Re: [PATCH mptcp-next RFC 2/3] mptcp: reject MP_JOIN earlier

[Matthieu Baerts <matttbe@kernel.org>]

Hi Paolo,

Thank you for having looked at this!

On 17/08/2026 09:27, Paolo Abeni wrote:
> On 8/14/26 8:31 PM, Matthieu Baerts (NGI0) wrote:
>> @@ -91,12 +92,22 @@ static struct mptcp_sock *subflow_token_join_request(struct request_sock *req)
>>  	msk = mptcp_token_get_sock(sock_net(req_to_sk(req)), subflow_req->token);
>>  	if (!msk) {
>>  		SUBFLOW_REQ_INC_STATS(req, MPTCP_MIB_JOINNOTOKEN);
>> +		*reason = MPTCP_RST_EMPTCP;
>> +		return NULL;
>> +	}
>> +
>> +	/* Stop it early if the subflow cannot be accepted */
>> +	if (!mptcp_can_accept_new_subflow(msk)) {
> 
> Side note: the above is inherently racy, the consistency safeguard is
> given by the fallback check at finish_join time.

Just to be sure: even if it is racy, is it OK to use it here but
continuing having the safe check at finish_join time?

In other words, having such a comment is enough?

  /* Stop it early if the subflow cannot be accepted.
   * This check is inherently racy, the consistency safeguard is
   * given by the fallback check at finish_join time.
   */

> Minor nit: I'm not sure this check belongs here or in
> subflow_check_req(). The latter option would avoid the additional
> argument and possibly produce a smaller diff (and smaller code).

After re-looking at this code yesterday, I agree with you. Also, I think
subflow_token_join_request() should only return the msk, and not
changing anything else from subflow_req.

Cheers,
Matt
-- 
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