mptcp: poll allow write call before actual connect

If fastopen is used, poll must allow a first write that will trigger
the SYN+data

Similar to what is done in tcp_poll().
Acked-by: Paolo Abeni <pabeni@redhat.com>
Reviewed-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: Benjamin Hesmans <benjamin.hesmans@tessares.net>
Signed-off-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

net/mptcp/protocol.c

@@ -3548,6 +3548,7 @@ static int mptcp_stream_connect(struct socket *sock, struct sockaddr *uaddr,
 
 do_connect:
 	err = ssock->ops->connect(ssock, uaddr, addr_len, flags);
+	inet_sk(sock->sk)->defer_connect = inet_sk(ssock->sk)->defer_connect;
 	sock->state = ssock->state;
 
 	/* on successful connect, the msk state will be moved to established by
@@ -3698,6 +3699,9 @@ static __poll_t mptcp_poll(struct file *file, struct socket *sock,
 	if (state != TCP_SYN_SENT && state != TCP_SYN_RECV) {
 		mask |= mptcp_check_readable(msk);
 		mask |= mptcp_check_writeable(msk);
+	} else if (state == TCP_SYN_SENT && inet_sk(sk)->defer_connect) {
+		/* cf tcp_poll() note about TFO */
+		mask |= EPOLLOUT | EPOLLWRNORM;
 	}
 	if (sk->sk_shutdown == SHUTDOWN_MASK || state == TCP_CLOSE)
 		mask |= EPOLLHUP;