mptcp: fix connect timeout handling

Ondrej reported a functional issue WRT timeout handling on connect
with a nice reproducer.

The problem is that the current mptcp connect waits for both the
MPTCP socket level timeout, and the first subflow socket timeout.
The latter is not influenced/touched by the exposed setsockopt().

Overall the above makes the SO_SNDTIMEO a no-op on connect.

Since mptcp_connect is invoked via inet_stream_connect and the
latter properly handle the MPTCP level timeout, we can address the
issue making the nested subflow level connect always unblocking.

This also allow simplifying a bit the code, dropping an ugly hack
to handle the fastopen and custom proto_ops connect.

The issues predates the blamed commit below, but the current resolution
requires the infrastructure introduced there.

Fixes: 54f1944 ("mptcp: factor out mptcp_connect()")
Reported-by: Ondrej Mosnacek <omosnace@redhat.com>
Closes: multipath-tcp/mptcp_net-next#399
Cc: stable@vger.kernel.org
Reviewed-by: Mat Martineau <martineau@kernel.org>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Mat Martineau <martineau@kernel.org>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Author: Paolo Abeni

net/mptcp/protocol.c

@@ -1702,7 +1702,6 @@ static int mptcp_sendmsg_fastopen(struct sock *sk, struct msghdr *msg,
 
 	lock_sock(ssk);
 	msg->msg_flags |= MSG_DONTWAIT;
-	msk->connect_flags = O_NONBLOCK;
 	msk->fastopening = 1;
 	ret = tcp_sendmsg_fastopen(ssk, msg, copied_syn, len, NULL);
 	msk->fastopening = 0;
@@ -3617,9 +3616,9 @@ static int mptcp_connect(struct sock *sk, struct sockaddr *uaddr, int addr_len)
 	 * acquired the subflow socket lock, too.
 	 */
 	if (msk->fastopening)
-		err = __inet_stream_connect(ssock, uaddr, addr_len, msk->connect_flags, 1);
+		err = __inet_stream_connect(ssock, uaddr, addr_len, O_NONBLOCK, 1);
 	else
-		err = inet_stream_connect(ssock, uaddr, addr_len, msk->connect_flags);
+		err = inet_stream_connect(ssock, uaddr, addr_len, O_NONBLOCK);
 	inet_sk(sk)->defer_connect = inet_sk(ssock->sk)->defer_connect;
 
 	/* on successful connect, the msk state will be moved to established by
@@ -3632,12 +3631,10 @@ static int mptcp_connect(struct sock *sk, struct sockaddr *uaddr, int addr_len)
 
 	mptcp_copy_inaddrs(sk, ssock->sk);
 
-	/* unblocking connect, mptcp-level inet_stream_connect will error out
-	 * without changing the socket state, update it here.
+	/* silence EINPROGRESS and let the caller inet_stream_connect
+	 * handle the connection in progress
 	 */
-	if (err == -EINPROGRESS)
-		sk->sk_socket->state = ssock->state;
-	return err;
+	return 0;
 }
 
 static struct proto mptcp_prot = {
@@ -3696,18 +3693,6 @@ static int mptcp_bind(struct socket *sock, struct sockaddr *uaddr, int addr_len)
 	return err;
 }
 
-static int mptcp_stream_connect(struct socket *sock, struct sockaddr *uaddr,
-				int addr_len, int flags)
-{
-	int ret;
-
-	lock_sock(sock->sk);
-	mptcp_sk(sock->sk)->connect_flags = flags;
-	ret = __inet_stream_connect(sock, uaddr, addr_len, flags, 0);
-	release_sock(sock->sk);
-	return ret;
-}
-
 static int mptcp_listen(struct socket *sock, int backlog)
 {
 	struct mptcp_sock *msk = mptcp_sk(sock->sk);
@@ -3859,7 +3844,7 @@ static const struct proto_ops mptcp_stream_ops = {
 	.owner		   = THIS_MODULE,
 	.release	   = inet_release,
 	.bind		   = mptcp_bind,
-	.connect	   = mptcp_stream_connect,
+	.connect	   = inet_stream_connect,
 	.socketpair	   = sock_no_socketpair,
 	.accept		   = mptcp_stream_accept,
 	.getname	   = inet_getname,
@@ -3954,7 +3939,7 @@ static const struct proto_ops mptcp_v6_stream_ops = {
 	.owner		   = THIS_MODULE,
 	.release	   = inet6_release,
 	.bind		   = mptcp_bind,
-	.connect	   = mptcp_stream_connect,
+	.connect	   = inet_stream_connect,
 	.socketpair	   = sock_no_socketpair,
 	.accept		   = mptcp_stream_accept,
 	.getname	   = inet6_getname,

net/mptcp/protocol.h

@@ -297,7 +297,6 @@ struct mptcp_sock {
 			nodelay:1,
 			fastopening:1,
 			in_accept_queue:1;
-	int		connect_flags;
 	struct work_struct work;
 	struct sk_buff  *ooo_last_skb;
 	struct rb_root  out_of_order_queue;