Re: [RFC] coredump: Do not interrupt dump for TIF_NOTIFY_SIGNAL

From: ebiederm@xmission.com (Eric W. Biederman)
To: Olivier Langlois <olivier@trillion01.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	linux-fsdevel <linux-fsdevel@vger.kernel.org>,
	io-uring <io-uring@vger.kernel.org>,
	Alexander Viro <viro@zeniv.linux.org.uk>,
	Jens Axboe <axboe@kernel.dk>,
	"Pavel Begunkov\>" <asml.silence@gmail.com>,
	Oleg Nesterov <oleg@redhat.com>
Subject: Re: [RFC] coredump: Do not interrupt dump for TIF_NOTIFY_SIGNAL
Date: Thu, 10 Jun 2021 09:26:47 -0500	[thread overview]
Message-ID: <87pmwt6biw.fsf@disp2133> (raw)
In-Reply-To: <b8434a8987672ab16f9fb755c1fc4d51e0f4004a.camel@trillion01.com> (Olivier Langlois's message of "Wed, 09 Jun 2021 17:26:30 -0400")

Olivier Langlois <olivier@trillion01.com> writes:

> On Wed, 2021-06-09 at 16:05 -0500, Eric W. Biederman wrote:
>> > 
>> > So the TIF_NOTIFY_SIGNAL does get set WHILE the core dump is
>> > written.
>> 
>> Did you mean?
>> 
>> So the TIF_NOTIFY_SIGNAL does _not_ get set WHILE the core dump is
>> written.
>> 
>> 
> Absolutely not. I did really mean what I have said. Bear with me that,
> I am not qualifying myself as an expert kernel dev yet so feel free to
> correct me if I say some heresy...

No.  I was just asking to make certain I understood what you said.

I thought you said you were getting a consistent 0 byte coredump,
and that implied that TIF_NOTIFY_SIGNAL was coming in before
the coredump even started.

> io_uring is placing my task in my TCP socket wait queue because it
> wants to read data from it.
>
> The task returns to user space and core dump with a SEGV.
>
> now my understanding is that the code that is waking up tasks, it is
> the NIC driver interrupt handler which can occur while the core dump is
> written.
>
> does that make sense?
>
> my testing is telling me that this is exactly what happens...

If you are getting partial coredumps that completely makes sense.

I was hoping that by this time Jens or Oleg would have been able to
chime in and at least confirm I am not missing something subtle.

I was afraid for a little bit that the file system code in called in
dump_emit would be checking signal_pending.  After looking into that I
see that the filesystem code very reasonably limits itself to testing
fatal_signal_pending (because by definition disk I/O on unix is not
interruptible).

So I will spin up a good version of my patch (based on your patch)
so we can unbreak coredumps.

Eric