linux-kernel - Re: [PATCH] signal: Set PF_SIGNALED flag for io workers during a group exit

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Date:   Mon, 07 Jun 2021 22:49:55 -0500
From:   ebiederm@...ssion.com (Eric W. Biederman)
To:     Olivier Langlois <olivier@...llion01.com>
Cc:     Oleg Nesterov <oleg@...hat.com>,
        "Peter Zijlstra \(Intel\)" <peterz@...radead.org>,
        Marco Elver <elver@...gle.com>,
        Peter Collingbourne <pcc@...gle.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Jens Axboe <axboe@...nel.dk>, linux-kernel@...r.kernel.org,
        io-uring@...r.kernel.org
Subject: Re: [PATCH] signal: Set PF_SIGNALED flag for io workers during a group exit

Olivier Langlois <olivier@...llion01.com> writes:

> io worker threads are in most regards userspace threads except
> that they never resume userspace. Therefore, they need to explicitly
> handle signals.
>
> On delivering a fatal signal generating a core dump to a thread of
> a group having 1 or more io workers, it is possible for the io_workers
> to exit with pending signals.
>
> One example of this is the io_wqe_worker() function thread in fs/io-wq.c
> This thread can exit the function with pending signals when its
> IO_WQ_BIT_EXIT bit is set.
>
> The consequence of exiting with pending signals is that PF_SIGNALED
> will not be set. This flag is used in exit_mm() to engage into
> the synchronization between do_coredump() and exit_mm().
>
> The purpose of this synchronization is not well documented and all
> that I have found is that it is used to avoid corruption in the core file
> in the section "Deleting a Process Address Space", chapter 9 of the
> Bovet & Cesati book.

We added the check just a little while ago.  I am surprised it shows up
in any book.  What is the Bovett & Cesati book?

The flag PF_SIGNALED today is set in exactly one place, and that
is in get_signal.  The meaning of PF_SIGNALED is that do_group_exit
was called from get_signal.  AKA your task was killed by a signal.

The check in exit_mm() that tests PF_SIGNALED is empirically testing
to see if all of the necessary state is saved on the kernel stack.
That state is the state accessed by fs/binfmt_elf.c:fill_note_info.

The very good description from the original change can be found in
the commit 123cbec460db ("signal: Remove the helper signal_group_exit").

For alpha it is has the assembly function do_switch_stack been called
before your code path was called in the kernel.

Since io_uring does not have a userspace  I don't know if testing
for PF_SIGNALED is at all meaningful to detect values saved on the
stack.

I suspect io_uring is simply broken on architectures that need
extra state saved on the stack, but I haven't looked yet.

> So I am not sure if the synchronizatin MUST be applied to io_workers
> or not but the proposed patch is making sure that it is applied in
> all cases if it is needed.

That patch is definitely wrong.  If anything the check in exit_mm
should be updated.

Can you share which code paths in io_uring exit with a
fatal_signal_pending and don't bother to call get_signal?

I am currently looking to see if the wait for a coredump to read
a threads data can be moved from exit_mm into get_signal.  Even
with that io_uring might need a some additional fixes.

Eric

> Signed-off-by: Olivier Langlois <olivier@...llion01.com>
> ---
>  kernel/signal.c | 9 +++++++++
>  1 file changed, 9 insertions(+)
>
> diff --git a/kernel/signal.c b/kernel/signal.c
> index f7c6ffcbd044..477bfe55fd3c 100644
> --- a/kernel/signal.c
> +++ b/kernel/signal.c
> @@ -2925,6 +2925,15 @@ void exit_signals(struct task_struct *tsk)
>  
>  	if (thread_group_empty(tsk) || signal_group_exit(tsk->signal)) {
>  		tsk->flags |= PF_EXITING;
> +		/*
> +		 * It is possible for an io worker thread to reach this
> +		 * function with a pending SIGKILL.
> +		 * Set PF_SIGNALED for proper core dump generation
> +		 * (See exit_mm())
> +		 */
> +		if (tsk->flags & PF_IO_WORKER &&
> +		    signal_group_exit(tsk->signal))
> +			tsk->flags |= PF_SIGNALED;
>  		cgroup_threadgroup_change_end(tsk);
>  		return;
>  	}