netdev - IPv6 TCP seems broken on 32bit x86 (bisected)

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Message-ID: <CABq1_viq9yMo9wZ2XkLg_45FaOcwL93qVhqFUZ9wTygKagnszg@mail.gmail.com>
Date: Thu, 24 Aug 2023 02:00:03 +0200
From: Klara Modin <klarasmodin@...il.com>
To: netdev@...r.kernel.org
Cc: asml.silence@...il.com, edumazet@...gle.com, davem@...emloft.net, 
	simon.horman@...igine.com
Subject: IPv6 TCP seems broken on 32bit x86 (bisected)

Hi,

I recently noticed that IPv6 stopped working properly on my 32 bit x86
machines with recent kernels. Bisecting leads to
"fe79bd65c819cc520aa66de65caae8e4cea29c5a net/tcp: refactor
tcp_inet6_sk()". Reverting this commit on top of
a5e505a99ca748583dbe558b691be1b26f05d678 fixes the issue.
64 bit x86 seems unaffected.

One of the symptoms seems to be that the hop limit is sometimes set to
zero. Attached is tcpdump output from trying to ssh into the machine
from a different subnet.

I also tried http which seems even weirder, the source address is cut
off or offsetted by 32 bits (results in
"a5c:1204:84c4:847e:950e:2b3d::", should be
"2001:678:a5c:1204:84c4:847e:950e:2b3d"). Trying to connect using
netcat sees the same result as with http.

Tested on two machines running Gentoo, one with a locally compiled
kernel and the other compiled from a x86_64 host, both using gcc 13.2.
If I remember correctly I was using gcc 12.3.1 on the first machine
when I initially noticed the issue but didn't troubleshoot further at
the time.

Please tell me if there's any other information you need.

Regards,
Klara Modin

View attachment "nc-tcpdump-output.log" of type "text/x-log" (2853 bytes)

View attachment "ssh-tcpdump-output.log" of type "text/x-log" (14066 bytes)

View attachment "http-tcpdump-output.log" of type "text/x-log" (1304 bytes)

View attachment "bisect-bad-ipv6-tcp.log" of type "text/x-log" (2560 bytes)