[PATCH net-next] tcp: clamp window like before the cleanup

[Matthieu Baerts (NGI0)]

A recent cleanup changed the behaviour of tcp_set_window_clamp(). This
looks unintentional, and affects MPTCP selftests, e.g. some tests
re-establishing a connection after a disconnect are now unstable.

Before the cleanup, this operation was done:

  new_rcv_ssthresh = min(tp->rcv_wnd, new_window_clamp);
  tp->rcv_ssthresh = max(new_rcv_ssthresh, tp->rcv_ssthresh);

The cleanup used the 'clamp' macro which takes 3 arguments -- value,
lowest, and highest -- and returns a value between the lowest and the
highest allowable values. This then assumes ...

  lowest (rcv_ssthresh) <= highest (rcv_wnd)

... which doesn't seem to be always the case here according to the MPTCP
selftests, even when running them without MPTCP, but only TCP.

For example, when we have ...

  rcv_wnd < rcv_ssthresh < new_rcv_ssthresh

... before the cleanup, the rcv_ssthresh was not changed, while after
the cleanup, it is lowered down to rcv_wnd (highest).

During a simple test with TCP, here are the values I observed:

  new_window_clamp (val)  rcv_ssthresh (lo)  rcv_wnd (hi)
      117760   (out)         65495         <  65536
      128512   (out)         109595        >  80256  => lo > hi
      1184975  (out)         328987        <  329088

      113664   (out)         65483         <  65536
      117760   (out)         110968        <  110976
      129024   (out)         116527        >  109696 => lo > hi

Here, we can see that it is not that rare to have rcv_ssthresh (lo)
higher than rcv_wnd (hi), so having a different behaviour when the
clamp() macro is used, even without MPTCP.

Note: new_window_clamp is always out of range (rcv_ssthresh < rcv_wnd)
here, which seems to be generally the case in my tests with small
connections.

I then suggests reverting this part, not to change the behaviour.

Fixes: 863a952eb79a ("tcp: tcp_set_window_clamp() cleanup")
Closes: https://github.com/multipath-tcp/mptcp_net-next/issues/551
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@xxxxxxxxxx>
---
Notes: the 'Fixes' commit is only in net-next
---
 net/ipv4/tcp.c | 12 ++++++------
 1 file changed, 6 insertions(+), 6 deletions(-)

diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
index eb5a60c7a9ccdd23fb78a74d614c18c4f7e281c9..46951e74930844af952dfbc57a107b504d4e296b 100644
--- a/net/ipv4/tcp.c
+++ b/net/ipv4/tcp.c
@@ -3693,7 +3693,7 @@ EXPORT_SYMBOL(tcp_sock_set_keepcnt);
 
 int tcp_set_window_clamp(struct sock *sk, int val)
 {
-	u32 old_window_clamp, new_window_clamp;
+	u32 old_window_clamp, new_window_clamp, new_rcv_ssthresh;
 	struct tcp_sock *tp = tcp_sk(sk);
 
 	if (!val) {
@@ -3714,12 +3714,12 @@ int tcp_set_window_clamp(struct sock *sk, int val)
 	/* Need to apply the reserved mem provisioning only
 	 * when shrinking the window clamp.
 	 */
-	if (new_window_clamp < old_window_clamp)
+	if (new_window_clamp < old_window_clamp) {
 		__tcp_adjust_rcv_ssthresh(sk, new_window_clamp);
-	else
-		tp->rcv_ssthresh = clamp(new_window_clamp,
-					 tp->rcv_ssthresh,
-					 tp->rcv_wnd);
+	} else {
+		new_rcv_ssthresh = min(tp->rcv_wnd, new_window_clamp);
+		tp->rcv_ssthresh = max(new_rcv_ssthresh, tp->rcv_ssthresh);
+	}
 	return 0;
 }
 

---
base-commit: c62e6f056ea308d6382450c1cb32e41727375885
change-id: 20250305-net-next-fix-tcp-win-clamp-9f4c417ff44d

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