linux-kernel - Re: [PATCH 1/4] INIT_TASK() should initialize ->thread

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Date:	Wed, 12 May 2010 17:54:53 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Sukadev Bhattiprolu <sukadev@...ux.vnet.ibm.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Cedric Le Goater <clg@...t.ibm.com>,
	Dave Hansen <haveblue@...ibm.com>,
	Eric Biederman <ebiederm@...ssion.com>,
	Herbert Poetzl <herbert@...hfloor.at>,
	Ingo Molnar <mingo@...e.hu>,
	Mathias Krause <Mathias.Krause@...unet.com>,
	Roland McGrath <roland@...hat.com>,
	Serge Hallyn <serue@...ibm.com>,
	Sukadev Bhattiprolu <sukadev@...ibm.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/4] INIT_TASK() should initialize ->thread_group list

On 05/11, Sukadev Bhattiprolu wrote:
>
> Oleg Nesterov [oleg@...hat.com] wrote:
> | The trivial /sbin/init doing
> |
> | 	int main(void)
> | 	{
> | 		kill(0, SIGKILL)
> | 	}
> |
> | crashes the kernel.
>
> Really subtle. Good catch.

Thanks to Mathias ;)

> So, now init is not part of any process group until it calls setsid().
> So the above SIGKILL is lost right ? - i.e it does not kill even init
> itself.

No, no. swapper != init. With or without these patches (more precisely,
the next patch) /sbin/init still belongs to the 0 pgrp/sid.

> In my quick test, the following init process lives on inspite of the
> SIGKILL.

Yes, /sbin/init is not killable, that is why it survies.

But:

> 	main()
> 	{
> 		kill(0, SIGKILL);
>
> 		while(1)
> 			sleep(1);
> 	}

Yes. if /sbin/init exits the kernel panics. The real test-case shouldn't
exit, like your example.

> I don't have a better solution. Maybe a hung init is better than a
> crashed kernel.

Agreed!!! I sent the patch a long ago. But security people do not
like it, they use exit() from init to provoke the crash intentionally.
And I still think they are wrong, but this is another story.

> Acked-by: Sukadev Bhattiprolu <sukadev@...ux.vnet.ibm.com>

Thanks!

Oleg.

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