Re: [PATCH mptcp-next v6 3/7] mptcp: remove CB offset field

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [High] Incorrect initialization of `subflow->map_subflow_seq` in `mptcp_propagate_state` completely breaks data reception when falling back to TCP.
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--- Patch [3]: [PATCH mptcp-next v6 3/7] mptcp: remove CB offset field ---
commit 8a2d837eb49f2826870e0f275e0beb7da32784d7
Author: Paolo Abeni <[email protected]>

mptcp: remove CB offset field

This commit replaces the SKB CB offset field with a new socket-level
copied_seq field to track bytes already consumed. Sequence number tracking
is updated to synchronize with IASN, and MPTCP sequence space is initialized
to 0 in fallback mode.

> diff --git a/net/mptcp/subflow.c b/net/mptcp/subflow.c
> index ed3a48cf9c973..6bffa02c69626 100644
> --- a/net/mptcp/subflow.c
> +++ b/net/mptcp/subflow.c
> @@ -520,6 +522,13 @@ static void mptcp_propagate_state(struct sock *sk, struct sock *ssk,
>  		WRITE_ONCE(msk->snd_una, subflow->idsn + 1);
>  		WRITE_ONCE(msk->wnd_end, subflow->idsn + 1 + tcp_sk(ssk)->snd_wnd);
>  		subflow_set_remote_key(msk, subflow, mp_opt);
> +	} else {
> +		/* Fallback: initialize sequence space to 0 (no remote key) */
> +		subflow->map_seq = 0;
> +		/* ensure mptcp_subflow_get_map_offset() returns 0 */
> +		subflow->map_subflow_seq = subflow->ssn_offset +
> +					   tcp_sk(ssk)->copied_seq;

[Severity: High]
Does this code incorrectly initialize map_subflow_seq during TCP fallback?

The comment says this should ensure mptcp_subflow_get_map_offset() returns
0, but the offset and copied_seq are added rather than subtracted.

Because mptcp_subflow_get_map_offset() evaluates
copied_seq - ssn_offset - map_subflow_seq, adding the terms here causes the
offset to evaluate to -2 * ISN.

When falling back to standard TCP, this miscalculation will cause incoming
packets to fail sequence bounds checks against msk->ack_seq, resulting in
data reception failures as packets are either dropped or permanently stuck
in the out-of-order queue.

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Sashiko AI review · https://sashiko.dev/#/patchset/[email protected]?part=3