netdev - Re: [PATCH net-next] tcp: be less liberal in tsecr received while in SYN

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Message-ID: <927c8b04-5944-4577-b6bd-3fc50ef55e7e@kernel.org>
Date: Tue, 25 Feb 2025 11:19:18 +0100
From: Matthieu Baerts <matttbe@...nel.org>
To: Eric Dumazet <edumazet@...gle.com>
Cc: Paolo Abeni <pabeni@...hat.com>, Kuniyuki Iwashima <kuniyu@...zon.com>,
 Simon Horman <horms@...nel.org>, Florian Westphal <fw@...len.de>,
 netdev@...r.kernel.org, eric.dumazet@...il.com,
 Jakub Kicinski <kuba@...nel.org>, Yong-Hao Zou <yonghaoz1994@...il.com>,
 "David S . Miller" <davem@...emloft.net>,
 Neal Cardwell <ncardwell@...gle.com>
Subject: Re: [PATCH net-next] tcp: be less liberal in tsecr received while in
 SYN_RECV state

Hi Eric,

On 25/02/2025 11:11, Eric Dumazet wrote:
> On Tue, Feb 25, 2025 at 11:09 AM Matthieu Baerts <matttbe@...nel.org> wrote:
>>
>> Hi Paolo, Eric,
>>
>> On 25/02/2025 10:59, Paolo Abeni wrote:
>>> On 2/24/25 12:06 PM, Eric Dumazet wrote:
>>>> Yong-Hao Zou mentioned that linux was not strict as other OS in 3WHS,
>>>> for flows using TCP TS option (RFC 7323)
>>>>
>>>> As hinted by an old comment in tcp_check_req(),
>>>> we can check the TSecr value in the incoming packet corresponds
>>>> to one of the SYNACK TSval values we have sent.
>>>>
>>>> In this patch, I record the oldest and most recent values
>>>> that SYNACK packets have used.
>>>>
>>>> Send a challenge ACK if we receive a TSecr outside
>>>> of this range, and increase a new SNMP counter.
>>>>
>>>> nstat -az | grep TcpExtTSECR_Rejected
>>>> TcpExtTSECR_Rejected            0                  0.0
>>
>> (...)
>>
>>> It looks like this change causes mptcp self-test failures:
>>>
>>> https://netdev-3.bots.linux.dev/vmksft-mptcp/results/6642/1-mptcp-join-sh/stdout
>>>
>>> ipv6 subflows creation fails due to the added check:
>>>
>>> # TcpExtTSECR_Rejected            3                  0.0
>>
>> You have been faster to report the issue :-)
>>
>>> (for unknown reasons the ipv4 variant of the test is successful)
>>
>> Please note that it is not the first time the MPTCP test suite caught
>> issues with the IPv6 stack. It is likely possible the IPv6 stack is less
>> covered than the v4 one in the net selftests. (Even if I guess here the
>> issue is only on MPTCP side.)
> 
> 
> subflow_prep_synack() does :
> 
>  /* clear tstamp_ok, as needed depending on cookie */
> if (foc && foc->len > -1)
>      ireq->tstamp_ok = 0;
> 
> I will double check fastopen code then.

Fastopen is not used in the failing tests. To be honest, it is not clear
to me why only the two tests I mentioned are failing, they are many
other tests using IPv6 in the MP_JOIN.

  045 add multiple subflows IPv6
  064 simult IPv4 and IPv6 subflows, fullmesh 2x2

Cheers,
Matt
-- 
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