[PATCH v2 2/3] powerpc/kprobes: un-blacklist system_call() from kprobes

[Michael Ellerman]

"Naveen N. Rao" <naveen.n.rao at linux.vnet.ibm.com> writes:
> On 2017/05/04 04:03PM, Michael Ellerman wrote:
>> Would this work?
>> 
>> diff --git a/arch/powerpc/kernel/entry_64.S b/arch/powerpc/kernel/entry_64.S
>> index 767ef6d68c9e..8d0fa4a2262a 100644
>> --- a/arch/powerpc/kernel/entry_64.S
>> +++ b/arch/powerpc/kernel/entry_64.S
>> @@ -207,6 +207,7 @@ system_call:			/* label this so stack traces look sane */
>>  	mtmsrd	r11,1
>>  #endif /* CONFIG_PPC_BOOK3E */
>> 
>> +syscall_exit:
>>  	ld	r9,TI_FLAGS(r12)
>>  	li	r11,-MAX_ERRNO
>>  	andi.	r0,r9,(_TIF_SYSCALL_DOTRACE|_TIF_SINGLESTEP|_TIF_USER_WORK_MASK|_TIF_PERSYSCALL_MASK)
>
> Ah, nice. I previously incorrectly assumed that syscall_exit was not 
> desirable throughout this function. Your earlier patch was only about 
> what label showed up while _inside_ a syscall.

Yep. When you're somewhere in a syscall the LR on the stack points to
the instruction following the bctrl that called the syscall handler, so
as long as the label preceeding that is system_call then the backtrace
should look good.

We could even just have a nop after the bctrl and then the label, but
that would be a bit gross.

> So, syscall_exit post handling of a syscall is fine.
>
> This patch looks fine to me. I will test with this change and get back.

Thanks.

cheers