tcp: SYN packets are now simply consumed

We now have proper per-listener but also per network namespace counters
for SYN packets that might be dropped.

We replace the kfree_skb() by consume_skb() to be drop monitor [1]
friendly, and remove an obsolete comment.
FastOpen SYN packets can carry payload in them just fine.

[1] perf record -a -g -e skb:kfree_skb sleep 1; perf report

Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index dcad8f9..967520d 100644 (file)
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -5815,24 +5815,7 @@ int tcp_rcv_state_process(struct sock *sk, struct sk_buff *skb)
                        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);
+                       consume_skb(skb);
                        return 0;
                }
                goto discard;