af_unix: Remove dead code in unix_stream_read_generic().

When splice() support was added in commit 2b514574 ("net:
af_unix: implement splice for stream af_unix sockets"), we had
to release unix_sk(sk)->readlock (current iolock) before calling
splice_to_pipe().

Due to the unlock, commit 73ed5d25 ("af-unix: fix use-after-free
with concurrent readers while splicing") added a safeguard in
unix_stream_read_generic(); we had to bump the skb refcount before
calling ->recv_actor() and then check if the skb was consumed by a
concurrent reader.

However, the pipe side locking was refactored, and since commit
25869262 ("skb_splice_bits(): get rid of callback"), we can
call splice_to_pipe() without releasing unix_sk(sk)->iolock.

Now, the skb is always alive after the ->recv_actor() callback,
so let's remove the unnecessary drop_skb logic.

This is mostly the revert of commit 73ed5d25 ("af-unix: fix
use-after-free with concurrent readers while splicing").
Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Link: https://lore.kernel.org/r/20240529144648.68591-1-kuniyu@amazon.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>

net/unix/af_unix.c

@@ -654,8 +654,8 @@ static void unix_release_sock(struct sock *sk, int embrion)
 	while ((skb = skb_dequeue(&sk->sk_receive_queue)) != NULL) {
 		if (state == TCP_LISTEN)
 			unix_release_sock(skb->sk, 1);
+
 		/* passed fds are erased in the kfree_skb hook	      */
-		UNIXCB(skb).consumed = skb->len;
 		kfree_skb(skb);
 	}
 
@@ -2704,9 +2704,8 @@ static int unix_stream_read_generic(struct unix_stream_read_state *state,
 	skip = max(sk_peek_offset(sk, flags), 0);
 
 	do {
-		int chunk;
-		bool drop_skb;
 		struct sk_buff *skb, *last;
+		int chunk;
 
 redo:
 		unix_state_lock(sk);
@@ -2802,11 +2801,7 @@ static int unix_stream_read_generic(struct unix_stream_read_state *state,
 		}
 
 		chunk = min_t(unsigned int, unix_skb_len(skb) - skip, size);
-		skb_get(skb);
 		chunk = state->recv_actor(skb, skip, chunk, state);
-		drop_skb = !unix_skb_len(skb);
-		/* skb is only safe to use if !drop_skb */
-		consume_skb(skb);
 		if (chunk < 0) {
 			if (copied == 0)
 				copied = -EFAULT;
@@ -2815,18 +2810,6 @@ static int unix_stream_read_generic(struct unix_stream_read_state *state,
 		copied += chunk;
 		size -= chunk;
 
-		if (drop_skb) {
-			/* the skb was touched by a concurrent reader;
-			 * we should not expect anything from this skb
-			 * anymore and assume it invalid - we can be
-			 * sure it was dropped from the socket queue
-			 *
-			 * let's report a short read
-			 */
-			err = 0;
-			break;
-		}
-
 		/* Mark read part of skb as used */
 		if (!(flags & MSG_PEEK)) {
 			UNIXCB(skb).consumed += chunk;