netdev - [PATCH v1 net-next 04/12] af_unix: Set drop reason in __unix

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Message-ID: <20250110092641.85905-5-kuniyu@amazon.com>
Date: Fri, 10 Jan 2025 18:26:33 +0900
From: Kuniyuki Iwashima <kuniyu@...zon.com>
To: "David S. Miller" <davem@...emloft.net>, Eric Dumazet
	<edumazet@...gle.com>, Jakub Kicinski <kuba@...nel.org>, Paolo Abeni
	<pabeni@...hat.com>, Simon Horman <horms@...nel.org>
CC: Donald Hunter <donald.hunter@...hat.com>, Kuniyuki Iwashima
	<kuniyu@...zon.com>, Kuniyuki Iwashima <kuni1840@...il.com>,
	<netdev@...r.kernel.org>
Subject: [PATCH v1 net-next 04/12] af_unix: Set drop reason in __unix_gc().

Inflight file descriptors by SCM_RIGHTS hold references to the
struct file.

AF_UNIX sockets could hold references to each other, forming
reference cycles.

Once such sockets are close()d without the fd recv()ed, they
will be unaccessible from userspace but remain in kernel.

__unix_gc() garbage-collects skb with the dead file descriptors
and frees them by __skb_queue_purge().

Let's set SKB_DROP_REASON_SOCKET_CLOSE there.

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

  # python3
  >>> from socket import *
  >>> from array import array
  >>>
  >>> # Create a reference cycle
  >>> s1 = socket(AF_UNIX, SOCK_DGRAM)
  >>> s1.bind('')
  >>> s1.sendmsg([b"nop"], [(SOL_SOCKET, SCM_RIGHTS, array("i", [s1.fileno()]))], 0, s1.getsockname())
  >>> s1.close()
  >>>
  >>> # Trigger GC
  >>> s2 = socket(AF_UNIX)
  >>> s2.close()

  # cat /sys/kernel/tracing/trace_pipe
  ...
     kworker/u16:2-42 ... kfree_skb: ... location=__unix_gc+0x4ad/0x580 reason: SOCKET_CLOSE

Signed-off-by: Kuniyuki Iwashima <kuniyu@...zon.com>
---
 net/unix/garbage.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/net/unix/garbage.c b/net/unix/garbage.c
index 0068e758be4d..9848b7b78701 100644
--- a/net/unix/garbage.c
+++ b/net/unix/garbage.c
@@ -573,7 +573,7 @@ static void __unix_gc(struct work_struct *work)
 			UNIXCB(skb).fp->dead = true;
 	}
 
-	__skb_queue_purge(&hitlist);
+	__skb_queue_purge_reason(&hitlist, SKB_DROP_REASON_SOCKET_CLOSE);
 skip_gc:
 	WRITE_ONCE(gc_in_progress, false);
 }
-- 
2.39.5 (Apple Git-154)