Re: [PATCH export v3 2/4] mptcp: reject joins after fallback in mptcp_is_fully_established

[Matthieu Baerts <matttbe@kernel.org>]

Hi Paolo, Chenguang,

On 12/08/2026 11:45, Paolo Abeni wrote:
> On 8/12/26 7:46 AM, Chenguang Zhao wrote:
>> From: Chenguang Zhao <zhaochenguang@kylinos.cn>
>>
>> After fallback, treat the connection as not fully established so later
>> MP_JOIN attempts are rejected.

@Chenguang: please first reply to the questions and comments from the
previous versions, then only send a new version when the discussions are
over.

>> Signed-off-by: Chenguang Zhao <zhaochenguang@kylinos.cn>
>> ---
>>  net/mptcp/protocol.h | 5 ++++-
>>  1 file changed, 4 insertions(+), 1 deletion(-)
>>
>> diff --git a/net/mptcp/protocol.h b/net/mptcp/protocol.h
>> index 7e168e450fb0..bf2483a7ed92 100644
>> --- a/net/mptcp/protocol.h
>> +++ b/net/mptcp/protocol.h
>> @@ -957,8 +957,11 @@ static inline void mptcp_start_tout_timer(struct sock *sk)
>>  
>>  static inline bool mptcp_is_fully_established(struct sock *sk)
>>  {
>> +	struct mptcp_sock *msk = mptcp_sk(sk);
>> +
>>  	return inet_sk_state_load(sk) == TCP_ESTABLISHED &&
>> -	       READ_ONCE(mptcp_sk(sk)->fully_established);
>> +	       READ_ONCE(msk->fully_established) &&
>> +	       !test_bit(MPTCP_FALLBACK_DONE, &msk->flags);
> 
> Does the above improve actually anything? The test is inherently racy,
> as lack the fallback_lock, and AFAICS all critical paths have already
> explicit checks under such lock, see i.e.  mptcp_finish_join().

Indeed, the fallback lock is missing.

Note: Chenguang is looking at a bug where, when an MP_FAIL is received
before being in fully_established, the kernel accept going to fully
established, and creating new subflows, then reject them once done.

Same when receiving a 4th packet without MPTCP options.

I think the MP_FAIL should do a fallback if possible, but it is probably
also required to check the fallback status or something similar when
receiving an MPJ.

> I would prefer avoiding additional conditionals, if not well reasoned.

Agreed, the commit messages in this series are not explaining (enough)
the reason and the context.

BTW, Chenguang, please use 'mptcp-net' for fixes (and mptcp-next for
features). Also fixes should have a Fixes tag.

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