Re: [RFC] coredump: Do not interrupt dump for TIF_NOTIFY_SIGNAL

["Eric W. Biederman" <ebiederm@xmission.com>]

Pavel Begunkov <asml.silence@gmail.com> writes:

> On 12/24/21 19:52, Eric W. Biederman wrote:
>> Pavel Begunkov <asml.silence@gmail.com> writes:
> [...]
>>> FWIW, I worked it around in io_uring back then by breaking the
>>> dependency.
>> I am in the middle of untangling the dependencies between ptrace,
>> coredump, signal handling and maybe a few related things.
>
> Sounds great
>
>> Do folks have a reproducer I can look at?  Pavel especially if you have
>> something that reproduces on the current kernels.
>
> A syz reproducer was triggering it reliably, I'd try to revert the
> commit below and test:
> https://syzkaller.appspot.com/text?tag=ReproC&x=15d3600cb00000
>
> It should hung a task. Syzbot report for reference:
> https://syzkaller.appspot.com/bug?extid=27d62ee6f256b186883e
>
>
> commit 1d5f5ea7cb7d15b9fb1cc82673ebb054f02cd7d2
> Author: Pavel Begunkov <asml.silence@gmail.com>
> Date:   Fri Oct 29 13:11:33 2021 +0100
>
>     io-wq: remove worker to owner tw dependency
>
>     INFO: task iou-wrk-6609:6612 blocked for more than 143 seconds.
>           Not tainted 5.15.0-rc5-syzkaller #0
>     "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
>     task:iou-wrk-6609    state:D stack:27944 pid: 6612 ppid:  6526 flags:0x00004006
>     Call Trace:
>      context_switch kernel/sched/core.c:4940 [inline]
>      __schedule+0xb44/0x5960 kernel/sched/core.c:6287
>      schedule+0xd3/0x270 kernel/sched/core.c:6366
>      schedule_timeout+0x1db/0x2a0 kernel/time/timer.c:1857
>      do_wait_for_common kernel/sched/completion.c:85 [inline]
>      __wait_for_common kernel/sched/completion.c:106 [inline]
>      wait_for_common kernel/sched/completion.c:117 [inline]
>      wait_for_completion+0x176/0x280 kernel/sched/completion.c:138
>      io_worker_exit fs/io-wq.c:183 [inline]
>      io_wqe_worker+0x66d/0xc40 fs/io-wq.c:597
>      ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:295
>      ...

Thank you very much for this.  There were some bugs elsewhere I had to
deal with so I am slower looking at this part of the code than I was
expecting.

I have now reproduced this with the commit reverted on current kernels
and the repro.c from the syzcaller report.  I am starting to look into
how this interacts with my planned code changes in this area.

In combination with my other planned changes I think all that needs to
happen in do_coredump is to clear TIF_NOTIFY_SIGNAL along with
TIF_SIGPENDING to prevent io_uring interaction problems.  But we will
see.

The deadlock you demonstrate here shows that it is definitely not enough
to clear TIF_NOTIFY_SIGNAL (without other changes) so that
signal_pending returns false, which I was hoping was be the case.

Eric