Re: [PATCH v2 mptcp-net 1/2] mptcp: do not fallback when OoO is present

[Matthieu Baerts <matttbe@kernel.org>]

Hi Paolo,

On 11/11/2025 08:24, Paolo Abeni wrote:
> In case of DSS corruption, the MPTCP protocol tries to avoid the
> subflow reset if fallback is possible. Such corruptions happen in
> the receive path; to ensure fallback is possible the stack additionally
> need to check for OoO data, otherwise the fallback will break the data
> stream.

Thank you for the fix!
> Fixes: e32d262c89e2 ("mptcp: handle consistently DSS corruption")
> Closes: https://github.com/multipath-tcp/mptcp_net-next/issues/598
> Signed-off-by: Paolo Abeni <pabeni@redhat.com>
> ---
> Note: this does not avoid the WARN(), but fixes the inconsistend
> read() behavior; the ingress data is OoO, we should not ack it
> ---
>  net/mptcp/protocol.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/net/mptcp/protocol.c b/net/mptcp/protocol.c
> index d6b08e1de358..7b966f105f89 100644
> --- a/net/mptcp/protocol.c
> +++ b/net/mptcp/protocol.c
> @@ -646,7 +646,8 @@ static void mptcp_check_data_fin(struct sock *sk)
>  
>  static void mptcp_dss_corruption(struct mptcp_sock *msk, struct sock *ssk)
>  {
> -	if (!mptcp_try_fallback(ssk, MPTCP_MIB_DSSCORRUPTIONFALLBACK)) {
> +	if (!RB_EMPTY_ROOT(&msk->out_of_order_queue) ||
> +	    !mptcp_try_fallback(ssk, MPTCP_MIB_DSSCORRUPTIONFALLBACK)) {

Does it mean we should check the OoO queue each time mptcp_try_fallback
is called?

Should we not eventually set msk->allow_infinite_fallback to false in
mptcp_data_queue_ofo()?

>  		MPTCP_INC_STATS(sock_net(ssk), MPTCP_MIB_DSSCORRUPTIONRESET);
>  		mptcp_subflow_reset(ssk);
>  	}

Cheers,
Matt
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