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

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Message-ID: <4A6564F2.10404@jp.fujitsu.com>
Date:	Tue, 21 Jul 2009 15:49:22 +0900
From:	Hidetoshi Seto <seto.hidetoshi@...fujitsu.com>
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>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	LKML <linux-kernel@...r.kernel.org>,
	"Ken'ichi Ohmichi" <oomichi@....nes.nec.co.jp>
Subject: Re: [PATCH] sysrq, kdump: fix regression, revert "simplify sysrq-c
 handler"

Lai Jiangshan wrote:
> 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.
> 
> 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.
> 
> |commit d6580a9f15238b87e618310c862231ae3f352d2d
> |Author: Neil Horman <nhorman@...driver.com>
> |Date:   Wed Jun 17 16:28:17 2009 -0700
> 
> |    kexec: sysrq: simplify sysrq-c handler
> 
> |    Currently the sysrq-c handler is bit over-engineered.  Its behavior is
> |    dependent on a few compile time and run time factors that alter its
> |    behavior which is really unnecessecary.
> 
> |    If CONFIG_KEXEC is not configured, sysrq-c, crashes the system with a NULL
> |    pointer dereference.  If CONFIG_KEXEC is configured, it calls crash_kexec
> |    directly, which implies that the kexec kernel will either be booted (if
> |    its been previously loaded), or it will simply do nothing (the no kexec
> |    kernel has been loaded).
> 
> |    It would be much easier to just simplify the whole thing to dereference a
> |    NULL pointer all the time regardless of configuration.  That way, it will
> |    always try to crash the system, and if a kexec kernel has been loaded into
> |    reserved space, it will still boot from the page fault trap handler
> |    (assuming panic_on_oops is set appropriately).
> 
> 
> Signed-off-by: Lai Jiangshan <laijs@...fujitsu.com>
> ---

FYI, this problem has already pointed by Ohmichi-san and this will be an
another patch for the following discussion:
  http://lists.infradead.org/pipermail/kexec/2009-July/003433.html
You can find my sloppy memo in:
  http://lists.infradead.org/pipermail/kexec/2009-July/003443.html

I agree with you that SysRq-'c' is well known as for 'C'rashdump, and it is
not expected as 'C'rash without dump.


Thanks,
H.Seto

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