linux-kernel - Re: [PATCH V3] exit: trigger panic when global init has exited

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Date:   Thu, 18 Mar 2021 19:04:51 +0100
From:   Oleg Nesterov <oleg@...hat.com>
To:     qianli zhao <zhaoqianligood@...il.com>
Cc:     christian@...uner.io, axboe@...nel.dk,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Peter Collingbourne <pcc@...gle.com>,
        linux-kernel@...r.kernel.org, Qianli Zhao <zhaoqianli@...omi.com>
Subject: Re: [PATCH V3] exit: trigger panic when global init has exited

On 03/18, qianli zhao wrote:
>
> Hi,Oleg
>
> Thank you for your reply.
>
> >> When init sub-threads running on different CPUs exit at the same time,
> >> zap_pid_ns_processe()->BUG() may be happened.
>
> > and why do you think your patch can't prevent this?
>
> > Sorry, I must have missed something. But it seems to me that you are trying
> > to fix the wrong problem. Yes, zap_pid_ns_processes() must not be called in
> > the root namespace, and this has nothing to do with CONFIG_PID_NS.
>
> Yes, i try to fix this exception by test SIGNAL_GROUP_EXIT and call
> panic before setting PF_EXITING to prevent zap_pid_ns_processes()
> being called when init do_exit().

Ah, I didn't notice your patch does atomic_dec_and_test(signal->live)
before exit_signals() which sets PF_EXITING. Thanks for correcting me.

So yes, I was wrong, your patch can prevent this. Although I'd like to
recheck if every do-something-if-group-dead action is correct in the
case we have a non-PF_EXITING thread...

But then I don't understand the SIGNAL_GROUP_EXIT check added by your
patch. Do we really need it if we want to avoid zap_pid_ns_processes()
when the global init exits?

> In addition, the patch also protects the init process state to
> successfully get usable init coredump.

Could you spell please?

Does this connect to SIGNAL_GROUP_EXIT check? Do you mean that you want
to panic earlier, before other init's sub-threads exit?

Thanks,

Oleg.