[PATCH net] tcp: keep the route window metric when SO_RCVBUF is set before connect

[Shashank Mohan Jain <jain.sm@gmail.com>]

Since commit b025461303d8 ("tcp: update window_clamp when SO_RCVBUF is
set") SO_RCVBUF sets tp->window_clamp from the new buffer size in every
socket state. For a socket without a connection this has a side
effect. tcp_connect_init() and tcp_openreq_init_rwin() use the window
metric of the route (RTAX_WINDOW, "ip route ... window N") only if
window_clamp is zero. The clamp left behind by SO_RCVBUF makes the
metric ineffective for that socket, or for the connections accepted
on a listener.

Example: two network namespaces connected by a veth pair, "window
4096" on the route to the peer on both sides, SO_RCVBUF set to 1 MiB
before connect():

  v7.3-rc6:                    SYN win 65535, wscale 5
                               TCP_WINDOW_CLAMP 1047128
  with that commit reverted:   SYN win 2920, no window scaling
                               TCP_WINDOW_CLAMP 4096

With SO_RCVBUF set on the listener the SYN-ACK and the clamp of the
accepted socket differ in the same way.

The update is not needed in these states. When the connection is set
up with SOCK_RCVBUF_LOCK set, both functions limit a zero or too
large clamp to tcp_full_space(), which is the value tcp_set_rcvbuf()
computes. Skip the update for sockets in TCP_CLOSE and TCP_LISTEN.

Tested in qemu with the setup above. With the route metric, the SYN
and SYN-ACK windows and TCP_WINDOW_CLAMP are the same as with the
commit above reverted. Without the metric they are the same as before
this change, with and without SO_RCVBUF on either side.

Fixes: b025461303d8 ("tcp: update window_clamp when SO_RCVBUF is set")
Assisted-by: LLM
Signed-off-by: Shashank Mohan Jain <jain.sm@gmail.com>
---
Prepared with Claude Code (Anthropic), model Claude Opus 5.5
(claude-opus-5-5).

 net/ipv4/tcp.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
index 87ef6d5cbf..9b6f214411 100644
--- a/net/ipv4/tcp.c
+++ b/net/ipv4/tcp.c
@@ -1856,6 +1856,14 @@ int tcp_set_rcvlowat(struct sock *sk, int val)
 
 void tcp_set_rcvbuf(struct sock *sk, int val)
 {
+	/* Without a connection there is no clamp to update: it is chosen
+	 * from the route's window metric and the buffer size when the
+	 * connection is set up, see tcp_connect_init() and
+	 * tcp_openreq_init_rwin().
+	 */
+	if ((1 << sk->sk_state) & (TCPF_CLOSE | TCPF_LISTEN))
+		return;
+
 	tcp_set_window_clamp(sk, tcp_win_from_space(sk, val));
 }
 

base-commit: a5ebb76233b79db01e82e061173cb73c6d2b5c6b
-- 
2.43.0