kernel/linux.git/fs/eventpoll.c, branch v5.15.209

eventpoll: defer struct eventpoll free to RCU grace period

2026-06-01T15:35:02+00:00

[ Upstream commit 07712db80857d5d09ae08f3df85a708ecfc3b61f ] In certain situations, ep_free() in eventpoll.c will kfree the epi->ep eventpoll struct while it still being used by another concurrent thread. Defer the kfree() to an RCU callback to prevent UAF. Fixes: f2e467a48287 ("eventpoll: Fix semi-unbounded recursion") Signed-off-by: Nicholas Carlini Signed-off-by: Christian Brauner Signed-off-by: Sasha Levin

epoll: use refcount to reduce ep_mutex contention

2026-06-01T15:35:02+00:00

[ Upstream commit 58c9b016e12855286370dfb704c08498edbc857a ] We are observing huge contention on the epmutex during an http connection/rate test: 83.17% 0.25% nginx [kernel.kallsyms] [k] entry_SYSCALL_64_after_hwframe [...] |--66.96%--__fput |--60.04%--eventpoll_release_file |--58.41%--__mutex_lock.isra.6 |--56.56%--osq_lock The application is multi-threaded, creates a new epoll entry for each incoming connection, and does not delete it before the connection shutdown - that is, before the connection's fd close(). Many different threads compete frequently for the epmutex lock, affecting the overall performance. To reduce the contention this patch introduces explicit reference counting for the eventpoll struct. Each registered event acquires a reference, and references are released at ep_remove() time. The eventpoll struct is released by whoever - among EP file close() and and the monitored file close() drops its last reference. Additionally, this introduces a new 'dying' flag to prevent races between the EP file close() and the monitored file close(). ep_eventpoll_release() marks, under f_lock spinlock, each epitem as dying before removing it, while EP file close() does not touch dying epitems. The above is needed as both close operations could run concurrently and drop the EP reference acquired via the epitem entry. Without the above flag, the monitored file close() could reach the EP struct via the epitem list while the epitem is still listed and then try to put it after its disposal. An alternative could be avoiding touching the references acquired via the epitems at EP file close() time, but that could leave the EP struct alive for potentially unlimited time after EP file close(), with nasty side effects. With all the above in place, we can drop the epmutex usage at disposal time. Overall this produces a significant performance improvement in the mentioned connection/rate scenario: the mutex operations disappear from the topmost offenders in the perf report, and the measured connections/rate grows by ~60%. To make the change more readable this additionally renames ep_free() to ep_clear_and_put(), and moves the actual memory cleanup in a separate ep_free() helper. Link: https://lkml.kernel.org/r/4a57788dcaf28f5eb4f8dfddcc3a8b172a7357bb.1679504153.git.pabeni@redhat.com Signed-off-by: Paolo Abeni Co-developed-by: Eric Dumazet Signed-off-by: Eric Dumazet Tested-by: Xiumei Mu Acked-by: Soheil Hassas Yeganeh Reviewed-by: Davidlohr Bueso Cc: Alexander Viro Cc: Carlos Maiolino Cc: Christian Brauner Cc: Eric Biggers Cc: Jacob Keller Cc: Jens Axboe Signed-off-by: Andrew Morton Stable-dep-of: 07712db80857 ("eventpoll: defer struct eventpoll free to RCU grace period") Signed-off-by: Sasha Levin

eventpoll: Fix integer overflow in ep_loop_check_proc()

2026-04-18T08:33:08+00:00

commit fdcfce93073d990ed4b71752e31ad1c1d6e9d58b upstream. If a recursive call to ep_loop_check_proc() hits the `result = INT_MAX`, an integer overflow will occur in the calling ep_loop_check_proc() at `result = max(result, ep_loop_check_proc(ep_tovisit, depth + 1) + 1)`, breaking the recursion depth check. Fix it by using a different placeholder value that can't lead to an overflow. Reported-by: Guenter Roeck Fixes: f2e467a48287 ("eventpoll: Fix semi-unbounded recursion") Cc: stable@vger.kernel.org Signed-off-by: Jann Horn Link: https://patch.msgid.link/20260223-epoll-int-overflow-v1-1-452f35132224@google.com Signed-off-by: Christian Brauner Signed-off-by: Greg Kroah-Hartman

eventpoll: Fix semi-unbounded recursion

2025-08-28T14:24:15+00:00

commit f2e467a48287c868818085aa35389a224d226732 upstream. Ensure that epoll instances can never form a graph deeper than EP_MAX_NESTS+1 links. Currently, ep_loop_check_proc() ensures that the graph is loop-free and does some recursion depth checks, but those recursion depth checks don't limit the depth of the resulting tree for two reasons: - They don't look upwards in the tree. - If there are multiple downwards paths of different lengths, only one of the paths is actually considered for the depth check since commit 28d82dc1c4ed ("epoll: limit paths"). Essentially, the current recursion depth check in ep_loop_check_proc() just serves to prevent it from recursing too deeply while checking for loops. A more thorough check is done in reverse_path_check() after the new graph edge has already been created; this checks, among other things, that no paths going upwards from any non-epoll file with a length of more than 5 edges exist. However, this check does not apply to non-epoll files. As a result, it is possible to recurse to a depth of at least roughly 500, tested on v6.15. (I am unsure if deeper recursion is possible; and this may have changed with commit 8c44dac8add7 ("eventpoll: Fix priority inversion problem").) To fix it: 1. In ep_loop_check_proc(), note the subtree depth of each visited node, and use subtree depths for the total depth calculation even when a subtree has already been visited. 2. Add ep_get_upwards_depth_proc() for similarly determining the maximum depth of an upwards walk. 3. In ep_loop_check(), use these values to limit the total path length between epoll nodes to EP_MAX_NESTS edges. Fixes: 22bacca48a17 ("epoll: prevent creating circular epoll structures") Cc: stable@vger.kernel.org Signed-off-by: Jann Horn Link: https://lore.kernel.org/20250711-epoll-recursion-fix-v1-1-fb2457c33292@google.com Signed-off-by: Christian Brauner Signed-off-by: Greg Kroah-Hartman

epoll: Add synchronous wakeup support for ep_poll_callback

2025-01-09T12:28:36+00:00

commit 900bbaae67e980945dec74d36f8afe0de7556d5a upstream. Now, the epoll only use wake_up() interface to wake up task. However, sometimes, there are epoll users which want to use the synchronous wakeup flag to hint the scheduler, such as Android binder driver. So add a wake_up_sync() define, and use the wake_up_sync() when the sync is true in ep_poll_callback(). Co-developed-by: Jing Xia Signed-off-by: Jing Xia Signed-off-by: Xuewen Yan Link: https://lore.kernel.org/r/20240426080548.8203-1-xuewen.yan@unisoc.com Tested-by: Brian Geffon Reviewed-by: Brian Geffon Reported-by: Benoit Lize Signed-off-by: Christian Brauner Cc: Brian Geffon Signed-off-by: Greg Kroah-Hartman

epoll: annotate racy check

2024-12-14T18:51:34+00:00

[ Upstream commit 6474353a5e3d0b2cf610153cea0c61f576a36d0a ] Epoll relies on a racy fastpath check during __fput() in eventpoll_release() to avoid the hit of pointlessly acquiring a semaphore. Annotate that race by using WRITE_ONCE() and READ_ONCE(). Link: https://lore.kernel.org/r/66edfb3c.050a0220.3195df.001a.GAE@google.com Link: https://lore.kernel.org/r/20240925-fungieren-anbauen-79b334b00542@brauner Reviewed-by: Jan Kara Reported-by: syzbot+3b6b32dc50537a49bb4a@syzkaller.appspotmail.com Signed-off-by: Christian Brauner Signed-off-by: Sasha Levin

epoll: be better about file lifetimes

2024-06-16T11:39:15+00:00

[ Upstream commit 4efaa5acf0a1d2b5947f98abb3acf8bfd966422b ] epoll can call out to vfs_poll() with a file pointer that may race with the last 'fput()'. That would make f_count go down to zero, and while the ep->mtx locking means that the resulting file pointer tear-down will be blocked until the poll returns, it means that f_count is already dead, and any use of it won't actually get a reference to the file any more: it's dead regardless. Make sure we have a valid ref on the file pointer before we call down to vfs_poll() from the epoll routines. Link: https://lore.kernel.org/lkml/0000000000002d631f0615918f1e@google.com/ Reported-by: syzbot+045b454ab35fd82a35fb@syzkaller.appspotmail.com Reviewed-by: Jens Axboe Signed-off-by: Linus Torvalds Signed-off-by: Sasha Levin

epoll: ep_autoremove_wake_function should use list_del_init_careful

2023-06-21T13:59:14+00:00

commit 2192bba03d80f829233bfa34506b428f71e531e7 upstream. autoremove_wake_function uses list_del_init_careful, so should epoll's more aggressive variant. It only doesn't because it was copied from an older wait.c rather than the most recent. [bsegall@google.com: add comment] Link: https://lkml.kernel.org/r/xm26bki0ulsr.fsf_-_@google.com Link: https://lkml.kernel.org/r/xm26pm6hvfer.fsf@google.com Fixes: a16ceb139610 ("epoll: autoremove wakers even more aggressively") Signed-off-by: Ben Segall Cc: Al Viro Cc: Christian Brauner Cc: Signed-off-by: Andrew Morton Signed-off-by: Greg Kroah-Hartman

eventpoll: add EPOLL_URING_WAKE poll wakeup flag

2023-01-24T06:22:43+00:00

[ Upstream commit caf1aeaffc3b09649a56769e559333ae2c4f1802 ] We can have dependencies between epoll and io_uring. Consider an epoll context, identified by the epfd file descriptor, and an io_uring file descriptor identified by iofd. If we add iofd to the epfd context, and arm a multishot poll request for epfd with iofd, then the multishot poll request will repeatedly trigger and generate events until terminated by CQ ring overflow. This isn't a desired behavior. Add EPOLL_URING so that io_uring can pass it in as part of the poll wakeup key, and io_uring can check for that to detect a potential recursive invocation. Cc: stable@vger.kernel.org # 6.0 Signed-off-by: Jens Axboe Signed-off-by: Sasha Levin

epoll: autoremove wakers even more aggressively

2022-08-17T12:22:59+00:00

commit a16ceb13961068f7209e34d7984f8e42d2c06159 upstream. If a process is killed or otherwise exits while having active network connections and many threads waiting on epoll_wait, the threads will all be woken immediately, but not removed from ep->wq. Then when network traffic scans ep->wq in wake_up, every wakeup attempt will fail, and will not remove the entries from the list. This means that the cost of the wakeup attempt is far higher than usual, does not decrease, and this also competes with the dying threads trying to actually make progress and remove themselves from the wq. Handle this by removing visited epoll wq entries unconditionally, rather than only when the wakeup succeeds - the structure of ep_poll means that the only potential loss is the timed_out->eavail heuristic, which now can race and result in a redundant ep_send_events attempt. (But only when incoming data and a timeout actually race, not on every timeout) Shakeel added: : We are seeing this issue in production with real workloads and it has : caused hard lockups. Particularly network heavy workloads with a lot : of threads in epoll_wait() can easily trigger this issue if they get : killed (oom-killed in our case). Link: https://lkml.kernel.org/r/xm26fsjotqda.fsf@google.com Signed-off-by: Ben Segall Tested-by: Shakeel Butt Cc: Alexander Viro Cc: Linus Torvalds Cc: Shakeel Butt Cc: Eric Dumazet Cc: Roman Penyaev Cc: Jason Baron Cc: Khazhismel Kumykov Cc: Heiher Cc: Signed-off-by: Andrew Morton Signed-off-by: Greg Kroah-Hartman