[PATCH net v2 2/2] tcp: limit wake-up for crossed SYN cases to SYN-ACK

From: Matthieu Baerts (NGI0)
Date: Thu Jul 18 2024 - 06:35:06 EST


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, it is then
required to also limit the check to packets containing the SYN-ACK
flags.

While at it, also update the attached comment: 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").

Fixes: 168a8f58059a ("tcp: TCP Fast Open Server - main code path")
Suggested-by: Kuniyuki Iwashima <kuniyu@xxxxxxxxxx>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@xxxxxxxxxx>
---
Notes:
- The above 'Fixes' tag should correspond to the commit introducing the
possibility to have sk->sk_socket being set there in other cases than
the crossed SYN one. But I might have missed other cases. Maybe
1da177e4c3f4 ("Linux-2.6.12-rc2") might be safer? On the other hand,
I don't think this wake-up was causing any visible issue, apart from
not being needed.
---
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 bfe1bc69dc3e..5cebb389bf71 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;

--
2.45.2