tcp: Return user_mss for TCP_MAXSEG in CLOSE/LISTEN state if user_mss set

This patch replaces the tp->mss_cache check in getting TCP_MAXSEG
with tp->rx_opt.user_mss check for CLOSE/LISTEN sock. Since
tp->mss_cache is initialized with TCP_MSS_DEFAULT, checking if
it's zero is probably a bug.

With this change, getting TCP_MAXSEG before connecting will return
default MSS normally, and return user_mss if user_mss is set.

Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Reported-by: Jack Yang <mingliang@linux.alibaba.com>
Suggested-by: Eric Dumazet <edumazet@google.com>
Link: https://lore.kernel.org/netdev/CANn89i+3kL9pYtkxkwxwNMzvC_w3LNUum_2=3u+UyLBmGmifHA@mail.gmail.com/#t
Signed-off-by: Cambda Zhu <cambda@linux.alibaba.com>
Link: https://lore.kernel.org/netdev/14D45862-36EA-4076-974C-EA67513C92F6@linux.alibaba.com/
Reviewed-by: Jason Xing <kerneljasonxing@gmail.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Link: https://lore.kernel.org/r/20230527040317.68247-1-cambda@linux.alibaba.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
index 635607ac37e4585a3f730bf93e80e6673733cd55..8d20d9221238cac81b84aa5c6267d20a13bc23a6 100644 (file)
--- a/net/ipv4/tcp.c
+++ b/net/ipv4/tcp.c
@@ -4078,7 +4078,8 @@ int do_tcp_getsockopt(struct sock *sk, int level,
        switch (optname) {
        case TCP_MAXSEG:
                val = tp->mss_cache;
-               if (!val && ((1 << sk->sk_state) & (TCPF_CLOSE | TCPF_LISTEN)))
+               if (tp->rx_opt.user_mss &&
+                   ((1 << sk->sk_state) & (TCPF_CLOSE | TCPF_LISTEN)))
                        val = tp->rx_opt.user_mss;
                if (tp->repair)
                        val = tp->rx_opt.mss_clamp;