af_unix: Define locking order for U_LOCK_SECOND in unix_stream_connect().

While a SOCK_(STREAM|SEQPACKET) socket connect()s to another, we hold
two locks of them by unix_state_lock() and unix_state_lock_nested() in
unix_stream_connect().

Before unix_state_lock_nested(), the following is guaranteed by checking
sk->sk_state:

  1. The first socket is TCP_LISTEN
  2. The second socket is not the first one
  3. Simultaneous connect() must fail

So, the client state can be TCP_CLOSE or TCP_LISTEN or TCP_ESTABLISHED.

Let's define the expected states as unix_state_lock_cmp_fn() instead of
using unix_state_lock_nested().

Note that 2. is detected by debug_spin_lock_before() and 3. cannot be
expressed as lock_cmp_fn.
Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

include/net/af_unix.h

@@ -98,7 +98,6 @@ struct unix_sock {
 #define unix_state_unlock(s)	spin_unlock(&unix_sk(s)->lock)
 enum unix_socket_lock_class {
 	U_LOCK_NORMAL,
-	U_LOCK_SECOND,	/* for double locking, see unix_state_double_lock(). */
 	U_LOCK_DIAG, /* used while dumping icons, see sk_diag_dump_icons(). */
 	U_LOCK_GC_LISTENER, /* used for listening socket while determining gc
 			     * candidates to close a small race window.

net/unix/af_unix.c

@@ -143,6 +143,41 @@ static int unix_state_lock_cmp_fn(const struct lockdep_map *_a,
 	a = container_of(_a, struct unix_sock, lock.dep_map);
 	b = container_of(_b, struct unix_sock, lock.dep_map);
 
+	if (a->sk.sk_state == TCP_LISTEN) {
+		/* unix_stream_connect(): Before the 2nd unix_state_lock(),
+		 *
+		 *   1. a is TCP_LISTEN.
+		 *   2. b is not a.
+		 *   3. concurrent connect(b -> a) must fail.
+		 *
+		 * Except for 2. & 3., the b's state can be any possible
+		 * value due to concurrent connect() or listen().
+		 *
+		 * 2. is detected in debug_spin_lock_before(), and 3. cannot
+		 * be expressed as lock_cmp_fn.
+		 */
+		switch (b->sk.sk_state) {
+		case TCP_CLOSE:
+		case TCP_ESTABLISHED:
+		case TCP_LISTEN:
+			return -1;
+		default:
+			/* Invalid case. */
+			return 0;
+		}
+	}
+
+	/* Should never happen.  Just to be symmetric. */
+	if (b->sk.sk_state == TCP_LISTEN) {
+		switch (b->sk.sk_state) {
+		case TCP_CLOSE:
+		case TCP_ESTABLISHED:
+			return 1;
+		default:
+			return 0;
+		}
+	}
+
 	/* unix_state_double_lock(): ascending address order. */
 	return cmp_ptr(a, b);
 }
@@ -1585,7 +1620,7 @@ static int unix_stream_connect(struct socket *sock, struct sockaddr *uaddr,
 		goto out_unlock;
 	}
 
-	unix_state_lock_nested(sk, U_LOCK_SECOND);
+	unix_state_lock(sk);
 
 	if (unlikely(sk->sk_state != TCP_CLOSE)) {
 		err = sk->sk_state == TCP_ESTABLISHED ? -EISCONN : -EINVAL;