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

[Jens Axboe]

On 8/17/21 1:29 PM, Tony Battersby wrote:
> 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.

That sounds like 5.10-stable is missing some of the cancelation
backports, and your setup makes the cancelation stall because of that.
Need to go over the 11/12/13 fixes and ensure that we've got everything
we need for those stable versions, particularly 5.10.

> 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.

OK, so that one is also using task_work for the retry based async
buffered reads, so it makes sense.

Maybe a temporary work-around is to use 06af8679449d and eliminate the
pipe based coredump?

-- 
Jens Axboe