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Date: Fri, 22 Oct 2021 20:30:09 +0300
From: Alexander Popov <alex.popov@...ux.com>
To: "Eric W. Biederman" <ebiederm@...ssion.com>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>,
 Petr Mladek <pmladek@...e.com>, "Paul E. McKenney" <paulmck@...nel.org>,
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Subject: Re: [PATCH] Introduce the pkill_on_warn boot parameter

On 05.10.2021 22:48, Eric W. Biederman wrote:
> Especially as calling do_group_exit(SIGKILL) from a random location is
> not a clean way to kill a process.  Strictly speaking it is not even
> killing the process.
> 
> Partly this is just me seeing the introduction of a
> do_group_exit(SIGKILL) call and not likely the maintenance that will be
> needed.  I am still sorting out the problems with other randomly placed
> calls to do_group_exit(SIGKILL) and interactions with ptrace and
> PTRACE_EVENT_EXIT in particular.
> 
> Which is a long winded way of saying if I can predictably trigger a
> warning that calls do_group_exit(SIGKILL), on some architectures I can
> use ptrace and  can convert that warning into a way to manipulate the
> kernel stack to have the contents of my choice.
> 
> If anyone goes forward with this please use the existing oops
> infrastructure so the ptrace interactions and anything else that comes
> up only needs to be fixed once.

Hello Eric, hello everyone.

I learned the oops infrastructure and see that it's arch-specific.
The architectures have separate implementations of the die() function with 
different prototypes. I don't see how to use the oops infrastructure for killing 
all threads in a process that hits a kernel warning.

What do you think about doing the same as the oom_killer (and some other 
subsystems)? It kills all threads in a process this way:
   do_send_sig_info(SIGKILL, SEND_SIG_PRIV, current, PIDTYPE_TGID).

The oom_killer also shows a nice way to avoid killing init and kthreads:
	static bool oom_unkillable_task(struct task_struct *p)
	{
		if (is_global_init(p))
			return true;
		if (p->flags & PF_KTHREAD)
			return true;
		return false;
	}
I want to do something similar.

I would appreciate your comments.
Best regards,
Alexander

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