Re: [PATCH mptcp-net] mptcp: options: fix uninit-value in mptcp_write_data_fin

[Matthieu Baerts <matttbe@kernel.org>]

Hi Geliang,

On 19/08/2026 11:09, Geliang Tang wrote:
> Hi Matt,
> 
> On Fri, 2026-08-14 at 23:38 +0200, Matthieu Baerts (NGI0) wrote:
>> When sending a DATA_FIN without data, and because the DATA_FIN
>> occupies
>> 1 octet of the connection-level sequence space [1], it is then
>> required
>> to add a DSS mapping with specific values.
>>
>> If the checksum has been negotiated, it also needs to be computed,
>> and
>> included in the outgoing packet, and thus the initial csum data needs
>> to
>> be reset to 0 as well. This is no longer the case since commit
>> cfcceb7a39fc ("tcp: shrink per-packet memset in
>> __tcp_transmit_skb()"),
>> because the whole ext_copy structure is no longer zeroed by default.
>>
>> This seems to be the only case where use_map is changed and set
>> afterwards, so initialising the csum field only in this case, along
>> with
>> other fields for this specific case.
> 
> Initially, I was wondering if we could skip calling mptcp_make_csum()
> for data_fin in mptcp_write_options(), similar to how we skip it for
> the infinite mapping:
> 
>         /* data_len == 0 is reserved for the infinite mapping,
>          * the checksum will also be set to 0.
>          */
>         put_len_csum(mpext->data_len,
>                      (mpext->data_len ? mptcp_make_csum(mpext) : 0),
>                      ptr);
> 
> But I realized that your modification is more straightforward.
> 
>>
>> Fixes: cfcceb7a39fc ("tcp: shrink per-packet memset in
>> __tcp_transmit_skb()")
>> Link: https://datatracker.ietf.org/doc/html/rfc8684#section-3.3.3 [1]
>> Link:
>> https://sashiko.dev/#/patchset/20260812-net-next-mptcp-misc-feat-7-3-v1-0-1905a818f6cb%40kernel.org?part=2
>> Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
> 
> Looks good to me!
> 
>     Reviewed-by: Geliang Tang <geliang@kernel.org>

Thank you for the review!

New patches for t/upstream-net and t/upstream:
- fbdc41264992: mptcp: options: fix uninit-value in mptcp_write_data_fin
- Results: e9b9aadd7dfb..d251bdc7d65e (export-net)
- Results: 8cefef708ced..77661003be12 (export)

Tests are now in progress:

- export-net:
https://github.com/multipath-tcp/mptcp_net-next/commit/439ace86ec3085a503e16283cf2719876d9a9d7e/checks
- export:
https://github.com/multipath-tcp/mptcp_net-next/commit/7c1c363f2cd4fb1dbd7edb83dc2d57313289d200/checks

Cheers,
Matt
-- 
Sponsored by the NGI0 Core fund.