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tcp: take care of zero tp->window_clamp in tcp_set_rcvlowat() 

Some applications (like selftests/net/tcp_mmap.c) call SO_RCVLOWAT
on their listener, before accept().

This has an unfortunate effect on wscale selection in
tcp_select_initial_window() during 3WHS.

For instance, tcp_mmap was negotiating wscale 4, regardless
of tcp_rmem[2] and sysctl_rmem_max.

Do not change tp->window_clamp if it is zero
or bigger than our computed value.

Zero value is special, it allows tcp_select_initial_window()
to enable autotuning.

Note that SO_RCVLOWAT use on listener is probably not wise,
because tp->scaling_ratio has a default value, possibly wrong.

Fixes: d1361840f8 ("tcp: fix SO_RCVLOWAT and RCVBUF autotuning")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Kuniyuki Iwashima <kuniyu@google.com>
Reviewed-by: Neal Cardwell <ncardwell@google.com>
Link: https://patch.msgid.link/20251003184119.2526655-1-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
This commit is contained in:
Eric Dumazet 2025-10-03 18:41:19 +00:00 committed by Jakub Kicinski
parent 95920c2ed0
commit 21b29e74ff
1 changed files with 4 additions and 1 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

5
net/ipv4/tcp.c
View File

@ -1791,6 +1791,7 @@ EXPORT_IPV6_MOD(tcp_peek_len);
/* Make sure sk_rcvbuf is big enough to satisfy SO_RCVLOWAT hint */
int tcp_set_rcvlowat(struct sock *sk, int val)
{
struct tcp_sock *tp = tcp_sk(sk);
int space, cap;
if (sk->sk_userlocks & SOCK_RCVBUF_LOCK)
@ -1809,7 +1810,9 @@ int tcp_set_rcvlowat(struct sock *sk, int val)
space = tcp_space_from_win(sk, val);
if (space > sk->sk_rcvbuf) {
WRITE_ONCE(sk->sk_rcvbuf, space);
WRITE_ONCE(tcp_sk(sk)->window_clamp, val);
if (tp->window_clamp && tp->window_clamp < val)
WRITE_ONCE(tp->window_clamp, val);
}
return 0;
}