From: ebiederm@xmission.com (Eric W. Biederman)
To: Oleg Nesterov <oleg@redhat.com>
Cc: qianli zhao <zhaoqianligood@gmail.com>,
christian@brauner.io, axboe@kernel.dk,
Thomas Gleixner <tglx@linutronix.de>,
Peter Collingbourne <pcc@google.com>,
linux-kernel@vger.kernel.org, Qianli Zhao <zhaoqianli@xiaomi.com>
Subject: Re: [PATCH V3] exit: trigger panic when global init has exited
Date: Thu, 18 Mar 2021 14:08:46 -0500 [thread overview]
Message-ID: <m1pmzwb7pd.fsf@fess.ebiederm.org> (raw)
In-Reply-To: <20210318180450.GA9977@redhat.com> (Oleg Nesterov's message of "Thu, 18 Mar 2021 19:04:51 +0100")
Oleg Nesterov <oleg@redhat.com> writes:
> 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?
That is my understanding.
As I understand it this patch has two purposes:
1. Avoid the BUG_ON in zap_pid_ns_processes when !CONFIG_PID_NS
2. panic as early as possible so exiting threads don't removing
interesting debugging state.
It is a bit tricky to tell if the movement of the decrement of
signal->live is safe. That affects current_is_single threaded
which is used by unshare, setns of the time namespace, and setting
the selinux part of creds.
The usage in kernel/cgroup/cgroup.c:css_task_iter_advance seems safe.
Hmm, Maybe not. Today cgroup_thread_change_begin is held around
setting PF_EXITING before signal->live is decremented. So there seem to
be some subtle cgroup dependencies.
The usages of group_dead in do_exit seem safe, as except for the new
one everything is the same.
We could definitely take advantage of knowing group_dead in exit_signals
to simplify it's optimization to not rerouting signals to living
threads.
I think if we are going to move the decrement of signal->live that
should be it's own patch and be accompanied with a good description of
why it is safe instead of having the decrement of signal->live be there
as a side effect of another change.
Eric
next prev parent reply other threads:[~2021-03-18 19:10 UTC|newest]
Thread overview: 20+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-03-17 12:51 [PATCH V3] exit: trigger panic when global init has exited Qianli Zhao
2021-03-17 14:38 ` Oleg Nesterov
2021-03-18 2:47 ` qianli zhao
2021-03-18 18:04 ` Oleg Nesterov
2021-03-18 19:08 ` Eric W. Biederman [this message]
2021-03-19 6:33 ` qianli zhao
2021-03-19 16:34 ` Oleg Nesterov
2021-03-19 16:26 ` Oleg Nesterov
2021-03-21 16:00 ` qianli zhao
2021-03-22 17:07 ` Oleg Nesterov
2021-03-22 17:09 ` Oleg Nesterov
2021-03-19 5:08 ` qianli zhao
2021-03-19 16:32 ` Oleg Nesterov
2021-03-21 13:04 ` qianli zhao
2021-03-22 16:37 ` Oleg Nesterov
2021-03-23 3:14 ` qianli zhao
2021-03-23 9:00 ` Oleg Nesterov
2021-03-23 11:23 ` qianli zhao
2021-03-24 18:12 ` Oleg Nesterov
2021-03-25 3:00 ` qianli zhao
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