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: d136184 ("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>

Author: edumazet

net/ipv4/tcp.c

@@ -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;
 }