text: // SPDX-License-Identifier: GPL-2.0
  //
  // Taken from https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/tools/testing/selftests/bpf/progs/test_tc_tunnel.c?h=v5.12
  
  /* In-place tunneling */
  
  #include <stdbool.h>
  #include <string.h>
  
  #include <linux/stddef.h>
  #include <linux/bpf.h>
  #include <linux/if_ether.h>
  #include <linux/in.h>
  #include <linux/ip.h>
  #include <linux/ipv6.h>
  #include <linux/mpls.h>
  #include <linux/tcp.h>
  #include <linux/udp.h>
  #include <linux/pkt_cls.h>
  #include <linux/types.h>
  
  #include <bpf/bpf_endian.h>
  #include <bpf/bpf_helpers.h>
  
  #define ERROR(ret) do {\
  		char fmt[] = "ERROR line:%d ret:%d\n";\
  		bpf_trace_printk(fmt, sizeof(fmt), __LINE__, ret); \
  	} while (0)
  
  static const int cfg_port = 8000;
  
  static const int cfg_udp_src = 20000;
  
  #define	UDP_PORT		5555
  #define	MPLS_OVER_UDP_PORT	6635
  #define	ETH_OVER_UDP_PORT	7777
  
  /* MPLS label 1000 with S bit (last label) set and ttl of 255. */
  static const __u32 mpls_label = __bpf_constant_htonl(1000 << 12 |
  						     MPLS_LS_S_MASK | 0xff);
  
  struct gre_hdr {
  	__be16 flags;
  	__be16 protocol;
  } __attribute__((packed));
  
  union l4hdr {
  	struct udphdr udp;
  	struct gre_hdr gre;
  };
  
  struct v4hdr {
  	struct iphdr ip;
  	union l4hdr l4hdr;
  	__u8 pad[16];			/* enough space for L2 header */
  } __attribute__((packed));
  
  struct v6hdr {
  	struct ipv6hdr ip;
  	union l4hdr l4hdr;
  	__u8 pad[16];			/* enough space for L2 header */
  } __attribute__((packed));
  
  static __always_inline void set_ipv4_csum(struct iphdr *iph)
  {
  	__u16 *iph16 = (__u16 *)iph;
  	__u32 csum;
  	int i;
  
  	iph->check = 0;
  
  #pragma clang loop unroll(full)
  	for (i = 0, csum = 0; i < sizeof(*iph) >> 1; i++)
  		csum += *iph16++;
  
  	iph->check = ~((csum & 0xffff) + (csum >> 16));
  }
  
  static __always_inline int encap_ipv4(struct __sk_buff *skb, __u8 encap_proto,
  				      __u16 l2_proto)
  {
  	__u16 udp_dst = UDP_PORT;
  	struct iphdr iph_inner;
  	struct v4hdr h_outer;
  	struct tcphdr tcph;
  	int olen, l2_len;
  	int tcp_off;
  	__u64 flags;
  
  	/* Most tests encapsulate a packet into a tunnel with the same
  	 * network protocol, and derive the outer header fields from
  	 * the inner header.
  	 *
  	 * The 6in4 case tests different inner and outer protocols. As
  	 * the inner is ipv6, but the outer expects an ipv4 header as
  	 * input, manually build a struct iphdr based on the ipv6hdr.
  	 */
  	if (encap_proto == IPPROTO_IPV6) {
  		const __u32 saddr = (192 << 24) | (168 << 16) | (1 << 8) | 1;
  		const __u32 daddr = (192 << 24) | (168 << 16) | (1 << 8) | 2;
  		struct ipv6hdr iph6_inner;
  
  		/* Read the IPv6 header */
  		if (bpf_skb_load_bytes(skb, ETH_HLEN, &iph6_inner,
  				       sizeof(iph6_inner)) < 0)
  			return TC_ACT_OK;
  
  		/* Derive the IPv4 header fields from the IPv6 header */
  		memset(&iph_inner, 0, sizeof(iph_inner));
  		iph_inner.version = 4;
  		iph_inner.ihl = 5;
  		iph_inner.tot_len = bpf_htons(sizeof(iph6_inner) +
  				    bpf_ntohs(iph6_inner.payload_len));
  		iph_inner.ttl = iph6_inner.hop_limit - 1;
  		iph_inner.protocol = iph6_inner.nexthdr;
  		iph_inner.saddr = __bpf_constant_htonl(saddr);
  		iph_inner.daddr = __bpf_constant_htonl(daddr);
  
  		tcp_off = sizeof(iph6_inner);
  	} else {
  		if (bpf_skb_load_bytes(skb, ETH_HLEN, &iph_inner,
  				       sizeof(iph_inner)) < 0)
  			return TC_ACT_OK;
  
  		tcp_off = sizeof(iph_inner);
  	}
  
  	/* filter only packets we want */
  	if (iph_inner.ihl != 5 || iph_inner.protocol != IPPROTO_TCP)
  		return TC_ACT_OK;
  
  	if (bpf_skb_load_bytes(skb, ETH_HLEN + tcp_off,
  			       &tcph, sizeof(tcph)) < 0)
  		return TC_ACT_OK;
  
  	if (tcph.dest != __bpf_constant_htons(cfg_port))
  		return TC_ACT_OK;
  
  	olen = sizeof(h_outer.ip);
  	l2_len = 0;
  
  	flags = BPF_F_ADJ_ROOM_FIXED_GSO | BPF_F_ADJ_ROOM_ENCAP_L3_IPV4;
  
  	switch (l2_proto) {
  	case ETH_P_MPLS_UC:
  		l2_len = sizeof(mpls_label);
  		udp_dst = MPLS_OVER_UDP_PORT;
  		break;
  	case ETH_P_TEB:
  		l2_len = ETH_HLEN;
  		udp_dst = ETH_OVER_UDP_PORT;
  		break;
  	}
  	flags |= BPF_F_ADJ_ROOM_ENCAP_L2(l2_len);
  
  	switch (encap_proto) {
  	case IPPROTO_GRE:
  		flags |= BPF_F_ADJ_ROOM_ENCAP_L4_GRE;
  		olen += sizeof(h_outer.l4hdr.gre);
  		h_outer.l4hdr.gre.protocol = bpf_htons(l2_proto);
  		h_outer.l4hdr.gre.flags = 0;
  		break;
  	case IPPROTO_UDP:
  		flags |= BPF_F_ADJ_ROOM_ENCAP_L4_UDP;
  		olen += sizeof(h_outer.l4hdr.udp);
  		h_outer.l4hdr.udp.source = __bpf_constant_htons(cfg_udp_src);
  		h_outer.l4hdr.udp.dest = bpf_htons(udp_dst);
  		h_outer.l4hdr.udp.check = 0;
  		h_outer.l4hdr.udp.len = bpf_htons(bpf_ntohs(iph_inner.tot_len) +
  						  sizeof(h_outer.l4hdr.udp) +
  						  l2_len);
  		break;
  	case IPPROTO_IPIP:
  	case IPPROTO_IPV6:
  		break;
  	default:
  		return TC_ACT_OK;
  	}
  
  	/* add L2 encap (if specified) */
  	switch (l2_proto) {
  	case ETH_P_MPLS_UC:
  		*((__u32 *)((__u8 *)&h_outer + olen)) = mpls_label;
  		break;
  	case ETH_P_TEB:
  		if (bpf_skb_load_bytes(skb, 0, (__u8 *)&h_outer + olen,
  				       ETH_HLEN))
  			return TC_ACT_SHOT;
  		break;
  	}
  	olen += l2_len;
  
  	/* add room between mac and network header */
  	if (bpf_skb_adjust_room(skb, olen, BPF_ADJ_ROOM_MAC, flags))
  		return TC_ACT_SHOT;
  
  	/* prepare new outer network header */
  	h_outer.ip = iph_inner;
  	h_outer.ip.tot_len = bpf_htons(olen +
  				       bpf_ntohs(h_outer.ip.tot_len));
  	h_outer.ip.protocol = encap_proto;
  
  	set_ipv4_csum((void *)&h_outer.ip);
  
  	/* store new outer network header */
  	if (bpf_skb_store_bytes(skb, ETH_HLEN, &h_outer, olen,
  				BPF_F_INVALIDATE_HASH) < 0)
  		return TC_ACT_SHOT;
  
  	/* if changing outer proto type, update eth->h_proto */
  	if (encap_proto == IPPROTO_IPV6) {
  		struct ethhdr eth;
  
  		if (bpf_skb_load_bytes(skb, 0, &eth, sizeof(eth)) < 0)
  			return TC_ACT_SHOT;
  		eth.h_proto = bpf_htons(ETH_P_IP);
  		if (bpf_skb_store_bytes(skb, 0, &eth, sizeof(eth), 0) < 0)
  			return TC_ACT_SHOT;
  	}
  
  	return TC_ACT_OK;
  }
  
  static __always_inline int encap_ipv6(struct __sk_buff *skb, __u8 encap_proto,
  				      __u16 l2_proto)
  {
  	__u16 udp_dst = UDP_PORT;
  	struct ipv6hdr iph_inner;
  	struct v6hdr h_outer;
  	struct tcphdr tcph;
  	int olen, l2_len;
  	__u16 tot_len;
  	__u64 flags;
  
  	if (bpf_skb_load_bytes(skb, ETH_HLEN, &iph_inner,
  			       sizeof(iph_inner)) < 0)
  		return TC_ACT_OK;
  
  	/* filter only packets we want */
  	if (bpf_skb_load_bytes(skb, ETH_HLEN + sizeof(iph_inner),
  			       &tcph, sizeof(tcph)) < 0)
  		return TC_ACT_OK;
  
  	if (tcph.dest != __bpf_constant_htons(cfg_port))
  		return TC_ACT_OK;
  
  	olen = sizeof(h_outer.ip);
  	l2_len = 0;
  
  	flags = BPF_F_ADJ_ROOM_FIXED_GSO | BPF_F_ADJ_ROOM_ENCAP_L3_IPV6;
  
  	switch (l2_proto) {
  	case ETH_P_MPLS_UC:
  		l2_len = sizeof(mpls_label);
  		udp_dst = MPLS_OVER_UDP_PORT;
  		break;
  	case ETH_P_TEB:
  		l2_len = ETH_HLEN;
  		udp_dst = ETH_OVER_UDP_PORT;
  		break;
  	}
  	flags |= BPF_F_ADJ_ROOM_ENCAP_L2(l2_len);
  
  	switch (encap_proto) {
  	case IPPROTO_GRE:
  		flags |= BPF_F_ADJ_ROOM_ENCAP_L4_GRE;
  		olen += sizeof(h_outer.l4hdr.gre);
  		h_outer.l4hdr.gre.protocol = bpf_htons(l2_proto);
  		h_outer.l4hdr.gre.flags = 0;
  		break;
  	case IPPROTO_UDP:
  		flags |= BPF_F_ADJ_ROOM_ENCAP_L4_UDP;
  		olen += sizeof(h_outer.l4hdr.udp);
  		h_outer.l4hdr.udp.source = __bpf_constant_htons(cfg_udp_src);
  		h_outer.l4hdr.udp.dest = bpf_htons(udp_dst);
  		tot_len = bpf_ntohs(iph_inner.payload_len) + sizeof(iph_inner) +
  			  sizeof(h_outer.l4hdr.udp);
  		h_outer.l4hdr.udp.check = 0;
  		h_outer.l4hdr.udp.len = bpf_htons(tot_len);
  		break;
  	case IPPROTO_IPV6:
  		break;
  	default:
  		return TC_ACT_OK;
  	}
  
  	/* add L2 encap (if specified) */
  	switch (l2_proto) {
  	case ETH_P_MPLS_UC:
  		*((__u32 *)((__u8 *)&h_outer + olen)) = mpls_label;
  		break;
  	case ETH_P_TEB:
  		if (bpf_skb_load_bytes(skb, 0, (__u8 *)&h_outer + olen,
  				       ETH_HLEN))
  			return TC_ACT_SHOT;
  		break;
  	}
  	olen += l2_len;
  
  	/* add room between mac and network header */
  	if (bpf_skb_adjust_room(skb, olen, BPF_ADJ_ROOM_MAC, flags))
  		return TC_ACT_SHOT;
  
  	/* prepare new outer network header */
  	h_outer.ip = iph_inner;
  	h_outer.ip.payload_len = bpf_htons(olen +
  					   bpf_ntohs(h_outer.ip.payload_len));
  
  	h_outer.ip.nexthdr = encap_proto;
  
  	/* store new outer network header */
  	if (bpf_skb_store_bytes(skb, ETH_HLEN, &h_outer, olen,
  				BPF_F_INVALIDATE_HASH) < 0)
  		return TC_ACT_SHOT;
  
  	return TC_ACT_OK;
  }
  
  SEC("encap_ipip_none")
  int __encap_ipip_none(struct __sk_buff *skb)
  {
  	if (skb->protocol == __bpf_constant_htons(ETH_P_IP))
  		return encap_ipv4(skb, IPPROTO_IPIP, ETH_P_IP);
  	else
  		return TC_ACT_OK;
  }
  
  SEC("encap_gre_none")
  int __encap_gre_none(struct __sk_buff *skb)
  {
  	if (skb->protocol == __bpf_constant_htons(ETH_P_IP))
  		return encap_ipv4(skb, IPPROTO_GRE, ETH_P_IP);
  	else
  		return TC_ACT_OK;
  }
  
  SEC("encap_gre_mpls")
  int __encap_gre_mpls(struct __sk_buff *skb)
  {
  	if (skb->protocol == __bpf_constant_htons(ETH_P_IP))
  		return encap_ipv4(skb, IPPROTO_GRE, ETH_P_MPLS_UC);
  	else
  		return TC_ACT_OK;
  }
  
  SEC("encap_gre_eth")
  int __encap_gre_eth(struct __sk_buff *skb)
  {
  	if (skb->protocol == __bpf_constant_htons(ETH_P_IP))
  		return encap_ipv4(skb, IPPROTO_GRE, ETH_P_TEB);
  	else
  		return TC_ACT_OK;
  }
  
  SEC("encap_udp_none")
  int __encap_udp_none(struct __sk_buff *skb)
  {
  	if (skb->protocol == __bpf_constant_htons(ETH_P_IP))
  		return encap_ipv4(skb, IPPROTO_UDP, ETH_P_IP);
  	else
  		return TC_ACT_OK;
  }
  
  SEC("encap_udp_mpls")
  int __encap_udp_mpls(struct __sk_buff *skb)
  {
  	if (skb->protocol == __bpf_constant_htons(ETH_P_IP))
  		return encap_ipv4(skb, IPPROTO_UDP, ETH_P_MPLS_UC);
  	else
  		return TC_ACT_OK;
  }
  
  SEC("encap_udp_eth")
  int __encap_udp_eth(struct __sk_buff *skb)
  {
  	if (skb->protocol == __bpf_constant_htons(ETH_P_IP))
  		return encap_ipv4(skb, IPPROTO_UDP, ETH_P_TEB);
  	else
  		return TC_ACT_OK;
  }
  
  SEC("encap_sit_none")
  int __encap_sit_none(struct __sk_buff *skb)
  {
  	if (skb->protocol == __bpf_constant_htons(ETH_P_IPV6))
  		return encap_ipv4(skb, IPPROTO_IPV6, ETH_P_IP);
  	else
  		return TC_ACT_OK;
  }
  
  SEC("encap_ip6tnl_none")
  int __encap_ip6tnl_none(struct __sk_buff *skb)
  {
  	if (skb->protocol == __bpf_constant_htons(ETH_P_IPV6))
  		return encap_ipv6(skb, IPPROTO_IPV6, ETH_P_IPV6);
  	else
  		return TC_ACT_OK;
  }
  
  SEC("encap_ip6gre_none")
  int __encap_ip6gre_none(struct __sk_buff *skb)
  {
  	if (skb->protocol == __bpf_constant_htons(ETH_P_IPV6))
  		return encap_ipv6(skb, IPPROTO_GRE, ETH_P_IPV6);
  	else
  		return TC_ACT_OK;
  }
  
  SEC("encap_ip6gre_mpls")
  int __encap_ip6gre_mpls(struct __sk_buff *skb)
  {
  	if (skb->protocol == __bpf_constant_htons(ETH_P_IPV6))
  		return encap_ipv6(skb, IPPROTO_GRE, ETH_P_MPLS_UC);
  	else
  		return TC_ACT_OK;
  }
  
  SEC("encap_ip6gre_eth")
  int __encap_ip6gre_eth(struct __sk_buff *skb)
  {
  	if (skb->protocol == __bpf_constant_htons(ETH_P_IPV6))
  		return encap_ipv6(skb, IPPROTO_GRE, ETH_P_TEB);
  	else
  		return TC_ACT_OK;
  }
  
  SEC("encap_ip6udp_none")
  int __encap_ip6udp_none(struct __sk_buff *skb)
  {
  	if (skb->protocol == __bpf_constant_htons(ETH_P_IPV6))
  		return encap_ipv6(skb, IPPROTO_UDP, ETH_P_IPV6);
  	else
  		return TC_ACT_OK;
  }
  
  SEC("encap_ip6udp_mpls")
  int __encap_ip6udp_mpls(struct __sk_buff *skb)
  {
  	if (skb->protocol == __bpf_constant_htons(ETH_P_IPV6))
  		return encap_ipv6(skb, IPPROTO_UDP, ETH_P_MPLS_UC);
  	else
  		return TC_ACT_OK;
  }
  
  SEC("encap_ip6udp_eth")
  int __encap_ip6udp_eth(struct __sk_buff *skb)
  {
  	if (skb->protocol == __bpf_constant_htons(ETH_P_IPV6))
  		return encap_ipv6(skb, IPPROTO_UDP, ETH_P_TEB);
  	else
  		return TC_ACT_OK;
  }
  
  static int decap_internal(struct __sk_buff *skb, int off, int len, char proto)
  {
  	struct gre_hdr greh;
  	struct udphdr udph;
  	int olen = len;
  
  	switch (proto) {
  	case IPPROTO_IPIP:
  	case IPPROTO_IPV6:
          //return TC_ACT_SHOT;
  		break;
  	case IPPROTO_GRE:
  		olen += sizeof(struct gre_hdr);
  		if (bpf_skb_load_bytes(skb, off + len, &greh, sizeof(greh)) < 0)
  			return TC_ACT_OK;
  		switch (bpf_ntohs(greh.protocol)) {
  		case ETH_P_MPLS_UC:
  			olen += sizeof(mpls_label);
  			break;
  		case ETH_P_TEB:
  			olen += ETH_HLEN;
  			break;
  		}
  		break;
  	case IPPROTO_UDP:
  		olen += sizeof(struct udphdr);
  		if (bpf_skb_load_bytes(skb, off + len, &udph, sizeof(udph)) < 0)
  			return TC_ACT_OK;
  		switch (bpf_ntohs(udph.dest)) {
  		case MPLS_OVER_UDP_PORT:
  			olen += sizeof(mpls_label);
  			break;
  		case ETH_OVER_UDP_PORT:
  			olen += ETH_HLEN;
  			break;
  		}
  		break;
  	default:
  		return TC_ACT_OK;
  	}
  
     	if (bpf_skb_adjust_room(skb, -olen, BPF_ADJ_ROOM_MAC, BPF_F_ADJ_ROOM_FIXED_GSO))
      	return TC_ACT_SHOT;
  
  	return TC_ACT_OK;
  }
  
  static int decap_ipv4(struct __sk_buff *skb)
  {
  	struct iphdr iph_outer;
  
  	if (bpf_skb_load_bytes(skb, ETH_HLEN, &iph_outer,
  			       sizeof(iph_outer)) < 0)
  		return TC_ACT_OK;
  
  	if (iph_outer.ihl != 5)
  		return TC_ACT_OK;
  
  	return decap_internal(skb, ETH_HLEN, sizeof(iph_outer),
  			      iph_outer.protocol);
  }
  
  static int decap_ipv6(struct __sk_buff *skb)
  {
  	struct ipv6hdr iph_outer;
  
  	if (bpf_skb_load_bytes(skb, ETH_HLEN, &iph_outer,
  			       sizeof(iph_outer)) < 0)
  		return TC_ACT_OK;
  
  	return decap_internal(skb, ETH_HLEN, sizeof(iph_outer),
  			      iph_outer.nexthdr);
  }
  
  SEC("decap")
  int decap_f(struct __sk_buff *skb)
  {
  	switch (skb->protocol) {
  	case __bpf_constant_htons(ETH_P_IP):
  		return decap_ipv4(skb);
  	case __bpf_constant_htons(ETH_P_IPV6):
  		return decap_ipv6(skb);
  	default:
  		/* does not match, ignore */
  		return TC_ACT_OK;
  	}
  }
  
  char __license[] SEC("license") = "GPL";
  