linux-kernel - Re: [PATCH] coredump: Limit what can interrupt coredumps

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Date:   Mon, 16 Aug 2021 14:02:30 +0100
From:   Pavel Begunkov <asml.silence@...il.com>
To:     Olivier Langlois <olivier@...llion01.com>,
        Jens Axboe <axboe@...nel.dk>,
        Tony Battersby <tonyb@...ernetics.com>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        Oleg Nesterov <oleg@...hat.com>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        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>
Subject: Re: [PATCH] coredump: Limit what can interrupt coredumps

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