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Date: Mon, 6 Nov 2017 15:31:38 +0100
From: Oleg Nesterov <oleg@redhat.com>
To: Jamie Iles <jamie.iles@oracle.com>
Cc: Dmitry Vyukov <dvyukov@google.com>,
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Subject: Re: WARNING in task_participate_group_stop
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On 11/06, Jamie Iles wrote:
>
> I'm unable to reproduce the warning in qemu with SMP (on a 32 CPU VM).

Neither me. Perhaps because I tried this test-case on the minimal system
with /bin/sh running as init process.

> Instead I get the following instant traceback which is different to what 
> you report when run as root:
>
> [   45.018469] Kernel panic - not syncing: Attempted to kill init! exitcode=0x00000013
> [   45.018469]
> [   45.019669] CPU: 19 PID: 1 Comm: systemd Not tainted 4.14.0-rc8 #7
> [   45.021094] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.10.1-1ubuntu1 04/01/2014
> [   45.022768] Call Trace:
> [   45.023076]  dump_stack+0x12e/0x188
> [   45.023481]  panic+0x1e4/0x417

This is fine and hopefully confirms the theory. let me quote my previous email:

		line 111    r[8] = syscall(__NR_ptrace, 0x10ul, r[7]);

	this is PTRACE_ATTACH

		line 115        syscall(__NR_ptrace, 0x4200ul, r[7], 0x40000012ul, 0x100012ul);
		
	this is PTRACE_SETOPTIONS and "data" includes PTRACE_O_EXITKILL.

	r[7] is initialized at

		line 110      r[7] = *(uint32_t*)0x20f9cffc;

	so if it is eq to 1 then it can attach to init and in this case the problem
	can be explained by the wrong SIGNAL_UNKILLABLE/SIGKILL logic.

So, if it is eq to 1 then init will be killed after the child process created
by loop() function exits (see PTRACE_O_EXITKILL above).

This is correct, only the warning is not.

For example, this command does ptrace(PTRACE_SEIZE, 1,0, PTRACE_O_EXITKILL)

	# perl -e 'syscall 101, 0x4206, 1, 0, 0x100000'

and crashes the kernel the same way, this is correct.

Oleg.