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To: Olivier Langlois <olivier@trillion01.com>,
Jens Axboe <axboe@kernel.dk>,
Tony Battersby <tonyb@cybernetics.com>,
"Eric W. Biederman" <ebiederm@xmission.com>,
Oleg Nesterov <oleg@redhat.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>
References: <CAHk-=wjC7GmCHTkoz2_CkgSc_Cgy19qwSQgJGXz+v2f=KT3UOw@mail.gmail.com>
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From: Pavel Begunkov <asml.silence@gmail.com>
Subject: Re: [PATCH] coredump: Limit what can interrupt coredumps
Message-ID: <b9f92bf3-77aa-8cdd-6db7-95c86e5a6946@gmail.com>
Date: Mon, 16 Aug 2021 14:02:30 +0100
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On 8/15/21 9:42 PM, Olivier Langlois wrote:
[...]
> When I have first encountered the issue, the very first thing that I
> did try was to create a simple test program that would synthetize the
> problem.
>
> After few time consumming failed attempts, I just gave up the idea and
> simply settle to my prod program that showcase systematically the
> problem every time that I kill the process with a SEGV signal.
>
> In a nutshell, all the program does is to issue read operations with
> io_uring on a TCP socket on which there is a constant data stream.
>
> Now that I have a better understanding of what is going on, I think
> that one way that could reproduce the problem consistently could be
> along those lines:
>
> 1. Create a pipe
> 2. fork a child
> 3. Initiate a read operation on the pipe with io_uring from the child
> 4. Let the parent kill its child with a core dump generating signal.
> 5. Write something in the pipe from the parent so that the io_uring
> read operation completes while the core dump is generated.
>
> I guess that I'll end up doing that if I cannot fix the issue with my
> current setup but here is what I have attempted so far:
>
> 1. Call io_uring_files_cancel from do_coredump
> 2. Same as #1 but also make sure that TIF_NOTIFY_SIGNAL is cleared on
> returning from io_uring_files_cancel
>
> Those attempts didn't work but lurking in the io_uring dev mailing list
> is starting to pay off. I thought that I did reach the bottom of the
> rabbit hole in my journey of understanding io_uring but the recent
> patch set sent by Hao Xu
>
> https://lore.kernel.org/io-uring/90fce498-968e-6812-7b6a-fdf8520ea8d9@kernel.dk/T/#t
>
> made me realize that I still haven't assimilated all the small io_uring
> nuances...
>
> Here is my feedback. From my casual io_uring code reader point of view,
> it is not 100% obvious what the difference is between
> io_uring_files_cancel and io_uring_task_cancel
As you mentioned, io_uring_task_cancel() cancels and waits for all
requests submitted by current task, used in exec() and SQPOLL because
of potential races.
io_uring_task_cancel() cancels only selected ones and
io_uring_files_cancel()
cancels and waits only some specific requests that we absolutely have
to, e.g. in 5.15 it'll be only requests referencing the ring itself.
It's used on normal task exit.
io_uring_task_cancel() cancels and waits all requests submitted by
current task, used on exec() because of races.
As you mentioned
>
> It seems like io_uring_files_cancel is cancelling polls only if they
> have the REQ_F_INFLIGHT flag set.
>
> I have no idea what an inflight request means and why someone would
> want to call io_uring_files_cancel over io_uring_task_cancel.
>
> I guess that if I was to meditate on the question for few hours, I
> would at some point get some illumination strike me but I believe that
> it could be a good idea to document in the code those concepts for
> helping casual readers...
>
> Bottomline, I now understand that io_uring_files_cancel does not cancel
> all the requests. Therefore, without fully understanding what I am
> doing, I am going to replace my call to io_uring_files_cancel from
> do_coredump with io_uring_task_cancel and see if this finally fix the
> issue for good.
>
> What I am trying to do is to cancel pending io_uring requests to make
> sure that TIF_NOTIFY_SIGNAL isn't set while core dump is generated.
>
> Maybe another solution would simply be to modify __dump_emit to make it
> resilient to TIF_NOTIFY_SIGNAL as Eric W. Biederman originally
> suggested.
>
> or maybe do both...
>
> Not sure which approach is best. If someone has an opinion, I would be
> curious to hear it.
>
> Greetings,
>
>
--
Pavel Begunkov