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

On 8/12/26 7:46 AM, Chenguang Zhao wrote:
> From: Chenguang Zhao <[email protected]>
> 
> After fallback, treat the connection as not fully established so later
> MP_JOIN attempts are rejected.
> 
> Signed-off-by: Chenguang Zhao <[email protected]>
> ---
>  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