af_unix: Set drop reason in manage_oob().

AF_UNIX SOCK_STREAM socket supports MSG_OOB.

When OOB data is sent to a socket, recv() will break at that point.

If the next recv() does not have MSG_OOB, the normal data following
the OOB data is returned.

Then, the OOB skb is dropped.

Let's define a new drop reason for that case in manage_oob().

  # echo 1 > /sys/kernel/tracing/events/skb/kfree_skb/enable

  # python3
  >>> from socket import *
  >>> s1, s2 = socketpair(AF_UNIX)
  >>> s1.send(b'a', MSG_OOB)
  >>> s1.send(b'b')
  >>> s2.recv(2)
  b'b'

  # cat /sys/kernel/tracing/trace_pipe
  ...
     python3-223 ... kfree_skb: ... location=unix_stream_read_generic+0x59e/0xc20 reason: UNIX_SKIP_OOB

Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Link: https://patch.msgid.link/20250116053441.5758-6-kuniyu@amazon.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

include/net/dropreason-core.h

@@ -9,6 +9,7 @@
 	FN(SOCKET_CLOSE)		\
 	FN(SOCKET_FILTER)		\
 	FN(SOCKET_RCVBUFF)		\
+	FN(UNIX_SKIP_OOB)		\
 	FN(PKT_TOO_SMALL)		\
 	FN(TCP_CSUM)			\
 	FN(UDP_CSUM)			\
@@ -145,6 +146,11 @@ enum skb_drop_reason {
 	SKB_DROP_REASON_SOCKET_FILTER,
 	/** @SKB_DROP_REASON_SOCKET_RCVBUFF: socket receive buff is full */
 	SKB_DROP_REASON_SOCKET_RCVBUFF,
+	/**
+	 * @SKB_DROP_REASON_UNIX_SKIP_OOB: Out-Of-Band data is skipped by
+	 * recv() without MSG_OOB so dropped.
+	 */
+	SKB_DROP_REASON_UNIX_SKIP_OOB,
 	/** @SKB_DROP_REASON_PKT_TOO_SMALL: packet size is too small */
 	SKB_DROP_REASON_PKT_TOO_SMALL,
 	/** @SKB_DROP_REASON_TCP_CSUM: TCP checksum error */

net/unix/af_unix.c

@@ -2695,7 +2695,7 @@ static struct sk_buff *manage_oob(struct sk_buff *skb, struct sock *sk,
 	spin_unlock(&sk->sk_receive_queue.lock);
 
 	consume_skb(read_skb);
-	kfree_skb(unread_skb);
+	kfree_skb_reason(unread_skb, SKB_DROP_REASON_UNIX_SKIP_OOB);
 
 	return skb;
 }