netdev - [PATCH net v3] tcp: Return user_mss for TCP_MAXSEG in CLOSE/LISTEN state if user

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Date: Sat, 27 May 2023 12:03:17 +0800
From: Cambda Zhu <cambda@...ux.alibaba.com>
To: netdev@...r.kernel.org
Cc: Eric Dumazet <edumazet@...gle.com>,
	Paolo Abeni <pabeni@...hat.com>,
	Jason Xing <kerneljasonxing@...il.com>,
	"David S. Miller" <davem@...emloft.net>,
	David Ahern <dsahern@...nel.org>,
	Jakub Kicinski <kuba@...nel.org>,
	Xuan Zhuo <xuanzhuo@...ux.alibaba.com>,
	Dust Li <dust.li@...ux.alibaba.com>,
	Tony Lu <tonylu@...ux.alibaba.com>,
	Cambda Zhu <cambda@...ux.alibaba.com>,
	Jack Yang <mingliang@...ux.alibaba.com>
Subject: [PATCH net v3] 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@...ux.alibaba.com>
Suggested-by: Eric Dumazet <edumazet@...gle.com>
Link: https://lore.kernel.org/netdev/CANn89i+3kL9pYtkxkwxwNMzvC_w3LNUum_2=3u+UyLBmGmifHA@mail.gmail.com/#t
Signed-off-by: Cambda Zhu <cambda@...ux.alibaba.com>
Link: https://lore.kernel.org/netdev/14D45862-36EA-4076-974C-EA67513C92F6@linux.alibaba.com/
Reviewed-by: Jason Xing <kerneljasonxing@...il.com>
---
v3:
- Add CC maintainers.

v2:
- Update Fixes tag with commit in current tree.
- Add Jason's Reviewed-by tag.

v1:
- Return default MSS if user_mss not set for backwards compatibility.
- Send patch to net instead of net-next, with Fixes tag.
- Add Eric's tags.
---
 net/ipv4/tcp.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
index 4d6392c16b7a..3e01a58724b8 100644
--- a/net/ipv4/tcp.c
+++ b/net/ipv4/tcp.c
@@ -4081,7 +4081,8 @@ 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;
-- 
2.16.6