From: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
Dmitry Vyukov <dvyukov@google.com>,
syzbot <syzbot+b7c3ba8cdc2f6cf83c21@syzkaller.appspotmail.com>,
linux-kernel@vger.kernel.org, syzkaller-bugs@googlegroups.com,
Nathan Chancellor <nathan@kernel.org>,
Arnd Bergmann <arnd@kernel.org>,
Al Viro <viro@zeniv.linux.org.uk>,
Jiri Slaby <jirislaby@kernel.org>
Subject: Re: [PATCH v3] tty: tty_io: remove hung_up_tty_fops
Date: Sun, 28 Apr 2024 19:19:52 +0900 [thread overview]
Message-ID: <a3be44f9-64eb-42e8-bf01-8610548a68a7@I-love.SAKURA.ne.jp> (raw)
In-Reply-To: <CAHk-=wgSOa_g+bxjNi+HQpC=6sHK2yKeoW-xOhb0-FVGMTDWjg@mail.gmail.com>
On 2024/04/28 4:02, Linus Torvalds wrote:
> On Fri, 26 Apr 2024 at 23:21, Tetsuo Handa
> <penguin-kernel@i-love.sakura.ne.jp> wrote:
>>
>> syzbot is reporting data race between __tty_hangup() and __fput(), for
>> filp->f_op readers are not holding tty->files_lock.
>
> Hmm. I looked round, and we actually have another case of this:
> snd_card_disconnect() also does
>
> mfile->file->f_op = &snd_shutdown_f_ops;
OK. That one needs to be fixed as well.
>
> and I don't think tty->files_lock (or, in the sound case,
> &card->files_lock) is at all relevant, since the users of f_ops don't
> use it or care.
More precisely, the users of f_op can't access it. For example,
do_splice_read() cannot understand that "in" argument refers to a tty
device and therefore will not know about tty->files_lock.
>
> That said, I really think we'd be better off just keeping the current
> model, and have the "you get one or the other". For the two cases that
> do this, do that f_op replacement with a WRITE_ONCE(), and just make
> the rule be that you have to have all the same ops in both the
> original and the shutdown version.
If we keep the current model, WRITE_ONCE() is not sufficient.
My understanding is that KCSAN's report like
https://elixir.bootlin.com/linux/v6.9-rc5/source/Documentation/dev-tools/kcsan.rst#L56
will remain unless we wrap all f_op readers using data_race() macro. That is,
we will need to define a wrapper like
static inline struct file_operations *f_op(struct file *file)
{
/*
* Ignore race in order to silence KCSAN, for __tty_hangup() or
* snd_card_disconnect() might update f_op while file is in use.
*/
return data_race(file->f_op);
}
and do for example
- if (unlikely(!in->f_op->splice_read))
+ if (unlikely(!f_op(in)->splice_read))
for https://elixir.bootlin.com/linux/v6.9-rc5/source/fs/splice.c#L977 and
- return in->f_op->splice_read(in, ppos, pipe, len, flags);
+ return f_op(in)->splice_read(in, ppos, pipe, len, flags);
for https://elixir.bootlin.com/linux/v6.9-rc5/source/fs/splice.c#L985 .
Are VFS people happy with such change? I guess that VFS people assume that
file->f_op does not get updated while file is in use. Also, such data_race()
usage does not match one of situations listed in
https://elixir.bootlin.com/linux/v6.9-rc5/source/tools/memory-model/Documentation/access-marking.txt#L58 .
>
> I do *not* think it's at all better to replace (in two different
> places) the racy f_op thing with another racy 'hungup' flag.
This approach allows VFS people to assume that file->f_op does not
get updated while file is in use.
>
> The sound case is actually a bit more involved, since it tries to deal
> with module counts. That looks potentially bogus. It does
>
> fops_get(mfile->file->f_op);
>
> after it has installed the snd_shutdown_f_ops, but in snd_open() it
> has done the proper
>
> replace_fops(file, new_fops);
replace_fops() is intended to be used *ONLY* from ->open() instances.
>
> which actually drops the module count for the old one. So the sound
> case seems to possibly leak a module ref on disconnect. That's a
> separate issue, though.
>
> Linus
>
> Linus
next prev parent reply other threads:[~2024-04-28 10:20 UTC|newest]
Thread overview: 59+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-04-21 8:18 [syzbot] [kernel?] KCSAN: data-race in __fput / __tty_hangup (4) syzbot
2023-04-21 8:21 ` Dmitry Vyukov
2023-04-21 15:12 ` Tetsuo Handa
2023-04-21 16:02 ` Tetsuo Handa
2023-04-23 23:34 ` Al Viro
2023-04-23 23:55 ` Tetsuo Handa
2023-04-24 0:44 ` Al Viro
2023-04-24 1:09 ` Tetsuo Handa
2023-04-25 14:47 ` Tetsuo Handa
2023-04-25 16:03 ` Al Viro
2023-04-25 22:09 ` Tetsuo Handa
2023-04-26 11:05 ` [PATCH] tty: tty_io: remove hung_up_tty_fops Tetsuo Handa
2023-04-28 16:27 ` Nathan Chancellor
2023-04-28 16:41 ` Tetsuo Handa
2023-04-28 17:11 ` Al Viro
2023-04-29 10:43 ` Tetsuo Handa
2023-04-28 17:31 ` Greg Kroah-Hartman
2023-04-29 15:21 ` Guenter Roeck
2023-05-01 18:42 ` Geert Uytterhoeven
2023-05-14 1:02 ` [PATCH v2] " Tetsuo Handa
2023-05-30 10:44 ` Greg Kroah-Hartman
2023-05-30 11:57 ` Tetsuo Handa
2023-05-30 12:51 ` Greg Kroah-Hartman
2024-04-27 6:20 ` [PATCH v3] " Tetsuo Handa
2024-04-27 19:02 ` Linus Torvalds
2024-04-28 10:19 ` Tetsuo Handa [this message]
2024-04-28 18:50 ` Linus Torvalds
2024-04-29 13:55 ` Marco Elver
2024-04-29 15:38 ` Linus Torvalds
2024-05-01 18:45 ` Paul E. McKenney
2024-05-01 18:56 ` Linus Torvalds
2024-05-01 19:02 ` Paul E. McKenney
2024-05-01 20:14 ` Marco Elver
2024-05-01 21:06 ` Linus Torvalds
2024-05-01 21:20 ` Linus Torvalds
2024-05-01 21:49 ` Paul E. McKenney
2024-05-01 22:32 ` Paul E. McKenney
2024-05-02 16:37 ` Boqun Feng
2024-05-03 23:59 ` Paul E. McKenney
2024-05-04 0:14 ` Linus Torvalds
2024-05-04 5:08 ` Paul E. McKenney
2024-05-04 17:50 ` Linus Torvalds
2024-05-04 18:18 ` Paul E. McKenney
2024-05-04 19:11 ` Linus Torvalds
2024-05-04 19:25 ` Linus Torvalds
2024-05-04 22:17 ` Paul E. McKenney
2024-05-04 22:04 ` Paul E. McKenney
2024-05-02 14:14 ` Marco Elver
2024-05-02 16:42 ` Tetsuo Handa
2024-05-02 17:20 ` Marco Elver
2024-05-02 17:29 ` Linus Torvalds
2024-05-02 18:14 ` Al Viro
2024-05-02 19:29 ` Marco Elver
2024-05-02 23:54 ` Tetsuo Handa
2024-05-03 1:12 ` Linus Torvalds
2023-04-23 13:28 ` [syzbot] [kernel?] KCSAN: data-race in __fput / __tty_hangup (4) Tetsuo Handa
2023-04-23 14:00 ` Greg Kroah-Hartman
2023-04-23 14:03 ` Greg Kroah-Hartman
2023-04-23 14:17 ` Tetsuo Handa
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