sctp: cancel a blocking accept when shutdown a listen socket

As David Laight noticed,

"In a multithreaded program it is reasonable to have a thread blocked in
 accept(). With TCP a subsequent shutdown(listen_fd, SHUT_RDWR) causes
 the accept to fail. But nothing happens for SCTP."

sctp_disconnect() is eventually called when shutdown a listen socket,
but nothing is done in this function. This patch sets RCV_SHUTDOWN
flag in sk->sk_shutdown there, and adds the check (sk->sk_shutdown &
RCV_SHUTDOWN) to break and return in sctp_accept().

Note that shutdown() is only supported on TCP-style SCTP socket.
Reported-by: David Laight <David.Laight@aculab.com>
Signed-off-by: Xin Long <lucien.xin@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/sctp/socket.c

@@ -4834,10 +4834,14 @@ int sctp_inet_connect(struct socket *sock, struct sockaddr *uaddr,
 	return sctp_connect(sock->sk, uaddr, addr_len, flags);
 }
 
-/* FIXME: Write comments. */
+/* Only called when shutdown a listening SCTP socket. */
 static int sctp_disconnect(struct sock *sk, int flags)
 {
-	return -EOPNOTSUPP; /* STUB */
+	if (!sctp_style(sk, TCP))
+		return -EOPNOTSUPP;
+
+	sk->sk_shutdown |= RCV_SHUTDOWN;
+	return 0;
 }
 
 /* 4.1.4 accept() - TCP Style Syntax
@@ -4866,7 +4870,8 @@ static struct sock *sctp_accept(struct sock *sk, struct proto_accept_arg *arg)
 		goto out;
 	}
 
-	if (!sctp_sstate(sk, LISTENING)) {
+	if (!sctp_sstate(sk, LISTENING) ||
+	    (sk->sk_shutdown & RCV_SHUTDOWN)) {
 		error = -EINVAL;
 		goto out;
 	}
@@ -9393,7 +9398,8 @@ static int sctp_wait_for_accept(struct sock *sk, long timeo)
 		}
 
 		err = -EINVAL;
-		if (!sctp_sstate(sk, LISTENING))
+		if (!sctp_sstate(sk, LISTENING) ||
+		    (sk->sk_shutdown & RCV_SHUTDOWN))
 			break;
 
 		err = 0;