From: Tony Battersby <[email protected]>
To: Jens Axboe <[email protected]>,
Olivier Langlois <[email protected]>,
"Eric W. Biederman" <[email protected]>,
Oleg Nesterov <[email protected]>
Cc: Linus Torvalds <[email protected]>,
Linux Kernel Mailing List <[email protected]>,
linux-fsdevel <[email protected]>,
io-uring <[email protected]>,
Alexander Viro <[email protected]>,
"Pavel Begunkov>" <[email protected]>
Subject: Re: [PATCH] coredump: Limit what can interrupt coredumps
Date: Tue, 17 Aug 2021 15:29:32 -0400 [thread overview]
Message-ID: <[email protected]> (raw)
In-Reply-To: <[email protected]>
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/[email protected]/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
next prev parent reply other threads:[~2021-08-17 19:43 UTC|newest]
Thread overview: 66+ messages / expand[flat|nested] mbox.gz Atom feed top
[not found] <[email protected]>
[not found] ` <[email protected]>
[not found] ` <87h7i694ij.fsf_-_@disp2133>
2021-06-09 20:33 ` [RFC] coredump: Do not interrupt dump for TIF_NOTIFY_SIGNAL Linus Torvalds
2021-06-09 20:48 ` Eric W. Biederman
2021-06-09 20:52 ` Linus Torvalds
2021-06-09 21:02 ` Olivier Langlois
2021-06-09 21:05 ` Eric W. Biederman
2021-06-09 21:26 ` Olivier Langlois
2021-06-09 21:56 ` Olivier Langlois
2021-06-10 14:26 ` Eric W. Biederman
2021-06-10 15:17 ` Olivier Langlois
2021-06-10 18:58 ` [CFT}[PATCH] coredump: Limit what can interrupt coredumps Eric W. Biederman
2021-06-10 19:10 ` Linus Torvalds
2021-06-10 19:18 ` Eric W. Biederman
2021-06-10 19:50 ` Linus Torvalds
2021-06-10 20:11 ` [PATCH] " Eric W. Biederman
2021-06-10 21:04 ` Linus Torvalds
2021-06-12 14:36 ` Olivier Langlois
2021-06-12 16:26 ` Jens Axboe
2021-06-14 14:10 ` Oleg Nesterov
2021-06-14 16:37 ` Eric W. Biederman
2021-06-14 16:59 ` Oleg Nesterov
2021-06-15 22:08 ` Eric W. Biederman
2021-06-16 19:23 ` Olivier Langlois
2021-06-16 20:00 ` Eric W. Biederman
2021-06-18 20:05 ` Olivier Langlois
2021-08-05 13:06 ` Olivier Langlois
2021-08-10 21:48 ` Tony Battersby
2021-08-11 20:47 ` Olivier Langlois
2021-08-12 1:55 ` Jens Axboe
2021-08-12 13:53 ` Tony Battersby
2021-08-15 20:42 ` Olivier Langlois
2021-08-16 13:02 ` Pavel Begunkov
2021-08-16 13:06 ` Pavel Begunkov
2021-08-17 18:15 ` Jens Axboe
2021-08-17 18:24 ` Jens Axboe
2021-08-17 19:29 ` Tony Battersby [this message]
2021-08-17 19:59 ` Jens Axboe
2021-08-17 21:28 ` Jens Axboe
2021-08-17 21:39 ` Tony Battersby
2021-08-17 22:05 ` Jens Axboe
2021-08-18 14:37 ` Tony Battersby
2021-08-18 14:46 ` Jens Axboe
2021-08-18 2:57 ` Jens Axboe
2021-08-18 2:58 ` Jens Axboe
2021-08-21 10:08 ` Olivier Langlois
2021-08-21 16:47 ` Olivier Langlois
2021-08-21 16:51 ` Jens Axboe
2021-08-21 17:21 ` Olivier Langlois
2021-08-21 9:52 ` Olivier Langlois
2021-08-21 9:48 ` Olivier Langlois
2021-10-22 14:13 ` [RFC] coredump: Do not interrupt dump for TIF_NOTIFY_SIGNAL Pavel Begunkov
2021-12-24 1:34 ` Olivier Langlois
2021-12-24 10:37 ` Pavel Begunkov
2021-12-24 19:52 ` Eric W. Biederman
2021-12-28 11:24 ` Pavel Begunkov
2022-03-14 23:58 ` Eric W. Biederman
[not found] ` <[email protected]>
2022-06-01 3:15 ` Jens Axboe
2022-07-20 16:49 ` [PATCH 0/2] coredump: Allow io_uring using apps to dump to pipes Eric W. Biederman
2022-07-20 16:50 ` [PATCH 1/2] signal: Move stopping for the coredump from do_exit into get_signal Eric W. Biederman
2022-07-20 16:51 ` [PATCH 2/2] coredump: Allow coredumps to pipes to work with io_uring Eric W. Biederman
2022-08-22 21:16 ` Olivier Langlois
2022-08-23 3:35 ` Olivier Langlois
2022-08-23 18:22 ` Eric W. Biederman
2022-08-23 18:27 ` Jens Axboe
2022-08-24 15:11 ` Eric W. Biederman
2022-08-24 15:51 ` Jens Axboe
2022-01-05 19:39 ` [RFC] coredump: Do not interrupt dump for TIF_NOTIFY_SIGNAL Olivier Langlois
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