Re: [PATCH v2 net 01/15] af_unix: Set sk->sk_state under unix_state_lock() for truly disconencted peer.

[Kuniyuki Iwashima <kuniyu@amazon.com>]

From: Michal Luczaj <mhal@rbox.co>
Date: Thu, 20 Jun 2024 22:35:55 +0200
> In fact, should I try to document those not-so-obvious OOB/sockmap
> interaction? And speaking of documentation, an astute reader noted that
> `man unix` is lying:

At least I wouldn't update man until we can say AF_UNIX MSG_OOB handling
is stable enough; the behaviour is not compliant with TCP now.

While rewriting the oob test thoroughly, I found few more weird behaviours,
and patches will follow.

For example:

  >>> from socket import *
  >>> c1, c2 = socketpair(AF_UNIX)
  >>> c1.send(b'hello', MSG_OOB)
  5
  >>> c1.send(b'world')
  5
  >>> c2.recv(10)
  b'hell'
  >>> c2.recv(1, MSG_OOB)
  b'o'
  >>> c2.setblocking(False)  # This causes -EAGAIN even with available data
  >>> c2.recv(5)
  Traceback (most recent call last):
    File "<stdin>", line 1, in <module>
  BlockingIOError: [Errno 11] Resource temporarily unavailable
  >>> c2.recv(5)
  b'world'


And we need

---8<---
diff --git a/net/unix/af_unix.c b/net/unix/af_unix.c
index 5e695a9a609c..2875a7ce1887 100644
--- a/net/unix/af_unix.c
+++ b/net/unix/af_unix.c
@@ -2614,9 +2614,20 @@ static struct sk_buff *manage_oob(struct sk_buff *skb, struct sock *sk,
 	struct unix_sock *u = unix_sk(sk);
 
 	if (!unix_skb_len(skb) && !(flags & MSG_PEEK)) {
-		skb_unlink(skb, &sk->sk_receive_queue);
-		consume_skb(skb);
-		skb = NULL;
+		struct sk_buff *unlinked_skb = skb;
+
+		spin_lock(&sk->sk_receive_queue.lock);
+
+		__skb_unlink(skb, &sk->sk_receive_queue);
+
+		if (copied)
+			skb = NULL;
+		else
+			skb = skb_peek(&sk->sk_receive_queue);
+
+		spin_unlock(&sk->sk_receive_queue.lock);
+
+		consume_skb(unlinked_skb);
 	} else {
 		struct sk_buff *unlinked_skb = NULL;
 
---8<---