mptcp: move fastopen subflow check inside mptcp_sendmsg_fastopen()

So that we can avoid a bunch of check in fastpath. Additionally we
can specialize such check according to the specific fastopen method
- defer_connect vs MSG_FASTOPEN.

The latter bits will simplify the next patches.
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Reviewed-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/mptcp/protocol.c

@@ -1662,13 +1662,27 @@ static void mptcp_set_nospace(struct sock *sk)
 
 static int mptcp_disconnect(struct sock *sk, int flags);
 
-static int mptcp_sendmsg_fastopen(struct sock *sk, struct sock *ssk, struct msghdr *msg,
+static int mptcp_sendmsg_fastopen(struct sock *sk, struct msghdr *msg,
 				  size_t len, int *copied_syn)
 {
 	unsigned int saved_flags = msg->msg_flags;
 	struct mptcp_sock *msk = mptcp_sk(sk);
+	struct sock *ssk;
 	int ret;
 
+	/* on flags based fastopen the mptcp is supposed to create the
+	 * first subflow right now. Otherwise we are in the defer_connect
+	 * path, and the first subflow must be already present.
+	 * Since the defer_connect flag is cleared after the first succsful
+	 * fastopen attempt, no need to check for additional subflow status.
+	 */
+	if (msg->msg_flags & MSG_FASTOPEN && !__mptcp_nmpc_socket(msk))
+		return -EINVAL;
+	if (!msk->first)
+		return -EINVAL;
+
+	ssk = msk->first;
+
 	lock_sock(ssk);
 	msg->msg_flags |= MSG_DONTWAIT;
 	msk->connect_flags = O_NONBLOCK;
@@ -1691,6 +1705,7 @@ static int mptcp_sendmsg_fastopen(struct sock *sk, struct sock *ssk, struct msgh
 	} else if (ret && ret != -EINPROGRESS) {
 		mptcp_disconnect(sk, 0);
 	}
+	inet_sk(sk)->defer_connect = 0;
 
 	return ret;
 }
@@ -1699,7 +1714,6 @@ static int mptcp_sendmsg(struct sock *sk, struct msghdr *msg, size_t len)
 {
 	struct mptcp_sock *msk = mptcp_sk(sk);
 	struct page_frag *pfrag;
-	struct socket *ssock;
 	size_t copied = 0;
 	int ret = 0;
 	long timeo;
@@ -1709,12 +1723,10 @@ static int mptcp_sendmsg(struct sock *sk, struct msghdr *msg, size_t len)
 
 	lock_sock(sk);
 
-	ssock = __mptcp_nmpc_socket(msk);
-	if (unlikely(ssock && (inet_sk(ssock->sk)->defer_connect ||
-			       msg->msg_flags & MSG_FASTOPEN))) {
+	if (unlikely(inet_sk(sk)->defer_connect || msg->msg_flags & MSG_FASTOPEN)) {
 		int copied_syn = 0;
 
-		ret = mptcp_sendmsg_fastopen(sk, ssock->sk, msg, len, &copied_syn);
+		ret = mptcp_sendmsg_fastopen(sk, msg, len, &copied_syn);
 		copied += copied_syn;
 		if (ret == -EINPROGRESS && copied_syn > 0)
 			goto out;