netdev - net: getsockopt(TCP_MAXSEG) on listen sock returns wrong MSS?

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Date: Wed, 17 May 2023 19:08:49 +0800
From: Cambda Zhu <cambda@...ux.alibaba.com>
To: netdev@...r.kernel.org,
 "David S. Miller" <davem@...emloft.net>,
 Eric Dumazet <edumazet@...gle.com>
Cc: 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>
Subject: net: getsockopt(TCP_MAXSEG) on listen sock returns wrong MSS?

I want to call setsockopt(TCP_MAXSEG) on a listen sock to let
all child socks have smaller MSS. And I found the child sock
MSS changed but getsockopt(TCP_MAXSEG) on the listen sock
returns 536 always.

It seems the tp->mss_cache is initialized with TCP_MSS_DEFAULT,
but getsockopt(TCP_MAXSEG) returns tp->rx_opt.user_mss only when
tp->mss_cache is 0. I don't understand the purpose of the mss_cache
check of TCP_MAXSEG. If getsockopt(TCP_MAXSEG) on listen sock makes
no sense, why does it have a branch for close/listen sock to return
user_mss? If getsockopt(TCP_MAXSEG) on listen sock is ok, why does
it check mss_cache for a listen sock?

I tried to find the commit log about TCP_MAXSEG, and found that
in commit 0c409e85f0ac ("Import 2.3.41pre2"), the mss_cache check
was added. No more detailed information found. Is this a bug or am
I misunderstanding something?

Regards,
Cambda