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[ Upstream commit21b29e74ff] 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> Signed-off-by: Sasha Levin <sashal@kernel.org>