linux-kernel - Re: [RFC] coredump: Do not interrupt dump for TIF_NOTIFY

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Date:   Wed, 09 Jun 2021 17:02:05 -0400
From:   Olivier Langlois <olivier@...llion01.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>,
        "Eric W. Biederman" <ebiederm@...ssion.com>
Cc:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        io-uring <io-uring@...r.kernel.org>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Jens Axboe <axboe@...nel.dk>,
        "Pavel Begunkov>" <asml.silence@...il.com>,
        Oleg Nesterov <oleg@...hat.com>
Subject: Re: [RFC] coredump: Do not interrupt dump for TIF_NOTIFY_SIGNAL

On Wed, 2021-06-09 at 13:33 -0700, Linus Torvalds wrote:
> Now, the fact that we haven't cleared TIF_NOTIFY_SIGNAL for the first
> signal is clearly the immediate cause of this, but at the same time I
> really get the feeling that that coredump aborting code should always
> had used fatal_signal_pending().

I need clarify what does happen with the io_uring situation. If
somehow, TIF_NOTIFY_SIGNAL wasn't cleared, I would get all the time a 0
byte size core dump because do_coredump() does check if the dump is
interrupted before writing a single byte.

io_uring is quite a strange animal. AFAIK, the common pattern to use a
wait_queue is to insert a task into it and then put that task to sleep
until the waited event occur.

io_uring place tasks into wait queues and then let the the task return
to user space to do some other stuff (like core dumping). I would guess
that it is the main reason for it using the task_work feature.

So the TIF_NOTIFY_SIGNAL does get set WHILE the core dump is written.

Greetings,
Olivier