Re: [PATCH] coredump: Limit what can interrupt coredumps

[Tony Battersby]

On 8/17/21 2:24 PM, Jens Axboe wrote:
> On 8/17/21 12:15 PM, Jens Axboe wrote:
>> On 8/15/21 2:42 PM, Olivier Langlois wrote:
>>> On Wed, 2021-08-11 at 19:55 -0600, Jens Axboe wrote:
>>>> On 8/10/21 3:48 PM, Tony Battersby wrote:
>>>>> On 8/5/21 9:06 AM, Olivier Langlois wrote:
>>>>>> Hi all,
>>>>>>
>>>>>> I didn't forgot about this remaining issue and I have kept thinking
>>>>>> about it on and off.
>>>>>>
>>>>>> I did try the following on 5.12.19:
>>>>>>
>>>>>> diff --git a/fs/coredump.c b/fs/coredump.c
>>>>>> index 07afb5ddb1c4..614fe7a54c1a 100644
>>>>>> --- a/fs/coredump.c
>>>>>> +++ b/fs/coredump.c
>>>>>> @@ -41,6 +41,7 @@
>>>>>>  #include <linux/fs.h>
>>>>>>  #include <linux/path.h>
>>>>>>  #include <linux/timekeeping.h>
>>>>>> +#include <linux/io_uring.h>
>>>>>>  
>>>>>>  #include <linux/uaccess.h>
>>>>>>  #include <asm/mmu_context.h>
>>>>>> @@ -625,6 +626,8 @@ void do_coredump(const kernel_siginfo_t
>>>>>> *siginfo)
>>>>>>                 need_suid_safe = true;
>>>>>>         }
>>>>>>  
>>>>>> +       io_uring_files_cancel(current->files);
>>>>>> +
>>>>>>         retval = coredump_wait(siginfo->si_signo, &core_state);
>>>>>>         if (retval < 0)
>>>>>>                 goto fail_creds;
>>>>>> --
>>>>>> 2.32.0
>>>>>>
>>>>>> with my current understanding, io_uring_files_cancel is supposed to
>>>>>> cancel everything that might set the TIF_NOTIFY_SIGNAL.
>>>>>>
>>>>>> I must report that in my testing with generating a core dump
>>>>>> through a
>>>>>> pipe with the modif above, I still get truncated core dumps.
>>>>>>
>>>>>> systemd is having a weird error:
>>>>>> [ 2577.870742] systemd-coredump[4056]: Failed to get COMM: No such
>>>>>> process
>>>>>>
>>>>>> and nothing is captured
>>>>>>
>>>>>> so I have replaced it with a very simple shell:
>>>>>> $ cat /proc/sys/kernel/core_pattern 
>>>>>>> /home/lano1106/bin/pipe_core.sh %e %p
>>>>>> ~/bin $ cat pipe_core.sh 
>>>>>> #!/bin/sh
>>>>>>
>>>>>> cat > /home/lano1106/core/core.$1.$2
>>>>>>
>>>>>> BFD: warning: /home/lano1106/core/core.test.10886 is truncated:
>>>>>> expected core file size >= 24129536, found: 61440
>>>>>>
>>>>>> I conclude from my attempt that maybe io_uring_files_cancel is not
>>>>>> 100%
>>>>>> cleaning everything that it should clean.
>>>>>>
>>>>>>
>>>>>>
>>>>> I just ran into this problem also - coredumps from an io_uring
>>>>> program
>>>>> to a pipe are truncated.  But I am using kernel 5.10.57, which does
>>>>> NOT
>>>>> have commit 12db8b690010 ("entry: Add support for TIF_NOTIFY_SIGNAL")
>>>>> or
>>>>> commit 06af8679449d ("coredump: Limit what can interrupt coredumps").
>>>>> Kernel 5.4 works though, so I bisected the problem to commit
>>>>> f38c7e3abfba ("io_uring: ensure async buffered read-retry is setup
>>>>> properly") in kernel 5.9.  Note that my io_uring program uses only
>>>>> async
>>>>> buffered reads, which may be why this particular commit makes a
>>>>> difference to my program.
>>>>>
>>>>> My io_uring program is a multi-purpose long-running program with many
>>>>> threads.  Most threads don't use io_uring but a few of them do. 
>>>>> Normally, my core dumps are piped to a program so that they can be
>>>>> compressed before being written to disk, but I can also test writing
>>>>> the
>>>>> core dumps directly to disk.  This is what I have found:
>>>>>
>>>>> *) Unpatched 5.10.57: if a thread that doesn't use io_uring triggers
>>>>> a
>>>>> coredump, the core file is written correctly, whether it is written
>>>>> to
>>>>> disk or piped to a program, even if another thread is using io_uring
>>>>> at
>>>>> the same time.
>>>>>
>>>>> *) Unpatched 5.10.57: if a thread that uses io_uring triggers a
>>>>> coredump, the core file is truncated, whether written directly to
>>>>> disk
>>>>> or piped to a program.
>>>>>
>>>>> *) 5.10.57+backport 06af8679449d: if a thread that uses io_uring
>>>>> triggers a coredump, and the core is written directly to disk, then
>>>>> it
>>>>> is written correctly.
>>>>>
>>>>> *) 5.10.57+backport 06af8679449d: if a thread that uses io_uring
>>>>> triggers a coredump, and the core is piped to a program, then it is
>>>>> truncated.
>>>>>
>>>>> *) 5.10.57+revert f38c7e3abfba: core dumps are written correctly,
>>>>> whether written directly to disk or piped to a program.
>>>> That is very interesting. Like Olivier mentioned, it's not that actual
>>>> commit, but rather the change of behavior implemented by it. Before
>>>> that
>>>> commit, we'd hit the async workers more often, whereas after we do the
>>>> correct retry method where it's driven by the wakeup when the page is
>>>> unlocked. This is purely speculation, but perhaps the fact that the
>>>> process changes state potentially mid dump is why the dump ends up
>>>> being
>>>> truncated?
>>>>
>>>> I'd love to dive into this and try and figure it out. Absent a test
>>>> case, at least the above gives me an idea of what to try out. I'll see
>>>> if it makes it easier for me to create a case that does result in a
>>>> truncated core dump.
>>>>
>>> Jens,
>>>
>>> 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@xxxxxxxxx/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
>>>
>>> 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.
>> It does indeed sound like it's TIF_NOTIFY_SIGNAL that will trigger some
>> signal_pending() and cause an interruption of the core dump. Just out of
>> curiosity, what is your /proc/sys/kernel/core_pattern set to? If it's
>> set to some piped process, can you try and set it to 'core' and see if
>> that eliminates the truncation of the core dumps for your case?
> And assuming that works, then I suspect this one would fix your issue
> even with a piped core dump:
>
> diff --git a/fs/coredump.c b/fs/coredump.c
> index 07afb5ddb1c4..852737a9ccbf 100644
> --- a/fs/coredump.c
> +++ b/fs/coredump.c
> @@ -41,6 +41,7 @@
>  #include <linux/fs.h>
>  #include <linux/path.h>
>  #include <linux/timekeeping.h>
> +#include <linux/io_uring.h>
>  
>  #include <linux/uaccess.h>
>  #include <asm/mmu_context.h>
> @@ -603,6 +604,7 @@ void do_coredump(const kernel_siginfo_t *siginfo)
>  	};
>  
>  	audit_core_dumps(siginfo->si_signo);
> +	io_uring_task_cancel();
>  
>  	binfmt = mm->binfmt;
>  	if (!binfmt || !binfmt->core_dump)
>
FYI, I tested kernel 5.10.59 + backport 06af8679449d + the patch above
with my io_uring program.  The coredump locked up even when writing the
core file directly to disk; the zombie process could not be killed with
"kill -9".  Unfortunately I can't test with newer kernels without
spending some time on it, and I am too busy with other stuff right now.

My io_uring program does async buffered reads
(io_uring_prep_read()/io_uring_prep_readv()) from a raw disk partition
(no filesystem).  One thread submits I/Os while another thread calls
io_uring_wait_cqe() and processes the completions.  To trigger the
coredump, I added an intentional abort() in the thread that submits I/Os
after running for a second.

Tony Battersby