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

[Lai Jiangshan <laijs@cn.fujitsu.com>]

Eric W. Biederman wrote:
> Lai Jiangshan <laijs@cn.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?

> 
>> 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.