Re: [PATCH net v3] mptcp: only set DATA_FIN when a mapping is present

[Matthieu Baerts <matttbe@kernel.org>]

Hi Michael,

On 09/07/2026 21:19, Michael Bommarito wrote:
> mptcp_get_options() clears only the status group of struct
> mptcp_options_received; data_seq, subflow_seq and data_len are filled in
> by mptcp_parse_option() exclusively inside the DSS mapping block, which
> runs only when the DSS M (mapping present) bit is set.
> 
> A peer can send a DSS option with the DATA_FIN flag set but the mapping
> bit clear. The parser then records mp_opt->data_fin while leaving
> data_len and data_seq uninitialized. For a zero-length segment
> mptcp_incoming_options() evaluates
> 
> 	if (mp_opt.data_fin && mp_opt.data_len == 1 &&
> 	    mptcp_update_rcv_data_fin(msk, mp_opt.data_seq, mp_opt.dsn64))
> 
> which reads the uninitialized data_len and data_seq; KMSAN reports an
> uninit-value in mptcp_incoming_options(). The stale data_seq can also be
> fed into the receive-side DATA_FIN sequence tracking.
> 
> Record the DATA_FIN flag only when the DSS option carries a mapping, so
> data_fin is never set without data_seq and data_len also being present.
> data_fin is part of the status group that mptcp_get_options() clears up
> front, so on the no-map path it stays zero and the zero-length DATA_FIN
> branch is simply skipped. A DATA_FIN is always transmitted together with
> a mapping (mptcp_write_data_fin() sets use_map along with data_seq and
> data_len), so legitimate DATA_FIN handling is unaffected.
> 
> Move the pr_debug() that logs the parsed DSS flags below the mapping
> block, so it reports the final data_fin value instead of the stale one
> it would otherwise print before the assignment.

Thank you for the v3, it looks good to me:

Reviewed-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>

I tried to reproduce it on my side using packetdrill-mptcp [1], but I
was not able to test with KMSAN: my kernel boot, but is stuck when KMSAN
is enabled... By chance, if you can try this reproducer on your side,
with and without your patched kernel, that would be great :)

[1] https://github.com/multipath-tcp/packetdrill/pull/203

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