tcp: update window_clamp when SO_RCVBUF is set

Commit under Fixes moved recomputing the window clamp to
tcp_measure_rcv_mss() (when scaling_ratio changes).
I suspect it missed the fact that we don't recompute the clamp
when rcvbuf is set. Until scaling_ratio changes we are
stuck with the old window clamp which may be based on
the small initial buffer. scaling_ratio may never change.

Inspired by Eric's recent commit d1361840f8c5 ("tcp: fix
SO_RCVLOWAT and RCVBUF autotuning") plumb the user action
thru to TCP and have it update the clamp.

A smaller fix would be to just have tcp_rcvbuf_grow()
adjust the clamp even if SOCK_RCVBUF_LOCK is set.
But IIUC this is what we were trying to get away from
in the first place.

Fixes: a2cbb1603943 ("tcp: Update window clamping condition")
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Reviewed-by: Eric Dumazet <edumaze@google.com>
Link: https://patch.msgid.link/20260408001438.129165-1-kuba@kernel.org
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

1 files changed, 9 insertions, 0 deletions

diff --git a/net/core/sock.c b/net/core/sock.c
index 367fd7bad4ac..b37b664b6eb9 100644
--- a/net/core/sock.c
+++ b/net/core/sock.c
@@ -966,6 +966,8 @@ EXPORT_SYMBOL(sock_set_keepalive);
 
 static void __sock_set_rcvbuf(struct sock *sk, int val)
 {
+	struct socket *sock = sk->sk_socket;
+
 	/* Ensure val * 2 fits into an int, to prevent max_t() from treating it
 	 * as a negative value.
 	 */
@@ -983,6 +985,13 @@ static void __sock_set_rcvbuf(struct sock *sk, int val)
 	 * we actually used in getsockopt is the most desirable behavior.
 	 */
 	WRITE_ONCE(sk->sk_rcvbuf, max_t(int, val * 2, SOCK_MIN_RCVBUF));
+
+	if (sock) {
+		const struct proto_ops *ops = READ_ONCE(sock->ops);
+
+		if (ops->set_rcvbuf)
+			ops->set_rcvbuf(sk, sk->sk_rcvbuf);
+	}
 }
 
 void sock_set_rcvbuf(struct sock *sk, int val)