af_unix: Don't call skb_get() for OOB skb.

Since introduced, OOB skb holds an additional reference count with no
special reason and caused many issues.

Also, kfree_skb() and consume_skb() are used to decrement the count,
which is confusing.

Let's drop the unnecessary skb_get() in queue_oob() and corresponding
kfree_skb(), consume_skb(), and skb_unref().

Now unix_sk(sk)->oob_skb is just a pointer to skb in the receive queue,
so special handing is no longer needed in GC.
Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Link: https://patch.msgid.link/20240816233921.57800-1-kuniyu@amazon.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>

net/unix/af_unix.c

@@ -693,10 +693,7 @@ static void unix_release_sock(struct sock *sk, int embrion)
 	unix_state_unlock(sk);
 
 #if IS_ENABLED(CONFIG_AF_UNIX_OOB)
-	if (u->oob_skb) {
-		kfree_skb(u->oob_skb);
-		u->oob_skb = NULL;
-	}
+	u->oob_skb = NULL;
 #endif
 
 	wake_up_interruptible_all(&u->peer_wait);
@@ -2226,13 +2223,9 @@ static int queue_oob(struct socket *sock, struct msghdr *msg, struct sock *other
 	}
 
 	maybe_add_creds(skb, sock, other);
-	skb_get(skb);
-
 	scm_stat_add(other, skb);
 
 	spin_lock(&other->sk_receive_queue.lock);
-	if (ousk->oob_skb)
-		consume_skb(ousk->oob_skb);
 	WRITE_ONCE(ousk->oob_skb, skb);
 	__skb_queue_tail(&other->sk_receive_queue, skb);
 	spin_unlock(&other->sk_receive_queue.lock);
@@ -2640,8 +2633,6 @@ static int unix_stream_recv_urg(struct unix_stream_read_state *state)
 
 	if (!(state->flags & MSG_PEEK))
 		WRITE_ONCE(u->oob_skb, NULL);
-	else
-		skb_get(oob_skb);
 
 	spin_unlock(&sk->sk_receive_queue.lock);
 	unix_state_unlock(sk);
@@ -2651,8 +2642,6 @@ static int unix_stream_recv_urg(struct unix_stream_read_state *state)
 	if (!(state->flags & MSG_PEEK))
 		UNIXCB(oob_skb).consumed += 1;
 
-	consume_skb(oob_skb);
-
 	mutex_unlock(&u->iolock);
 
 	if (chunk < 0)
@@ -2694,12 +2683,10 @@ static struct sk_buff *manage_oob(struct sk_buff *skb, struct sock *sk,
 			if (copied) {
 				skb = NULL;
 			} else if (!(flags & MSG_PEEK)) {
-				if (sock_flag(sk, SOCK_URGINLINE)) {
-					WRITE_ONCE(u->oob_skb, NULL);
-					consume_skb(skb);
-				} else {
+				WRITE_ONCE(u->oob_skb, NULL);
+
+				if (!sock_flag(sk, SOCK_URGINLINE)) {
 					__skb_unlink(skb, &sk->sk_receive_queue);
-					WRITE_ONCE(u->oob_skb, NULL);
 					unlinked_skb = skb;
 					skb = skb_peek(&sk->sk_receive_queue);
 				}
@@ -2710,10 +2697,7 @@ static struct sk_buff *manage_oob(struct sk_buff *skb, struct sock *sk,
 
 		spin_unlock(&sk->sk_receive_queue.lock);
 
-		if (unlinked_skb) {
-			WARN_ON_ONCE(skb_unref(unlinked_skb));
-			kfree_skb(unlinked_skb);
-		}
+		kfree_skb(unlinked_skb);
 	}
 	return skb;
 }
@@ -2756,7 +2740,6 @@ static int unix_stream_read_skb(struct sock *sk, skb_read_actor_t recv_actor)
 		unix_state_unlock(sk);
 
 		if (drop) {
-			WARN_ON_ONCE(skb_unref(skb));
 			kfree_skb(skb);
 			return -EAGAIN;
 		}

net/unix/garbage.c

@@ -337,18 +337,6 @@ static bool unix_vertex_dead(struct unix_vertex *vertex)
 	return true;
 }
 
-static void unix_collect_queue(struct unix_sock *u, struct sk_buff_head *hitlist)
-{
-	skb_queue_splice_init(&u->sk.sk_receive_queue, hitlist);
-
-#if IS_ENABLED(CONFIG_AF_UNIX_OOB)
-	if (u->oob_skb) {
-		WARN_ON_ONCE(skb_unref(u->oob_skb));
-		u->oob_skb = NULL;
-	}
-#endif
-}
-
 static void unix_collect_skb(struct list_head *scc, struct sk_buff_head *hitlist)
 {
 	struct unix_vertex *vertex;
@@ -371,11 +359,11 @@ static void unix_collect_skb(struct list_head *scc, struct sk_buff_head *hitlist
 				struct sk_buff_head *embryo_queue = &skb->sk->sk_receive_queue;
 
 				spin_lock(&embryo_queue->lock);
-				unix_collect_queue(unix_sk(skb->sk), hitlist);
+				skb_queue_splice_init(embryo_queue, hitlist);
 				spin_unlock(&embryo_queue->lock);
 			}
 		} else {
-			unix_collect_queue(u, hitlist);
+			skb_queue_splice_init(queue, hitlist);
 		}
 
 		spin_unlock(&queue->lock);