linux-kernel - Re: [PATCH] sysrq, kdump: fix regression, revert "simplify sysrq-c handler"

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Message-ID: <m1vdlmmsmh.fsf@fess.ebiederm.org>
Date:	Mon, 20 Jul 2009 23:56:22 -0700
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Lai Jiangshan <laijs@...fujitsu.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Neil Horman <nhorman@...driver.com>,
	Vivek Goyal <vgoyal@...hat.com>,
	Brayan Arraes <brayan@...k.com.br>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] sysrq, kdump: fix regression, revert "simplify sysrq-c handler"

Lai Jiangshan <laijs@...fujitsu.com> writes:

> Eric W. Biederman wrote:
>> Lai Jiangshan <laijs@...fujitsu.com> writes:
>> 
>>> 1) This fix breaks our tools.
>>>       This fix changes the ABI. panic_on_oops is default 0,
>>>    and a lots system do not specify the boot option "panic",
>>>    thus, Sysrq-c will not cause CrashDump(Kdump) as expected.
>> 
>> How does it break your tools?
>
> Sysrq-c is known for causing a CrashDump.
> This fix make Sysrq-c just causing an oops.
> An oops in process context just kills current task and
> does nothing. (panic_on_oops=0)
>
> Why we let a cleanup patch changes the kernel behavior so much?

Because it seemed reasonable to be able to test the oops path
as well.

>>> 2) When CONFIG_KEXEC=n, Sysrq-c should become an invalid
>>>    command like Sysrq-D(CONFIG_LOCKDEP, show-all-locks).
>>>    But this fix makes it a valid command and let it do a
>>>    hazard thing: cause a page fault(NULL dereference) in kernel.
>>>
>>> So, we revert this fix.
>> 
>> The idea was to extend sysrq-d to also be a way of testing NULL
>> pointer dereferences.  How is that a bad idea?
>> 
>
> When CONFIG_KEXEC=n, Crashdump is not available,
> Sysrq-c should become an invalid command.

Why should sysrq-c become an invalid command?

What problem does this cause for you?

Eric

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