sashiko-bot@kernel.org writes:

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [High] Garbage collector is no longer scheduled when creating new inflight FDs, leading to permanent cycle accumulation and -ETOOMANYREFS failures in isolated environments.
--

commit 506189bd29f8ad56f5d9e51f3e0216203ebad5ec
Author: Nam Cao [off-list ref]

af_unix: Do not wait for garbage collector in sendmsg()

This patch removes the garbage collector scheduling and waiting from the
sendmsg() path for AF_UNIX sockets. It aims to prevent priority inversion
by relying on RLIMIT_NOFILE limits and asynchronous garbage collection.

quoted

diff --git a/net/unix/garbage.c b/net/unix/garbage.c
index 0783555e25266..f180c59b3da9d 100644
--- a/net/unix/garbage.c
+++ b/net/unix/garbage.c

@@ -300,8 +300,6 @@ int unix_prepare_fpl(struct scm_fp_list *fpl)
 	if (!fpl->edges)
 		goto err;
 
-	unix_schedule_gc(fpl->user);
-

[Severity: High]
Does this removal prevent the garbage collector from running when a process
creates isolated socket cycles?

If a process continuously creates socket cycles (e.g., passing FDs to each
other) and closes its local FDs, the sockets reference each other in their
receive queues, so their file reference counts will not reach zero.

Because the refcounts do not drop to zero, unix_release_sock() is never
triggered. By removing unix_schedule_gc() from unix_prepare_fpl() here,
unix_release_sock() is left as the only trigger for garbage collection.

This could allow the inflight count to accumulate indefinitely until it
hits RLIMIT_NOFILE, causing subsequent sendmsg() calls to fail with
-ETOOMANYREFS and permanently leaking the network namespace unless an
unrelated AF_UNIX socket is closed system-wide.

Sashiko found a valid issue here. But this is a pre-existing issue. It
is not introduced in this patch.

unix_schedule_gc() does not actually schedule the garbage collector
unless user's inflight unix socket count exceeds UNIX_INFLIGHT_SANE_USER
(2024). So user can already accumulate up to 2023 inflight socket counts
without the garbage collector running until unix_release_sock() is called.

Nam