text: // SPDX-License-Identifier: GPL-2.0
  // Copyright (c) 2018 Facebook
  // Copyright (c) 2019 Cloudflare
  
  #include <string.h>
  
  #include <linux/bpf.h>
  #include <linux/pkt_cls.h>
  #include <linux/if_ether.h>
  #include <linux/in.h>
  #include <linux/ip.h>
  #include <linux/ipv6.h>
  #include <sys/socket.h>
  #include <linux/tcp.h>
  
  #include <bpf/bpf_helpers.h>
  #include <bpf/bpf_endian.h>
  
  struct bpf_map_def SEC("maps") results = {
  	.type = BPF_MAP_TYPE_ARRAY,
  	.key_size = sizeof(__u32),
  	.value_size = sizeof(__u32),
  	.max_entries = 3,
  };
  
  static __always_inline __s64 gen_syncookie(void *data_end, struct bpf_sock *sk,
  					   void *iph, __u32 ip_size,
  					   struct tcphdr *tcph)
  {
  	__u32 thlen = tcph->doff * 4;
  
  	if (tcph->syn && !tcph->ack) {
  		// packet should only have an MSS option
  		if (thlen != 24)
  			return 0;
  
  		if ((void *)tcph + thlen > data_end)
  			return 0;
  
  		return bpf_tcp_gen_syncookie(sk, iph, ip_size, tcph, thlen);
  	}
  	return 0;
  }
  
  static __always_inline void check_syncookie(void *ctx, void *data,
  					    void *data_end)
  {
  	struct bpf_sock_tuple tup;
  	struct bpf_sock *sk;
  	struct ethhdr *ethh;
  	struct iphdr *ipv4h;
  	struct ipv6hdr *ipv6h;
  	struct tcphdr *tcph;
  	int ret;
  	__u32 key_mss = 2;
  	__u32 key_gen = 1;
  	__u32 key = 0;
  	__s64 seq_mss;
  
  	ethh = data;
  	if (ethh + 1 > data_end)
  		return;
  
  	switch (bpf_ntohs(ethh->h_proto)) {
  	case ETH_P_IP:
  		ipv4h = data + sizeof(struct ethhdr);
  		if (ipv4h + 1 > data_end)
  			return;
  
  		if (ipv4h->ihl != 5)
  			return;
  
  		tcph = data + sizeof(struct ethhdr) + sizeof(struct iphdr);
  		if (tcph + 1 > data_end)
  			return;
  
  		tup.ipv4.saddr = ipv4h->saddr;
  		tup.ipv4.daddr = ipv4h->daddr;
  		tup.ipv4.sport = tcph->source;
  		tup.ipv4.dport = tcph->dest;
  
  		sk = bpf_skc_lookup_tcp(ctx, &tup, sizeof(tup.ipv4),
  					BPF_F_CURRENT_NETNS, 0);
  		if (!sk)
  			return;
  
  		if (sk->state != BPF_TCP_LISTEN)
  			goto release;
  
  		seq_mss = gen_syncookie(data_end, sk, ipv4h, sizeof(*ipv4h),
  					tcph);
  
  		ret = bpf_tcp_check_syncookie(sk, ipv4h, sizeof(*ipv4h),
  					      tcph, sizeof(*tcph));
  		break;
  
  	case ETH_P_IPV6:
  		ipv6h = data + sizeof(struct ethhdr);
  		if (ipv6h + 1 > data_end)
  			return;
  
  		if (ipv6h->nexthdr != IPPROTO_TCP)
  			return;
  
  		tcph = data + sizeof(struct ethhdr) + sizeof(struct ipv6hdr);
  		if (tcph + 1 > data_end)
  			return;
  
  		memcpy(tup.ipv6.saddr, &ipv6h->saddr, sizeof(tup.ipv6.saddr));
  		memcpy(tup.ipv6.daddr, &ipv6h->daddr, sizeof(tup.ipv6.daddr));
  		tup.ipv6.sport = tcph->source;
  		tup.ipv6.dport = tcph->dest;
  
  		sk = bpf_skc_lookup_tcp(ctx, &tup, sizeof(tup.ipv6),
  					BPF_F_CURRENT_NETNS, 0);
  		if (!sk)
  			return;
  
  		if (sk->state != BPF_TCP_LISTEN)
  			goto release;
  
  		seq_mss = gen_syncookie(data_end, sk, ipv6h, sizeof(*ipv6h),
  					tcph);
  
  		ret = bpf_tcp_check_syncookie(sk, ipv6h, sizeof(*ipv6h),
  					      tcph, sizeof(*tcph));
  		break;
  
  	default:
  		return;
  	}
  
  	if (seq_mss > 0) {
  		__u32 cookie = (__u32)seq_mss;
  		__u32 mss = seq_mss >> 32;
  
  		bpf_map_update_elem(&results, &key_gen, &cookie, 0);
  		bpf_map_update_elem(&results, &key_mss, &mss, 0);
  	}
  
  	if (ret == 0) {
  		__u32 cookie = bpf_ntohl(tcph->ack_seq) - 1;
  
  		bpf_map_update_elem(&results, &key, &cookie, 0);
  	}
  
  release:
  	bpf_sk_release(sk);
  }
  
  SEC("tc")
  int check_syncookie_clsact(struct __sk_buff *skb)
  {
  	check_syncookie(skb, (void *)(long)skb->data,
  			(void *)(long)skb->data_end);
  	return TC_ACT_OK;
  }
  
  SEC("xdp")
  int check_syncookie_xdp(struct xdp_md *ctx)
  {
  	check_syncookie(ctx, (void *)(long)ctx->data,
  			(void *)(long)ctx->data_end);
  	return XDP_PASS;
  }
  
  char _license[] SEC("license") = "GPL";
  