nbd: Reject unconnected sockets in nbd_get_socket().

NBD requires a handshake, so passing unconnected or half-closed
sockets to NBD does not make sense.

Let's accept TCP_ESTABLISHED sockets only.

Note that AF_UNIX sockets remain in TCP_ESTABLISHED once connect()ed
regardless of shutdown(), but this is a prep patch for TCP, allowing
a subsequent patch to call tcp_sendmsg_locked() directly without extra
setup (e.g. inet_send_prepare()).

Signed-off-by: Kuniyuki Iwashima <[email protected]>

Author: q2ven

drivers/block/nbd.c

@@ -1228,6 +1228,13 @@ static struct socket *nbd_get_socket(struct nbd_device *nbd, unsigned long fd,
 		return NULL;
 	}
 
+	if (READ_ONCE(sock->sk->sk_state) != TCP_ESTABLISHED) {
+		dev_err(disk_to_dev(nbd->disk), "Socket does not have bi-directional stream.\n");
+		*err = -EPIPE;
+		sockfd_put(sock);
+		return NULL;
+	}
+
 	return sock;
 }