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Subject: Re: [PATCH] coredump: Limit what can interrupt coredumps
From:   Pavel Begunkov <asml.silence@gmail.com>
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>
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Date:   Mon, 16 Aug 2021 14:06:22 +0100
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On 8/16/21 2:02 PM, Pavel Begunkov wrote:
> 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.

Apologies for this draft rumbling...

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, e.g. in 5.15
will be only requests operating on io_uring, and used during normal
exit.

Agree that the names may be not too descriptive.

>>
>> 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