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Date:   Tue, 30 Jun 2020 16:34:53 -0400 (EDT)
From:   Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
To:     Linus Torvalds <torvalds@linux-foundation.org>,
        Eric Dumazet <edumazet@google.com>
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        linux-kernel <linux-kernel@vger.kernel.org>,
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        Yuchung Cheng <ycheng@google.com>,
        Jonathan Rajotte-Julien <joraj@efficios.com>
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Subject: Re: [regression] TCP_MD5SIG on established sockets
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----- On Jun 30, 2020, at 3:52 PM, Linus Torvalds torvalds@linux-foundation.org wrote:

> On Tue, Jun 30, 2020 at 12:43 PM Linus Torvalds
> <torvalds@linux-foundation.org> wrote:
>>
[...]
> So I think it's still wrong (clearly others do change passwords
> outside of listening state), but considering that it apparently took
> people two years to notice, at least some of the onus on figuring out
> a better morel is on people who didn't even bother to test things in a
> timely manner.

I'm fully willing to work with Eric on finding a way forward with a
fix which addresses the original issue Eric's patch was trying to
fix while preserving ABI compatibility.

The main thing we need to agree on at this stage is what is our goal. We
can either choose to restore the original ABI behavior entirely, or only
focus on what appears to be the most important use-cases.

AFAIU, restoring full ABI compatibility would require to re-enable all
the following scenarios:

A) Transition of live socket from no key -> MD5 key.
B) Transition of live socket from MD5 key -> no key.
C) Transition of live socket from MD5 key to a different MD5 key.

Scenario (C) appears to be the most important use-case, and probably the
easiest to restore to its original behavior.

AFAIU restoring scenarios A and B would require us to validate how
much header space is needed by each SACK, TS and MD5 option enabled
on the socket, and reject enabling any option that adds header space
requirement exceeding the available space.

I welcome advice on what should be the end goal here.

Thanks,

Mathieu

-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com