Re: [PATCH mptcp-net 1/2] mptcp: reset msk state on early connect failure

[Paolo Abeni <pabeni@redhat.com>]

On 9/10/26 2:23 PM, Quanye Yang via B4 Relay wrote:
> mptcp_connect() can fall back before the subflow SYN is sent, for
> example when the netns is in an MPTCP blackhole, token allocation
> fails, or MD5SIG is in use. If the subsequent subflow connect()
> fails immediately (EAFNOSUPPORT, ENETUNREACH, ...),
> __inet_stream_connect() returns while the socket is still
> SS_UNCONNECTED and never calls ->disconnect().
> 
> The error path only dropped the token and moved the msk back to
> TCP_CLOSE. MPTCP_FALLBACK_DONE, allow_subflows and
> request_mptcp were left as after early fallback, so a later
> connect() on the same fd stayed TCP-only.
> 
> Closes: https://github.com/multipath-tcp/mptcp_net-next/issues/633
> Fixes: 0235d075a592 ("mptcp: mark as fallback even early ones")
> Signed-off-by: Quanye Yang <quanyeyang@proton.me>
> ---
>  net/mptcp/protocol.c | 22 +++++++++++++++++++---
>  1 file changed, 19 insertions(+), 3 deletions(-)
> 
> diff --git a/net/mptcp/protocol.c b/net/mptcp/protocol.c
> index e1f08f71cdb1..ad476d29bf3d 100644
> --- a/net/mptcp/protocol.c
> +++ b/net/mptcp/protocol.c
> @@ -4129,6 +4129,24 @@ static int mptcp_ioctl(struct sock *sk, int cmd, int *karg)
>  	return 0;
>  }
>  
> +static void mptcp_connect_undo(struct sock *sk)
> +{
> +	struct mptcp_sock *msk = mptcp_sk(sk);
> +	struct sock *ssk = msk->first;
> +
> +	mptcp_token_destroy(msk);
> +	mptcp_set_state(sk, TCP_CLOSE);
> +
> +	spin_lock_bh(&msk->fallback_lock);
> +	msk->allow_subflows = true;
> +	msk->allow_infinite_fallback = true;
> +	clear_bit(MPTCP_FALLBACK_DONE, &msk->flags);
> +	spin_unlock_bh(&msk->fallback_lock);
> +
> +	if (ssk)
> +		mptcp_subflow_ctx_reset(mptcp_subflow_ctx(ssk));

Overall LGTM, but I think ssk should always be != NULL. Also it possibly
make sense to avoid entirely the additional helper, just open code it
into the mptcp_connect() error path.

/P