linux-kernel - Re: [PATCH 1/8] signal: Make SIGKILL during coredumps an explicit special case

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Date:   Sat, 15 Jan 2022 14:23:34 -0500
From:   Olivier Langlois <olivier@...llion01.com>
To:     "Eric W. Biederman" <ebiederm@...ssion.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Heiko Carstens <hca@...ux.ibm.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        "<linux-arch@...r.kernel.org>" <linux-arch@...r.kernel.org>,
        Linux API <linux-api@...r.kernel.org>,
        Alexey Gladkov <legion@...nel.org>,
        Kyle Huey <me@...ehuey.com>, Oleg Nesterov <oleg@...hat.com>,
        Kees Cook <keescook@...omium.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        Jens Axboe <axboe@...nel.dk>,
        Pavel Begunkov <asml.silence@...il.com>
Subject: Re: [PATCH 1/8] signal: Make SIGKILL during coredumps an explicit
 special case

On Fri, 2022-01-14 at 18:12 -0600, Eric W. Biederman wrote:
> Linus Torvalds <torvalds@...ux-foundation.org> writes:
> 
> > On Tue, Jan 11, 2022 at 10:51 AM Eric W. Biederman
> > <ebiederm@...ssion.com> wrote:
> > > 
> > > +       while ((n == -ERESTARTSYS) &&
> > > test_thread_flag(TIF_NOTIFY_SIGNAL)) {
> > > +               tracehook_notify_signal();
> > > +               n = __kernel_write(file, addr, nr, &pos);
> > > +       }
> > 
> > This reads horribly wrongly to me.
> > 
> > That "tracehook_notify_signal()" thing *has* to be renamed before
> > we
> > have anything like this that otherwise looks like "this will just
> > loop
> > forever".
> > 
> > I'm pretty sure we've discussed that "tracehook" thing before - the
> > whole header file is misnamed, and most of the functions in theer
> > are
> > too.
> > 
> > As an ugly alternative, open-code it, so that it's clear that "yup,
> > that clears the TIF_NOTIFY_SIGNAL flag".
> 
> A cleaner alternative looks like to modify the pipe code to use
> wake_up_XXX instead of wake_up_interruptible_XXX and then have code
> that does pipe_write_killable instead of pipe_write_interruptible.

Do not forget that the problem might not be limited to the pipe FS as
Oleg Nesterov pointed out here:

https://lore.kernel.org/io-uring/20210614141032.GA13677@redhat.com/

This is why I did like your patch fixing __dump_emit. If the only
problem is the tracehook_notify_signal() function unclear name, that
should be addressed instead of trying to fix the problem in a different
way.
> 
> There is also a question of how all of this should interact with the
> freezer, as I think changing from interruptible to killable means
> that
> the coredumps became unfreezable.
> 
> I am busily simmering this on my back burner and I hope I can come up
> with something sensible.

IMHO, fixing the problem on the emit function side has the merit of
being future proof if something else than io_uring in the future would
raise the TIF_NOTIFY_SIGNAL flag

but I am wondering why no one commented anything about my proposal of
cancelling io_uring before generating the core dump therefore stopping
it to flip TIF_NOTIFY_SIGNAL while the core dump is generated.

Is there something wrong with my proposed approach?
https://lore.kernel.org/lkml/cover.1629655338.git.olivier@trillion01.com/

It did flawlessly created many dozens of io_uring app core dumps in the
last months for me...

Olivier