Re: [PATCH mptcp_net-next 1/1] mptcp: fastopen: only mark MPTFO subflows with SYN data

On Wed, Jul 22, 2026 at 4:07 AM Matthieu Baerts (NGI0)
<[email protected]> wrote:
>
> Hi Ren, Wyatt, Yuan,
>
> > Passive TCP Fast Open accepts a valid-cookie SYN even when it carries
> > no data. In that case the child socket's receive queue is intentionally
> > left empty.
> >
> > mptcp_fastopen_subflow_synack_set_params() set is_mptfo before checking
> > for queued SYN data. That made data-less TFO SYNs hit a WARN and, if
> > the warning was non-fatal, left stale MPTFO state behind. The stale
> > flag could later trigger a state-confusion bug in
> > check_fully_established().
> >
> > Only mark the subflow as MPTFO after confirming that a SYN-data skb is
> > present. Return quietly when the receive queue is empty.
>
> Thank you for the fix, it looks good to me:
>
> Reviewed-by: Matthieu Baerts (NGI0) <[email protected]>
>
> @Netdev maintainers: this can be applied directly in net.
>
>
> Sorry for the delay, I initially wanted to understand the reproducer,
> then wrote a small packetdrill script, then I lost track of this patch.
>
> As I mentioned to Yuan last week at NetdevConf, it would be great if the
> PoC could first try to use Packetdrill when it is doable. For example,
> in this case, this packetdrill script appears to be far more readable
> than the .c files generated by AI:
>
>   https://github.com/multipath-tcp/packetdrill/pull/202/changes
>
> => It is easy to read the syscalls, the injected packets (<) and the
> expected ones (>) with an output similar to the one from tcpdump.
>
> Note that for MPTCP, this fork should be used:
>
>   https://github.com/multipath-tcp/packetdrill
>
> Otherwise, the official repo can be used:
>
>   https://github.com/google/packetdrill
>
> --
> Matthieu Baerts (NGI0) <[email protected]>

Thank you very much for your review. After receiving your suggestions,
I asked Wyatt to prepare a packetdrill version of the PoC for this
bug, but it is not ready yet. I apologize for taking up some of your
time on this review.

For future patches, whenever possible, we will include a packetdrill
PoC in the cover letter and send it to the public mailing list. We
will also use a new Reported-by: Vega tag instead of the tag we are
currently using.