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

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Message-ID: <CAPx_LQENxx0y5mFJjwRT2qMSLt7pbAmF30=eE-QduEwRVJEJ0Q@mail.gmail.com>
Date:   Fri, 19 Mar 2021 13:08:55 +0800
From:   qianli zhao <zhaoqianligood@...il.com>
To:     Oleg Nesterov <oleg@...hat.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

Hi,Oleg

> 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?

I think check SIGNAL_GROUP_EXIT is necessary,or panic() will happen
after all init sub-threads do_exit(),so the following two situations
will happen:
1.According to the timing in the changelog,
zap_pid_ns_processes()->BUG() maybe happened.
2.The key variables of each init sub-threads will be in the exit
state(such task->mm=NULL,task->flags=PF_EXITING,task->nsproxy=NULL),resulting
in the failure to parse coredump from fulldump.

So i think check SIGNAL_GROUP_EXIT is a simple and effective way to
prevent these

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

Yes, my patch just want panic earlier before other init's sub-threads exit

Oleg Nesterov <oleg@...hat.com> 于2021年3月19日周五 上午2:05写道：
>
> 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.
>