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

[Chenguang Zhao <chenguang.zhao@linux.dev>]

在 2026/8/12 17:45, Paolo Abeni 写道:
> 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.
>>
>> 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().
>
> I would prefer avoiding additional conditionals, if not well reasoned.
>
> /P
>
Hi Paolo

You are right: the extra FALLBACK_DONE check in
mptcp_is_fully_established() does not actually improve anything.
It is read without fallback_lock, so it is racy. The paths that must
not complete a join after MP_FAIL already reject it under that lock
via allow_subflows, e.g. mptcp_finish_join() / __mptcp_finish_join().
I will drop this patch in the next revision and leave
mptcp_is_fully_established() unchanged.

Thanks,
Chenguang