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From: Christian Brauner <[email protected]>
To: Linus Torvalds <[email protected]>
Cc: Al Viro <[email protected]>,
	[email protected],  [email protected],
	[email protected], [email protected],
	 [email protected], [email protected], [email protected],
	[email protected],  [email protected],
	[email protected], [email protected],
	 [email protected], [email protected],
	 [email protected],
	[email protected]
Subject: Re: [PATCH] epoll: try to be a _bit_ better about file lifetimes
Date: Mon, 6 May 2024 16:19:06 +0200	[thread overview]
Message-ID: <20240506-zerbrach-zehnkampf-80281b1452c8@brauner> (raw)
In-Reply-To: <20240506-zweibeinig-mahnen-daa579a233db@brauner>

On Mon, May 06, 2024 at 11:27:04AM +0200, Christian Brauner wrote:
> On Mon, May 06, 2024 at 10:45:35AM +0200, Christian Brauner wrote:
> > > The fact is, it's not dma-buf that is violating any rules. It's epoll.
> > 
> > I agree that epoll() not taking a reference on the file is at least
> > unexpected and contradicts the usual code patterns for the sake of
> > performance and that it very likely is the case that most callers of
> > f_op->poll() don't know this.
> > 
> > Note, I cleary wrote upthread that I'm ok to do it like you suggested
> > but raised two concerns a) there's currently only one instance of
> > prolonged @file lifetime in f_op->poll() afaict and b) that there's
> > possibly going to be some performance impact on epoll().
> > 
> > So it's at least worth discussing what's more important because epoll()
> > is very widely used and it's not that we haven't favored performance
> > before.
> > 
> > But you've already said that you aren't concerned with performance on
> > epoll() upthread. So afaict then there's really not a lot more to
> > discuss other than take the patch and see whether we get any complaints.
> 
> Two closing thoughts:
> 
> (1) I wonder if this won't cause userspace regressions for the semantics
>     of epoll because dying files are now silently ignored whereas before
>     they'd generated events.
> 
> (2) The other part is that this seems to me that epoll() will now
>     temporarly pin filesystems opening up the possibility for spurious
>     EBUSY errors.
> 
>     If you register a file descriptor in an epoll instance and then
>     close it and umount the filesystem but epoll managed to do an fget()
>     on that fd before that close() call then epoll will pin that
>     filesystem.
> 
>     If the f_op->poll() method does something that can take a while
>     (blocks on a shared mutex of that subsystem) that umount is very
>     likely going to return EBUSY suddenly.
> 
>     Afaict, before that this wouldn't have been an issue at all and is
>     likely more serious than performance.
> 
>     (One option would be to only do epi_fget() for stuff like
>     dma-buf that's never unmounted. That'll cover nearly every
>     driver out there. Only "real" filesystems would have to contend with
>     @file count going to zero but honestly they also deal with dentry
>     lookup under RCU which is way more adventurous than this.)
> 
>     Maybe I'm barking up the wrong tree though.

Sorry, had to step out for an appointment.

Under the assumption that I'm not entirely off with this - and I really
could be ofc - then one possibility would be that we enable persistence
of files from f_op->poll() for SB_NOUSER filesystems.

That'll catch everything that's relying on anonymous inodes (drm and all
drivers) and init_pseudo() so everything that isn't actually unmountable
(pipefs, pidfs, sockfs, etc.).

So something like the _completely untested_ diff on top of your proposal
above:

diff --git a/fs/eventpoll.c b/fs/eventpoll.c
index a3f0f868adc4..95968a462544 100644
--- a/fs/eventpoll.c
+++ b/fs/eventpoll.c
@@ -1018,8 +1018,24 @@ static struct file *epi_fget(const struct epitem *epi)
 static __poll_t ep_item_poll(const struct epitem *epi, poll_table *pt,
                                 int depth)
 {
-       struct file *file = epi_fget(epi);
+       struct file *file = epi->ffd.file;
        __poll_t res;
+       bool unrefd = false;
+
+       /*
+        * Taking a reference for anything that isn't mountable is fine
+        * because we don't have to worry about spurious EBUSY warnings
+        * from umount().
+        *
+        * File count might go to zero in f_op->poll() for mountable
+        * filesystems.
+        */
+       if (file->f_inode->i_sb->s_flags & SB_NOUSER) {
+               unrefd = true;
+               file = epi_fget(epi);
+       } else if (file_count(file) == 0) {
+               file = NULL;
+       }

        /*
         * We could return EPOLLERR | EPOLLHUP or something,
@@ -1034,7 +1050,9 @@ static __poll_t ep_item_poll(const struct epitem *epi, poll_table *pt,
                res = vfs_poll(file, pt);
        else
                res = __ep_eventpoll_poll(file, pt, depth);
-       fput(file);
+
+       if (unrefd)
+               fput(file);
        return res & epi->event.events;
 }

Basically, my worry is that we end up with really annoying to debug
EBUSYs caused by epoll(). I'd really like to avoid that. But again, I
might be wrong and this isn't an issue.

  reply	other threads:[~2024-05-06 14:19 UTC|newest]

Thread overview: 87+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2024-04-08  8:26 [syzbot] [fs?] [io-uring?] general protection fault in __ep_remove syzbot
2024-04-15 14:31 ` Jens Axboe
2024-04-15 14:57   ` Pavel Begunkov
2024-05-03 11:54 ` Bui Quang Minh
2024-05-03 18:26   ` get_file() unsafe under epoll (was Re: [syzbot] [fs?] [io-uring?] general protection fault in __ep_remove) Kees Cook
2024-05-03 18:49     ` Jens Axboe
2024-05-03 19:22       ` Kees Cook
2024-05-03 19:35         ` Jens Axboe
2024-05-03 19:59           ` Kees Cook
2024-05-03 20:28             ` Kees Cook
2024-05-03 21:11               ` Al Viro
2024-05-03 21:24                 ` Linus Torvalds
2024-05-03 21:30                   ` Al Viro
2024-05-06 17:46                   ` Stefan Metzmacher
2024-05-06 18:17                     ` Linus Torvalds
2024-05-08  8:47                       ` David Laight
2024-05-03 21:36                 ` Al Viro
2024-05-03 21:42                   ` Linus Torvalds
2024-05-03 21:53                     ` Al Viro
2024-05-06 12:23                       ` Daniel Vetter
2024-05-04  9:59             ` Christian Brauner
2024-05-03 21:11     ` [PATCH] epoll: try to be a _bit_ better about file lifetimes Linus Torvalds
2024-05-03 21:24       ` Al Viro
2024-05-03 21:33         ` Linus Torvalds
2024-05-03 21:45           ` Al Viro
2024-05-03 21:52             ` Linus Torvalds
2024-05-03 22:01               ` Al Viro
2024-05-03 22:07                 ` Al Viro
2024-05-03 23:16                   ` Linus Torvalds
2024-05-03 23:39                     ` Al Viro
2024-05-03 23:54                       ` Linus Torvalds
2024-05-04 10:44                       ` Christian Brauner
2024-05-03 22:46               ` Kees Cook
2024-05-03 23:03                 ` Al Viro
2024-05-03 23:23                   ` Kees Cook
2024-05-03 23:41                     ` Linus Torvalds
2024-05-04  9:19                       ` Christian Brauner
2024-05-06 12:37                       ` Daniel Vetter
2024-05-04  9:37           ` Christian Brauner
2024-05-04 15:32             ` Linus Torvalds
2024-05-04 15:40               ` Linus Torvalds
2024-05-04 15:53                 ` Linus Torvalds
2024-05-05 19:46                   ` Al Viro
2024-05-05 20:03                     ` Linus Torvalds
2024-05-05 20:30                       ` Al Viro
2024-05-05 20:53                         ` Linus Torvalds
2024-05-06 12:47                           ` Daniel Vetter
2024-05-06 14:46                             ` Christian Brauner
2024-05-07 10:58                               ` Daniel Vetter
2024-05-06 16:15                           ` Christian König
2024-05-05 10:50                 ` Christian Brauner
2024-05-05 16:46                   ` Linus Torvalds
2024-05-05 17:55                     ` [PATCH v2] epoll: be " Linus Torvalds
2024-05-05 18:04                       ` Jens Axboe
2024-05-05 20:01                       ` David Laight
2024-05-05 20:16                         ` Linus Torvalds
2024-05-05 20:12                     ` [PATCH] epoll: try to be a _bit_ " Al Viro
2024-05-06  8:45                     ` Christian Brauner
2024-05-06  9:26                       ` Christian Brauner
2024-05-06 14:19                         ` Christian Brauner [this message]
2024-05-07 21:02                       ` David Laight
2024-05-04 18:20               ` Linus Torvalds
2024-05-06 14:29                 ` [Linaro-mm-sig] " Christian König
2024-05-07 11:02                   ` Daniel Vetter
2024-05-07 16:46                     ` Linus Torvalds
2024-05-07 17:45                       ` Christian König
2024-05-08  7:51                         ` Michel Dänzer
2024-05-08  7:59                           ` Christian König
2024-05-08  8:23                         ` Christian Brauner
2024-05-08  9:10                           ` Christian König
2024-05-07 18:04                       ` Daniel Vetter
2024-05-07 19:07                         ` Linus Torvalds
2024-05-08  5:55                           ` Christian König
2024-05-08  8:32                             ` Daniel Vetter
2024-05-08 10:16                               ` Christian Brauner
2024-05-08  8:05                           ` Christian Brauner
2024-05-08 16:19                           ` Linus Torvalds
2024-05-08 17:14                             ` Linus Torvalds
2024-05-09 11:38                               ` Christian Brauner
2024-05-09 15:48                                 ` Linus Torvalds
2024-05-10  6:33                                   ` Christian Brauner
2024-05-08 10:08                   ` Christian Brauner
2024-05-08 15:45                     ` Daniel Vetter
2024-05-10 10:55                       ` Christian Brauner
2024-05-11 18:25                         ` David Laight
2024-05-05 17:31       ` Jens Axboe
2024-05-04  9:45     ` get_file() unsafe under epoll (was Re: [syzbot] [fs?] [io-uring?] general protection fault in __ep_remove) Christian Brauner

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