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@amazon.com>
Link: https://patch.msgid.link/20250116053441.5758-5-kuniyu@amazon.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
c49a157c33