Hi Petar,

Thank you for the patch! Yet something to improve:

[auto build test ERROR on bpf-next/master]

url:    https://github.com/0day-ci/linux/commits/Petar-Penkov/Introduce-a-BPF-helper-to-generate-SYN-cookies/20190723-235628
base:   https://kernel.googlesource.com/pub/scm/linux/kernel/git/bpf/bpf-next.git master
config: x86_64-randconfig-e004-201929 (attached as .config)
compiler: gcc-7 (Debian 7.4.0-10) 7.4.0
reproduce:
        # save the attached .config to linux build tree
        make ARCH=x86_64 

If you fix the issue, kindly add following tag
Reported-by: kbuild test robot <lkp@intel.com>

All errors (new ones prefixed by >>):

   ld: net/ipv4/tcp_input.o: in function `tcp_get_syncookie':
>> net/ipv4/tcp_input.c:6538: undefined reference to `__cookie_v6_init_sequence'

vim +6538 net/ipv4/tcp_input.c

  6509	
  6510	u16 tcp_get_syncookie(struct request_sock_ops *rsk_ops,
  6511			      const struct tcp_request_sock_ops *af_ops,
  6512			      struct sock *sk, void *iph, struct tcphdr *th,
  6513			      u32 *cookie)
  6514	{
  6515		u16 mss = 0;
  6516	#ifdef CONFIG_SYN_COOKIES
  6517		bool is_v4 = rsk_ops->family == AF_INET;
  6518		struct tcp_sock *tp = tcp_sk(sk);
  6519	
  6520		if (sock_net(sk)->ipv4.sysctl_tcp_syncookies != 2 &&
  6521		    !inet_csk_reqsk_queue_is_full(sk))
  6522			return 0;
  6523	
  6524		if (!tcp_syn_flood_action(sk, rsk_ops->slab_name))
  6525			return 0;
  6526	
  6527		if (sk_acceptq_is_full(sk)) {
  6528			NET_INC_STATS(sock_net(sk), LINUX_MIB_LISTENOVERFLOWS);
  6529			return 0;
  6530		}
  6531	
  6532		mss = tcp_parse_mss_option(th, tp->rx_opt.user_mss);
  6533		if (!mss)
  6534			mss = af_ops->mss_clamp;
  6535	
  6536		tcp_synq_overflow(sk);
  6537		*cookie = is_v4 ? __cookie_v4_init_sequence(iph, th, &mss)
> 6538				: __cookie_v6_init_sequence(iph, th, &mss);
  6539	#endif
  6540		return mss;
  6541	}
  6542	

---
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