[PATCH net-next] tcp: limit wake-up for crossed SYN cases with SYN-ACK

[Matthieu Baerts (NGI0)]

In TCP_SYN_RECV states, sk->sk_socket will be assigned in case of
marginal crossed SYN, but also in other cases, e.g.

 - With TCP Fast Open, if the connection got accept()'ed before
   receiving the 3rd ACK ;

 - With MPTCP, when accepting additional subflows to an existing MPTCP
   connection.

In these cases, the switch to TCP_ESTABLISHED is done when receiving the
3rd ACK, without the SYN flag then.

To properly restrict the wake-up to crossed SYN cases as expected there,
it is then required to also limit the check to packets containing the
SYN-ACK flags.

Without this modification, it looks like the wake-up was not causing any
visible issue with TFO and MPTCP, apart from not being needed. That's
why this patch doesn't contain a Cc to stable, and a Fixes tag.

While at it, the attached comment has also been updated: sk->sk_sleep
has been removed in 2010, and replaced by sk->sk_wq in commit
43815482370c ("net: sock_def_readable() and friends RCU conversion").

Suggested-by: Kuniyuki Iwashima <kuniyu@xxxxxxxxxx>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@xxxxxxxxxx>
---
Notes:
  - This is the same patch as the one suggested earlier in -net as part
    of another series, but targeting net-next (Eric), and with an
    updated commit message. The previous version was visible there:
    https://lore.kernel.org/20240718-upstream-net-next-20240716-tcp-3rd-ack-consume-sk_socket-v2-2-d653f85639f6@xxxxxxxxxx/
---
 net/ipv4/tcp_input.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index 454362e359da..b2d2c843ecd2 100644
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -6797,9 +6797,9 @@ tcp_rcv_state_process(struct sock *sk, struct sk_buff *skb)
 
 		/* 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.
+		 * sk->sk_wq == NULL and sk->sk_socket == NULL.
 		 */
-		if (sk->sk_socket)
+		if (sk->sk_socket && th->syn)
 			sk_wake_async(sk, SOCK_WAKE_IO, POLL_OUT);
 
 		tp->snd_una = TCP_SKB_CB(skb)->ack_seq;

---
base-commit: 743ff02152bc46bb4a2f2a49ec891c87eba6ab5b
change-id: 20240801-upstream-net-next-20240801-tcp-limit-wake-up-x-syn-62e971a2accc

Best regards,
-- 
Matthieu Baerts (NGI0) <matttbe@xxxxxxxxxx>