Re: [PATCH] mptcp: Do TCP fallback on early DSS checksum failure

[Matthieu Baerts <matthieu.baerts@tessares.net>]

Hi Greg, Mat,

On 25/05/2022 09:51, Greg KH wrote:
> On Tue, May 24, 2022 at 11:10:41AM -0700, Mat Martineau wrote:
>> [ Upstream commit ae66fb2ba6c3dcaf8b9612b65aa949a1a4bed150 ]
>>
>> RFC 8684 section 3.7 describes several opportunities for a MPTCP
>> connection to "fall back" to regular TCP early in the connection
>> process, before it has been confirmed that MPTCP options can be
>> successfully propagated on all SYN, SYN/ACK, and data packets. If a peer
>> acknowledges the first received data packet with a regular TCP header
>> (no MPTCP options), fallback is allowed.
>>
>> If the recipient of that first data packet finds a MPTCP DSS checksum
>> error, this provides an opportunity to fail gracefully with a TCP
>> fallback rather than resetting the connection (as might happen if a
>> checksum failure were detected later).
>>
>> This commit modifies the checksum failure code to attempt fallback on
>> the initial subflow of a MPTCP connection, only if it's a failure in the
>> first data mapping. In cases where the peer initiates the connection,
>> requests checksums, is the first to send data, and the peer is sending
>> incorrect checksums (see
>> https://github.com/multipath-tcp/mptcp_net-next/issues/275), this allows
>> the connection to proceed as TCP rather than reset.
>>
>> Cc: <stable@vger.kernel.org> # 5.17.x
>> Cc: <stable@vger.kernel.org> # 5.15.x
>> Fixes: dd8bcd1768ff ("mptcp: validate the data checksum")
>> Acked-by: Paolo Abeni <pabeni@redhat.com>
>> Signed-off-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
>> Signed-off-by: David S. Miller <davem@davemloft.net>
>> [mathew.j.martineau: backport: Resolved bitfield conflict in protocol.h]
>> Signed-off-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
>> ---
>>
>> This patch is already in 5.17.10-rc1 and 5.15.42-rc1, but involves a
>> context dependency on upstream commit 4cf86ae84c71 which I have
>> requested to be dropped from the stable queues.
>>
>> I'm posting this backport without the protocol.h conflict to
>> (hopefully?) make it easier for the stable maintainers to drop
>> 4cf86ae84c71.
>>
>> For context see https://lore.kernel.org/stable/fa953ec-288f-7715-c6fb-47a222e85270@linux.intel.com/
> 
> THanks, will take this after this round of releases.

It might already be too late but is it possible to have this patch
("mptcp: Do TCP fallback on early DSS checksum failure") and "mptcp: fix
checksum byte order" in the same stable release?

Note that "mptcp: fix checksum byte order" patch has been recently
queued by Sasha at the same time as "mptcp: Do TCP fallback on early DSS
checksum failure".

A bit of context: "mptcp: fix checksum byte order" fixes an important
encoding issue but it also breaks the interoperability with previous
Linux versions not having this patch.

The patch from Mat ("mptcp: Do TCP fallback on early DSS checksum
failure") improves the situation when there is this interoperability
issue with a previous Linux versions not implementing the RFC properly.
The improvement is there to make the MPTCP connections falling back to
TCP instead of resetting them: at least there is a connection.

In other words, that would be really nice to have these two commits
backported together. If it is easier, it looks best to me to delay the
main fix ("mptcp: fix checksum byte order") than having the two patches
in different stable versions. But I understand it was not clear and
maybe too late to do these modifications.

Anyway, thank you for your work maintaining these stable versions! :)

Cheers,
Matt
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