_timestamp) {
  		challenge_timestamp = now;
  		challenge_count = 0;
  	}
  	if (++challenge_count <= sysctl_tcp_challenge_ack_limit) {
  		NET_INC_STATS_BH(sock_net(sk), 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.
   * 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);
  	bool is_tlp_dupack = (ack == tp->tlp_high_seq) &&
  			     !(flag & (FLAG_SND_UNA_ADVANCED |
  				       FLAG_NOT_DUP | FLAG_DATA_SACKED));
  
  	/* Mark the end of TLP episode on receiving TLP dupack or when
  	 * ack is after tlp_high_seq.
  	 */
  	if (is_tlp_dupack) {
  		tp->tlp_high_seq = 0;
  		return;
  	}
  
  	if (after(ack, tp->tlp_high_seq)) {
  		tp->tlp_high_seq = 0;
  		/* Don't reduce cwnd if DSACK arrives for TLP retrans. */
  		if (!(flag & FLAG_DSACKING_ACK)) {
  			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_BH(sock_net(sk),
  					 LINUX_MIB_TCPLOSSPROBERECOVERY);
  		}
  	}
  }
  
  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);
  }
  
  /* 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);
  	u32 prior_snd_una = tp->snd_una;
  	u32 ack_seq = TCP_SKB_CB(skb)->seq;
  	u32 ack = TCP_SKB_CB(skb)->ack_seq;
  	bool is_dupack = false;
  	u32 prior_fackets;
  	int prior_packets = tp->packets_out;
  	const int prior_unsacked = tp->packets_out - tp->sacked_out;
  	int acked = 0; /* Number of packets newly acked */
  	long sack_rtt_us = -1L;
  
  	/* We very likely will need to access write queue head. */
  	prefetchw(sk->sk_write_queue.next);
  
  	/* 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)) {
  			tcp_send_challenge_ack(sk);
  			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 (icsk->icsk_pending == ICSK_TIME_EARLY_RETRANS ||
  	    icsk->icsk_pending == ICSK_TIME_LOSS_PROBE)
  		tcp_rearm_rto(sk);
  
  	if (after(ack, prior_snd_una)) {
  		flag |= FLAG_SND_UNA_ADVANCED;
  		icsk->icsk_retransmits = 0;
  	}
  
  	prior_fackets = tp->fackets_out;
  
  	/* 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);
  		tp->snd_una = ack;
  		flag |= FLAG_WIN_UPDATE;
  
  		tcp_in_ack_event(sk, CA_ACK_WIN_UPDATE);
  
  		NET_INC_STATS_BH(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_BH(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_rtt_us);
  
  		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_time_stamp;
  	if (!prior_packets)
  		goto no_queue;
  
  	/* See if we can take anything off of the retransmit queue. */
  	acked = tp->packets_out;
  	flag |= tcp_clean_rtx_queue(sk, prior_fackets, prior_snd_una,
  				    sack_rtt_us);
  	acked -= tp->packets_out;
  
  	/* Advance cwnd if state allows */
  	if (tcp_may_raise_cwnd(sk, flag))
  		tcp_cong_avoid(sk, ack, acked);
  
  	if (tcp_ack_is_dubious(sk, flag)) {
  		is_dupack = !(flag & (FLAG_SND_UNA_ADVANCED | FLAG_NOT_DUP));
  		tcp_fastretrans_alert(sk, acked, prior_unsacked,
  				      is_dupack, flag);
  	}
  	if (tp->tlp_high_seq)
  		tcp_process_tlp_ack(sk, ack, flag);
  
  	if ((flag & FLAG_FORWARD_PROGRESS) || !(flag & FLAG_NOT_DUP)) {
  		struct dst_entry *dst = __sk_dst_get(sk);
  		if (dst)
  			dst_confirm(dst);
  	}
  
  	if (icsk->icsk_pending == ICSK_TIME_RETRANS)
  		tcp_schedule_loss_probe(sk);
  	tcp_update_pacing_rate(sk);
  	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, acked, prior_unsacked,
  				      is_dupack, 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.
  	 */
  	if (tcp_send_head(sk))
  		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_rtt_us);
  		tcp_fastretrans_alert(sk, acked, prior_unsacked,
  				      is_dupack, flag);
  	}
  
  	SOCK_DEBUG(sk, "Ack %u before %u:%u\n", ack, tp->snd_una, tp->snd_nxt);
  	return 0;
  }
  
  /* 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 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 && sysctl_tcp_window_scaling) {
  					__u8 snd_wscale = *(__u8 *)ptr;
  					opt_rx->wscale_ok = 1;
  					if (snd_wscale > 14) {
  						net_info_ratelimited("%s: Illegal window scaling value %d >14 received\n",
  								     __func__,
  								     snd_wscale);
  						snd_wscale = 14;
  					}
  					opt_rx->snd_wscale = snd_wscale;
  				}
  				break;
  			case TCPOPT_TIMESTAMP:
  				if ((opsize == TCPOLEN_TIMESTAMP) &&
  				    ((estab && opt_rx->tstamp_ok) ||
  				     (!estab && 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 && 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_EXP:
  				/* Fast Open option shares code 254 using a
  				 * 16 bits magic number. It's valid only in
  				 * SYN or SYN-ACK with an even size.
  				 */
  				if (opsize < TCPOLEN_EXP_FASTOPEN_BASE ||
  				    get_unaligned_be16(ptr) != TCPOPT_FASTOPEN_MAGIC ||
  				    foc == NULL || !th->syn || (opsize & 1))
  					break;
  				foc->len = opsize - TCPOLEN_EXP_FASTOPEN_BASE;
  				if (foc->len >= TCP_FASTOPEN_COOKIE_MIN &&
  				    foc->len <= TCP_FASTOPEN_COOKIE_MAX)
  					memcpy(foc->val, ptr + 2, foc->len);
  				else if (foc->len != 0)
  					foc->len = -1;
  				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 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(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 the TCP option is too short, we can short cut */
  	if (length < TCPOLEN_MD5SIG)
  		return NULL;
  
  	while (length > 0) {
  		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)
  {
  	/* 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();
  
  	if (!sock_flag(sk, SOCK_DEAD))
  		sk->sk_error_report(sk);
  
  	tcp_done(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.
   */
  static void tcp_fin(struct sock *sk)
  {
  	struct tcp_sock *tp = tcp_sk(sk);
  	const struct dst_entry *dst;
  
  	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);
  		dst = __sk_dst_get(sk);
  		if (!dst || !dst_metric(dst, RTAX_QUICKACK))
  			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_queue_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) && 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_BH(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_BH(sock_net(sk), LINUX_MIB_DELAYEDACKLOST);
  		tcp_enter_quickack_mode(sk);
  
  		if (tcp_is_sack(tp) && 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) {
  		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 (skb_queue_empty(&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
   * @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_BH(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;
  	return true;
  }
  
  /* 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;
  	struct sk_buff *skb, *tail;
  	bool fragstolen, eaten;
  
  	while ((skb = skb_peek(&tp->out_of_order_queue)) != NULL) {
  		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);
  		}
  
  		__skb_unlink(skb, &tp->out_of_order_queue);
  		if (!after(TCP_SKB_CB(skb)->end_seq, tp->rcv_nxt)) {
  			SOCK_DEBUG(sk, "ofo packet was already received\n");
  			__kfree_skb(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);
  		tp->rcv_nxt = TCP_SKB_CB(skb)->end_seq;
  		if (!eaten)
  			__skb_queue_tail(&sk->sk_receive_queue, skb);
  		if (TCP_SKB_CB(skb)->tcp_flags & TCPHDR_FIN)
  			tcp_fin(sk);
  		if (eaten)
  			kfree_skb_partial(skb, fragstolen);
  	}
  }
  
  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;
  
  		if (!sk_rmem_schedule(sk, skb, size)) {
  			if (!tcp_prune_ofo_queue(sk))
  				return -1;
  
  			if (!sk_rmem_schedule(sk, skb, size))
  				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 sk_buff *skb1;
  	u32 seq, end_seq;
  
  	tcp_ecn_check_ce(tp, skb);
  
  	if (unlikely(tcp_try_rmem_schedule(sk, skb, skb->truesize))) {
  		NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_TCPOFODROP);
  		__kfree_skb(skb);
  		return;
  	}
  
  	/* Disable header prediction. */
  	tp->pred_flags = 0;
  	inet_csk_schedule_ack(sk);
  
  	NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_TCPOFOQUEUE);
  	SOCK_DEBUG(sk, "out of order segment: rcv_next %X seq %X - %X\n",
  		   tp->rcv_nxt, TCP_SKB_CB(skb)->seq, TCP_SKB_CB(skb)->end_seq);
  
  	skb1 = skb_peek_tail(&tp->out_of_order_queue);
  	if (!skb1) {
  		/* 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 = TCP_SKB_CB(skb)->seq;
  			tp->selective_acks[0].end_seq =
  						TCP_SKB_CB(skb)->end_seq;
  		}
  		__skb_queue_head(&tp->out_of_order_queue, skb);
  		goto end;
  	}
  
  	seq = TCP_SKB_CB(skb)->seq;
  	end_seq = TCP_SKB_CB(skb)->end_seq;
  
  	if (seq == TCP_SKB_CB(skb1)->end_seq) {
  		bool fragstolen;
  
  		if (!tcp_try_coalesce(sk, skb1, skb, &fragstolen)) {
  			__skb_queue_after(&tp->out_of_order_queue, skb1, skb);
  		} else {
  			tcp_grow_window(sk, skb);
  			kfree_skb_partial(skb, fragstolen);
  			skb = NULL;
  		}
  
  		if (!tp->rx_opt.num_sacks ||
  		    tp->selective_acks[0].end_seq != seq)
  			goto add_sack;
  
  		/* Common case: data arrive in order after hole. */
  		tp->selective_acks[0].end_seq = end_seq;
  		goto end;
  	}
  
  	/* Find place to insert this segment. */
  	while (1) {
  		if (!after(TCP_SKB_CB(skb1)->seq, seq))
  			break;
  		if (skb_queue_is_first(&tp->out_of_order_queue, skb1)) {
  			skb1 = NULL;
  			break;
  		}
  		skb1 = skb_queue_prev(&tp->out_of_order_queue, skb1);
  	}
  
  	/* Do skb overlap to previous one? */
  	if (skb1 && 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_BH(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 {
  			if (skb_queue_is_first(&tp->out_of_order_queue,
  					       skb1))
  				skb1 = NULL;
  			else
  				skb1 = skb_queue_prev(
  					&tp->out_of_order_queue,
  					skb1);
  		}
  	}
  	if (!skb1)
  		__skb_queue_head(&tp->out_of_order_queue, skb);
  	else
  		__skb_queue_after(&tp->out_of_order_queue, skb1, skb);
  
  	/* And clean segments covered by new one as whole. */
  	while (!skb_queue_is_last(&tp->out_of_order_queue, skb)) {
  		skb1 = skb_queue_next(&tp->out_of_order_queue, skb);
  
  		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;
  		}
  		__skb_unlink(skb1, &tp->out_of_order_queue);
  		tcp_dsack_extend(sk, TCP_SKB_CB(skb1)->seq,
  				 TCP_SKB_CB(skb1)->end_seq);
  		NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_TCPOFOMERGE);
  		__kfree_skb(skb1);
  	}
  
  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_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_sk(sk)->rcv_nxt = 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;
  	bool fragstolen;
  
  	if (size == 0)
  		return 0;
  
  	skb = alloc_skb(size, sk->sk_allocation);
  	if (!skb)
  		goto err;
  
  	if (tcp_try_rmem_schedule(sk, skb, skb->truesize))
  		goto err_free;
  
  	if (memcpy_fromiovec(skb_put(skb, size), msg->msg_iov, size))
  		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 -ENOMEM;
  }
  
  static void tcp_data_queue(struct sock *sk, struct sk_buff *skb)
  {
  	struct tcp_sock *tp = tcp_sk(sk);
  	int eaten = -1;
  	bool fragstolen = false;
  
  	if (TCP_SKB_CB(skb)->seq == TCP_SKB_CB(skb)->end_seq)
  		goto drop;
  
  	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. */
  		if (tp->ucopy.task == current &&
  		    tp->copied_seq == tp->rcv_nxt && tp->ucopy.len &&
  		    sock_owned_by_user(sk) && !tp->urg_data) {
  			int chunk = min_t(unsigned int, skb->len,
  					  tp->ucopy.len);
  
  			__set_current_state(TASK_RUNNING);
  
  			local_bh_enable();
  			if (!skb_copy_datagram_iovec(skb, 0, tp->ucopy.iov, chunk)) {
  				tp->ucopy.len -= chunk;
  				tp->copied_seq += chunk;
  				eaten = (chunk == skb->len);
  				tcp_rcv_space_adjust(sk);
  			}
  			local_bh_disable();
  		}
  
  		if (eaten <= 0) {
  queue_and_out:
  			if (eaten < 0 &&
  			    tcp_try_rmem_schedule(sk, skb, skb->truesize))
  				goto drop;
  
  			eaten = tcp_queue_rcv(sk, skb, 0, &fragstolen);
  		}
  		tp->rcv_nxt = 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 (!skb_queue_empty(&tp->out_of_order_queue)) {
  			tcp_ofo_queue(sk);
  
  			/* RFC2581. 4.2. SHOULD send immediate ACK, when
  			 * gap in queue is filled.
  			 */
  			if (skb_queue_empty(&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))
  			sk->sk_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_BH(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);
  		inet_csk_schedule_ack(sk);
  drop:
  		__kfree_skb(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;
  
  	tcp_enter_quickack_mode(sk);
  
  	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_collapse_one(struct sock *sk, struct sk_buff *skb,
  					struct sk_buff_head *list)
  {
  	struct sk_buff *next = NULL;
  
  	if (!skb_queue_is_last(list, skb))
  		next = skb_queue_next(list, skb);
  
  	__skb_unlink(skb, list);
  	__kfree_skb(skb);
  	NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_TCPRCVCOLLAPSED);
  
  	return next;
  }
  
  /* Collapse contiguous sequence of skbs head..tail with
   * sequence numbers start..end.
   *
   * If tail is NULL, this means until the end of the list.
   *
   * 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 sk_buff *head, struct sk_buff *tail,
  	     u32 start, u32 end)
  {
  	struct sk_buff *skb, *n;
  	bool end_of_skbs;
  
  	/* First, check that queue is collapsible and find
  	 * the point where collapsing can be useful. */
  	skb = head;
  restart:
  	end_of_skbs = true;
  	skb_queue_walk_from_safe(list, skb, n) {
  		if (skb == tail)
  			break;
  		/* 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);
  			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(skb->truesize) > skb->len ||
  		     before(TCP_SKB_CB(skb)->seq, start))) {
  			end_of_skbs = false;
  			break;
  		}
  
  		if (!skb_queue_is_last(list, skb)) {
  			struct sk_buff *next = skb_queue_next(list, skb);
  			if (next != tail &&
  			    TCP_SKB_CB(skb)->end_seq != TCP_SKB_CB(next)->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;
  
  	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)
  			return;
  
  		memcpy(nskb->cb, skb->cb, sizeof(skb->cb));
  		TCP_SKB_CB(nskb)->seq = TCP_SKB_CB(nskb)->end_seq = start;
  		__skb_queue_before(list, skb, nskb);
  		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);
  				if (!skb ||
  				    skb == tail ||
  				    (TCP_SKB_CB(skb)->tcp_flags & (TCPHDR_SYN | TCPHDR_FIN)))
  					return;
  			}
  		}
  	}
  }
  
  /* 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 = skb_peek(&tp->out_of_order_queue);
  	struct sk_buff *head;
  	u32 start, end;
  
  	if (skb == NULL)
  		return;
  
  	start = TCP_SKB_CB(skb)->seq;
  	end = TCP_SKB_CB(skb)->end_seq;
  	head = skb;
  
  	for (;;) {
  		struct sk_buff *next = NULL;
  
  		if (!skb_queue_is_last(&tp->out_of_order_queue, skb))
  			next = skb_queue_next(&tp->out_of_order_queue, skb);
  		skb = next;
  
  		/* Segment is terminated when we see gap or when
  		 * we are at the end of all the queue. */
  		if (!skb ||
  		    after(TCP_SKB_CB(skb)->seq, end) ||
  		    before(TCP_SKB_CB(skb)->end_seq, start)) {
  			tcp_collapse(sk, &tp->out_of_order_queue,
  				     head, skb, start, end);
  			head = skb;
  			if (!skb)
  				break;
  			/* Start new segment */
  			start = TCP_SKB_CB(skb)->seq;
  			end = TCP_SKB_CB(skb)->end_seq;
  		} else {
  			if (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;
  		}
  	}
  }
  
  /*
   * Purge the out-of-order queue.
   * Return true if queue was pruned.
   */
  static bool tcp_prune_ofo_queue(struct sock *sk)
  {
  	struct tcp_sock *tp = tcp_sk(sk);
  	bool res = false;
  
  	if (!skb_queue_empty(&tp->out_of_order_queue)) {
  		NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_OFOPRUNED);
  		__skb_queue_purge(&tp->out_of_order_queue);
  
  		/* 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);
  		sk_mem_reclaim(sk);
  		res = true;
  	}
  	return res;
  }
  
  /* 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_BH(sock_net(sk), LINUX_MIB_PRUNECALLED);
  
  	if (atomic_read(&sk->sk_rmem_alloc) >= sk->sk_rcvbuf)
  		tcp_clamp_window(sk);
  	else if (sk_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,
  			     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_BH(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 (sk_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 (tp->packets_out >= 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_time_stamp;
  	}
  
  	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);
  		if (sk->sk_socket &&
  		    test_bit(SOCK_NOSPACE, &sk->sk_socket->flags))
  			tcp_new_space(sk);
  	}
  }
  
  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);
  
  	    /* 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). Or...
  	      */
  	     __tcp_select_window(sk) >= tp->rcv_wnd) ||
  	    /* We ACK each frame or... */
  	    tcp_in_quickack_mode(sk) ||
  	    /* We have out of order data. */
  	    (ofo_possible && skb_peek(&tp->out_of_order_queue))) {
  		/* Then ack it now */
  		tcp_send_ack(sk);
  	} else {
  		/* Else, send delayed ack. */
  		tcp_send_delayed_ack(sk);
  	}
  }
  
  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 && !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);
  		}
  	}
  }
  
  static int tcp_copy_to_iovec(struct sock *sk, struct sk_buff *skb, int hlen)
  {
  	struct tcp_sock *tp = tcp_sk(sk);
  	int chunk = skb->len - hlen;
  	int err;
  
  	local_bh_enable();
  	if (skb_csum_unnecessary(skb))
  		err = skb_copy_datagram_iovec(skb, hlen, tp->ucopy.iov, chunk);
  	else
  		err = skb_copy_and_csum_datagram_iovec(skb, hlen,
  						       tp->ucopy.iov);
  
  	if (!err) {
  		tp->ucopy.len -= chunk;
  		tp->copied_seq += chunk;
  		tcp_rcv_space_adjust(sk);
  	}
  
  	local_bh_disable();
  	return err;
  }
  
  static __sum16 __tcp_checksum_complete_user(struct sock *sk,
  					    struct sk_buff *skb)
  {
  	__sum16 result;
  
  	if (sock_owned_by_user(sk)) {
  		local_bh_enable();
  		result = __tcp_checksum_complete(skb);
  		local_bh_disable();
  	} else {
  		result = __tcp_checksum_complete(skb);
  	}
  	return result;
  }
  
  static inline bool tcp_checksum_complete_user(struct sock *sk,
  					     struct sk_buff *skb)
  {
  	return !skb_csum_unnecessary(skb) &&
  	       __tcp_checksum_complete_user(sk, skb);
  }
  
  /* 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);
  
  	/* RFC1323: H1. Apply PAWS check first. */
  	if (tcp_fast_parse_options(skb, th, tp) && tp->rx_opt.saw_tstamp &&
  	    tcp_paws_discard(sk, skb)) {
  		if (!th->rst) {
  			NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_PAWSESTABREJECTED);
  			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;
  			tcp_send_dupack(sk, skb);
  		}
  		goto discard;
  	}
  
  	/* Step 2: check RST bit */
  	if (th->rst) {
  		/* RFC 5961 3.2 :
  		 * If sequence number exactly matches RCV.NXT, then
  		 *     RESET the connection
  		 * else
  		 *     Send a challenge ACK
  		 */
  		if (TCP_SKB_CB(skb)->seq == tp->rcv_nxt)
  			tcp_reset(sk);
  		else
  			tcp_send_challenge_ack(sk);
  		goto discard;
  	}
  
  	/* step 3: check security and precedence [ignored] */
  
  	/* step 4: Check for a SYN
  	 * RFC 5691 4.2 : Send a challenge ack
  	 */
  	if (th->syn) {
  syn_challenge:
  		if (syn_inerr)
  			TCP_INC_STATS_BH(sock_net(sk), TCP_MIB_INERRS);
  		NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_TCPSYNCHALLENGE);
  		tcp_send_challenge_ack(sk);
  		goto discard;
  	}
  
  	return true;
  
  discard:
  	__kfree_skb(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, unsigned int len)
  {
  	struct tcp_sock *tp = tcp_sk(sk);
  
  	if (unlikely(sk->sk_rx_dst == NULL))
  		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_BH(sock_net(sk), TCP_MIB_INERRS);
  				goto discard;
  			}
  		} else {
  			int eaten = 0;
  			bool fragstolen = false;
  
  			if (tp->ucopy.task == current &&
  			    tp->copied_seq == tp->rcv_nxt &&
  			    len - tcp_header_len <= tp->ucopy.len &&
  			    sock_owned_by_user(sk)) {
  				__set_current_state(TASK_RUNNING);
  
  				if (!tcp_copy_to_iovec(sk, skb, 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);
  
  					tcp_rcv_rtt_measure_ts(sk, skb);
  
  					__skb_pull(skb, tcp_header_len);
  					tp->rcv_nxt = TCP_SKB_CB(skb)->end_seq;
  					NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_TCPHPHITSTOUSER);
  					eaten = 1;
  				}
  			}
  			if (!eaten) {
  				if (tcp_checksum_complete_user(sk, 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_BH(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);
  			sk->sk_data_ready(sk);
  			return;
  		}
  	}
  
  slow_path:
  	if (len < (th->doff << 2) || tcp_checksum_complete_user(sk, 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_BH(sock_net(sk), TCP_MIB_CSUMERRORS);
  	TCP_INC_STATS_BH(sock_net(sk), TCP_MIB_INERRS);
  
  discard:
  	__kfree_skb(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);
  
  	if (skb != NULL) {
  		icsk->icsk_af_ops->sk_rx_dst_set(sk, skb);
  		security_inet_conn_established(sk, skb);
  	}
  
  	/* Make sure socket is routed, for correct metrics.  */
  	icsk->icsk_af_ops->rebuild_header(sk);
  
  	tcp_init_metrics(sk);
  
  	tcp_init_congestion_control(sk);
  
  	/* Prevent spurious tcp_cwnd_restart() on first data
  	 * packet.
  	 */
  	tp->lsndtime = tcp_time_stamp;
  
  	tcp_init_buffer_space(sk);
  
  	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;
  
  	if (!sock_flag(sk, SOCK_DEAD)) {
  		sk->sk_state_change(sk);
  		sk_wake_async(sk, SOCK_WAKE_IO, POLL_OUT);
  	}
  }
  
  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_write_queue_head(sk) : NULL;
  	u16 mss = tp->rx_opt.mss_clamp;
  	bool syn_drop;
  
  	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(synack, &opt, 0, NULL);
  		mss = opt.mss_clamp;
  	}
  
  	if (!tp->syn_fastopen)  /* Ignore an unsolicited cookie */
  		cookie->len = -1;
  
  	/* The SYN-ACK neither has cookie nor acknowledges the data. Presumably
  	 * the remote receives only the retransmitted (regular) SYNs: either
  	 * the original SYN-data or the corresponding SYN-ACK is lost.
  	 */
  	syn_drop = (cookie->len <= 0 && data && tp->total_retrans);
  
  	tcp_fastopen_cache_set(sk, mss, cookie, syn_drop);
  
  	if (data) { /* Retransmit unacked data in SYN */
  		tcp_for_write_queue_from(data, sk) {
  			if (data == tcp_send_head(sk) ||
  			    __tcp_retransmit_skb(sk, data))
  				break;
  		}
  		tcp_rearm_rto(sk);
  		NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_TCPFASTOPENACTIVEFAIL);
  		return true;
  	}
  	tp->syn_data_acked = tp->syn_data;
  	if (tp->syn_data_acked)
  		NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_TCPFASTOPENACTIVE);
  	return false;
  }
  
  static int tcp_rcv_synsent_state_process(struct sock *sk, struct sk_buff *skb,
  					 const struct tcphdr *th, unsigned int len)
  {
  	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;
  
  	tcp_parse_options(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)) {
  			NET_INC_STATS_BH(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);
  		}
  
  		if (tcp_is_sack(tp) && sysctl_tcp_fack)
  			tcp_enable_fack(tp);
  
  		tcp_mtup_init(sk);
  		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;
  
  		smp_mb();
  
  		tcp_finish_connect(sk, skb);
  
  		if ((tp->syn_fastopen || tp->syn_data) &&
  		    tcp_rcv_fastopen_synack(sk, skb, &foc))
  			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);
  			icsk->icsk_ack.lrcvtime = tcp_time_stamp;
  			tcp_enter_quickack_mode(sk);
  			inet_csk_reset_xmit_timer(sk, ICSK_TIME_DACK,
  						  TCP_DELACK_MAX, sysctl_tcp_rto_max);
  
  discard:
  			__kfree_skb(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->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,
  			  const struct tcphdr *th, unsigned int len)
  {
  	struct tcp_sock *tp = tcp_sk(sk);
  	struct inet_connection_sock *icsk = inet_csk(sk);
  	struct request_sock *req;
  	int queued = 0;
  	bool acceptable;
  	u32 synack_stamp;
  
  	tp->rx_opt.saw_tstamp = 0;
  
  	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;
  			if (icsk->icsk_af_ops->conn_request(sk, skb) < 0)
  				return 1;
  
  			/* Now we have several options: In theory there is
  			 * nothing else in the frame. KA9Q has an option to
  			 * send data with the syn, BSD accepts data with the
  			 * syn up to the [to be] advertised window and
  			 * Solaris 2.1 gives you a protocol error. For now
  			 * we just ignore it, that fits the spec precisely
  			 * and avoids incompatibilities. It would be nice in
  			 * future to drop through and process the data.
  			 *
  			 * Now that TTCP is starting to be used we ought to
  			 * queue this data.
  			 * But, this leaves one open to an easy denial of
  			 * service attack, and SYN cookies can't defend
  			 * against this problem. So, we drop the data
  			 * in the interest of security over speed unless
  			 * it's still in use.
  			 */
  			kfree_skb(skb);
  			return 0;
  		}
  		goto discard;
  
  	case TCP_SYN_SENT:
  		queued = tcp_rcv_synsent_state_process(sk, skb, th, len);
  		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;
  	}
  
  	req = tp->fastopen_rsk;
  	if (req != NULL) {
  		WARN_ON_ONCE(sk->sk_state != TCP_SYN_RECV &&
  		    sk->sk_state != TCP_FIN_WAIT1);
  
  		if (tcp_check_req(sk, skb, req, NULL, true) == NULL)
  			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) > 0;
  
  	switch (sk->sk_state) {
  	case TCP_SYN_RECV:
  		if (!acceptable)
  			return 1;
  
  		/* Once we leave TCP_SYN_RECV, we no longer need req
  		 * so release it.
  		 */
  		if (req) {
  			synack_stamp = tcp_rsk(req)->snt_synack;
  			tp->total_retrans = req->num_retrans;
  			reqsk_fastopen_remove(sk, req, false);
  		} else {
  			synack_stamp = tp->lsndtime;
  			/* Make sure socket is routed, for correct metrics. */
  			icsk->icsk_af_ops->rebuild_header(sk);
  			tcp_init_congestion_control(sk);
  
  			tcp_mtup_init(sk);
  			tp->copied_seq = tp->rcv_nxt;
  			tcp_init_buffer_space(sk);
  		}
  		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);
  		tcp_synack_rtt_meas(sk, synack_stamp);
  
  		if (tp->rx_opt.tstamp_ok)
  			tp->advmss -= TCPOLEN_TSTAMP_ALIGNED;
  
  		if (req) {
  			/* 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_metrics(sk);
  
  		tcp_update_pacing_rate(sk);
  
  		/* Prevent spurious tcp_cwnd_restart() on first data packet */
  		tp->lsndtime = tcp_time_stamp;
  
  		tcp_initialize_rcv_mss(sk);
  		tcp_fast_path_on(tp);
  		break;
  
  	case TCP_FIN_WAIT1: {
  		struct dst_entry *dst;
  		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 != NULL) {
  			/* Return RST if ack_seq is invalid.
  			 * Note that RFC793 only says to generate a
  			 * DUPACK for it but for TCP Fast Open it seems
  			 * better to treat this case like TCP_SYN_RECV
  			 * above.
  			 */
  			if (!acceptable)
  				return 1;
  			/* 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;
  
  		dst = __sk_dst_get(sk);
  		if (dst)
  			dst_confirm(dst);
  
  		if (!sock_flag(sk, SOCK_DEAD)) {
  			/* Wake up lingering close() */
  			sk->sk_state_change(sk);
  			break;
  		}
  
  		if (tp->linger2 < 0 ||
  		    (TCP_SKB_CB(skb)->end_seq != TCP_SKB_CB(skb)->seq &&
  		     after(TCP_SKB_CB(skb)->end_seq - th->fin, tp->rcv_nxt))) {
  			tcp_done(sk);
  			NET_INC_STATS_BH(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;
  	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_BH(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:
  		__kfree_skb(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)
  		LIMIT_NETDEBUG(KERN_DEBUG pr_fmt("drop open request from %pI4/%u\n"),
  			       &ireq->ir_rmt_addr, port);
  #if IS_ENABLED(CONFIG_IPV6)
  	else if (family == AF_INET6)
  		LIMIT_NETDEBUG(KERN_DEBUG pr_fmt("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 negociation.
   *
   * Exception: tcp_ca wants ECN. This is required for DCTCP
   * congestion control; it requires setting ECT on all packets,
   * including SYN. We inverse the test in this case: If our
   * local socket wants ECN, but peer only set ece/cwr (but not
   * ECT in IP header) its probably a non-DCTCP aware sender.
   */
  static void tcp_ecn_create_request(struct request_sock *req,
  				   const struct sk_buff *skb,
  				   const struct sock *listen_sk)
  {
  	const struct tcphdr *th = tcp_hdr(skb);
  	const struct net *net = sock_net(listen_sk);
  	bool th_ecn = th->ece && th->cwr;
  	bool ect, need_ecn;
  
  	if (!th_ecn)
  		return;
  
  	ect = !INET_ECN_is_not_ect(TCP_SKB_CB(skb)->ip_dsfield);
  	need_ecn = tcp_ca_needs_ecn(listen_sk);
  
  	if (!ect && !need_ecn && net->ipv4.sysctl_tcp_ecn)
  		inet_rsk(req)->ecn_ok = 1;
  	else if (ect && need_ecn)
  		inet_rsk(req)->ecn_ok = 1;
  }
  
  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_options_received tmp_opt;
  	struct request_sock *req;
  	struct tcp_sock *tp = tcp_sk(sk);
  	struct dst_entry *dst = NULL;
  	__u32 isn = TCP_SKB_CB(skb)->tcp_tw_isn;
  	bool want_cookie = false, fastopen;
  	struct flowi fl;
  	struct tcp_fastopen_cookie foc = { .len = -1 };
  	int err;
  
  
  	/* TW buckets are converted to open requests without
  	 * limitations, they conserve resources and peer is
  	 * evidently real one.
  	 */
  	if ((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;
  	}
  
  
  	/* Accept backlog is full. If we have already queued enough
  	 * of warm entries in syn queue, drop request. It is better than
  	 * clogging syn queue with openreqs with exponentially increasing
  	 * timeout.
  	 */
  	if (sk_acceptq_is_full(sk) && inet_csk_reqsk_queue_young(sk) > 1) {
  		NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_LISTENOVERFLOWS);
  		goto drop;
  	}
  
  	req = inet_reqsk_alloc(rsk_ops);
  	if (!req)
  		goto drop;
  
  	tcp_rsk(req)->af_specific = af_ops;
  
  	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(skb, &tmp_opt, 0, want_cookie ? NULL : &foc);
  
  	if (want_cookie && !tmp_opt.saw_tstamp)
  		tcp_clear_options(&tmp_opt);
  
  	tmp_opt.tstamp_ok = tmp_opt.saw_tstamp;
  	tcp_openreq_init(req, &tmp_opt, skb, sk);
  
  	af_ops->init_req(req, sk, skb);
  
  	if (security_inet_conn_request(sk, skb, req))
  		goto drop_and_free;
  
  	if (!want_cookie || tmp_opt.tstamp_ok)
  		tcp_ecn_create_request(req, skb, sk);
  
  	if (want_cookie) {
  		isn = cookie_init_sequence(af_ops, sk, skb, &req->mss);
  		req->cookie_ts = tmp_opt.tstamp_ok;
  	} else if (!isn) {
  		/* VJ's idea. We save last timestamp seen
  		 * from the destination in peer table, when entering
  		 * state TIME-WAIT, and check against it before
  		 * accepting new connection request.
  		 *
  		 * If "isn" is not zero, this request hit alive
  		 * timewait bucket, so that all the necessary checks
  		 * are made in the function processing timewait state.
  		 */
  		if (tcp_death_row.sysctl_tw_recycle) {
  			bool strict;
  
  			dst = af_ops->route_req(sk, &fl, req, &strict);
  
  			if (dst && strict &&
  			    !tcp_peer_is_proven(req, dst, true,
  						tmp_opt.saw_tstamp)) {
  				NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_PAWSPASSIVEREJECTED);
  				goto drop_and_release;
  			}
  		}
  		/* Kill the following clause, if you dislike this way. */
  		else if (!sysctl_tcp_syncookies &&
  			 (sysctl_max_syn_backlog - inet_csk_reqsk_queue_len(sk) <
  			  (sysctl_max_syn_backlog >> 2)) &&
  			 !tcp_peer_is_proven(req, dst, false,
  					     tmp_opt.saw_tstamp)) {
  			/* 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);
  	}
  	if (!dst) {
  		dst = af_ops->route_req(sk, &fl, req, NULL);
  		if (!dst)
  			goto drop_and_free;
  	}
  
  	tcp_rsk(req)->snt_isn = isn;
  	tcp_openreq_init_rwin(req, sk, dst);
  	fastopen = !want_cookie &&
  		   tcp_try_fastopen(sk, skb, req, &foc, dst);
  	err = af_ops->send_synack(sk, dst, &fl, req,
  				  skb_get_queue_mapping(skb), &foc);
  	if (!fastopen) {
  		if (err || want_cookie)
  			goto drop_and_free;
  
  		tcp_rsk(req)->listener = NULL;
  		af_ops->queue_hash_add(sk, req, TCP_TIMEOUT_INIT);
  	}
  
  	return 0;
  
  drop_and_release:
  	dst_release(dst);
  drop_and_free:
  	reqsk_free(req);
  drop:
  	NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_LISTENDROPS);
  	return 0;
  }
  EXPORT_SYMBOL(tcp_conn_request);
  