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From: Al Viro <[email protected]>
To: Linus Torvalds <[email protected]>
Cc: [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: Sat, 4 May 2024 00:39:00 +0100	[thread overview]
Message-ID: <20240503233900.GG2118490@ZenIV> (raw)
In-Reply-To: <CAHk-=whULchE1i5LA2Fa=ZndSAzPXGWh_e5+a=YV3qT1BEST7w@mail.gmail.com>

On Fri, May 03, 2024 at 04:16:15PM -0700, Linus Torvalds wrote:
> On Fri, 3 May 2024 at 15:07, Al Viro <[email protected]> wrote:
> >
> > Suppose your program calls select() on a pipe and dmabuf, sees data to be read
> > from pipe, reads it, closes both pipe and dmabuf and exits.
> >
> > Would you expect that dmabuf file would stick around for hell knows how long
> > after that?  I would certainly be very surprised by running into that...
> 
> Why?
> 
> That's the _point_ of refcounts. They make the thing they refcount
> stay around until it's no longer referenced.
> 
> Now, I agree that dmabuf's are a bit odd in how they use a 'struct
> file' *as* their refcount, but hey, it's a specialty use. Unusual
> perhaps, but not exactly wrong.
> 
> I suspect that if you saw a dmabuf just have its own 'refcount_t' and
> stay around until it was done, you wouldn't bat an eye at it, and it's
> really just the "it uses a struct file for counting" that you are
> reacting to.

*IF* those files are on purely internal filesystem, that's probably
OK; do that with something on something mountable (char device,
sysfs file, etc.) and you have a problem with filesystem staying
busy.

I'm really unfamiliar with the subsystem; it might be OK with all
objects that use that for ->poll(), but that's definitely not a good
thing to see in ->poll() instance in general.  And code gets copied,
so there really should be a big fat comment about the reasons why
it's OK in this particular case.

Said that, it seems that a better approach might be to have
their ->release() cancel callbacks and drop fence references.
Note that they *do* have refcounts - on fences.  The file
(well, dmabuf, really) is pinned only to protect against the
situation when pending callback is still around.  And Kees'
observation about multiple fences is also interesting - we don't
get extra fput(), but only because we get events only from one
fence, which does look fishy...

  reply	other threads:[~2024-05-03 23:39 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 [this message]
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
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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