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

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Date:   Mon, 22 Mar 2021 17:37:05 +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

Hi,

It seems that we don't understand each other.

If we move atomic_dec_and_test(signal->live) and do

	if (group_dead && is_global_init)
		panic(...);


before setting PF_EXITING like your patch does, then zap_pid_ns_processes()
simply won't be called.

Because:

On 03/21, qianli zhao wrote:
>
> Hi,Oleg
>
> > How? Perhaps I missed something again, but I don't think this is possible.
>
> > zap_pid_ns_processes() simply won't be called, find_child_reaper() will
> > see the !PF_EXITING thread which calls panic().
>
> > So I think this should be documented somehow, at least in the changelog.
>
> This problem occurs when both two init threads enter the do_exit,
> One of the init thread is syscall sys_exit_group,and set SIGNAL_GROUP_EXIT
> The other init thread perform ret_to_user()->get_signal() and found
> SIGNAL_GROUP_EXIT is set,then do_group_exit()->do_exit(),since there
> are no alive init threads it finally goes to
> zap_pid_ns_processes()

No, there is at least one alive init thread. If they all have exited, we have
the thread which calls panic() above.

> and BUG().

so we don't need the SIGNAL_GROUP_EXIT check to avoid this BUG().

What have I missed?

Oleg.