Subject: Re: [PATCH] powerpc/vdso: Fix incorrect CFI in gettimeofday.S

From: Michael Ellerman <[email-masked]>
Date: Tue, 17 May 2022 22:32:09 +1000

"Naveen N. Rao" <[email-masked]> writes:
> Michael Ellerman wrote:
>>
>> diff --git a/arch/powerpc/kernel/vdso/gettimeofday.S b/arch/powerpc/kernel/vdso/gettimeofday.S
>> index eb9c81e1c218..0aee255e9cbb 100644
>> --- a/arch/powerpc/kernel/vdso/gettimeofday.S
>> +++ b/arch/powerpc/kernel/vdso/gettimeofday.S
>> @@ -22,12 +22,15 @@
>>  .macro cvdso_call funct call_time=0
>>    .cfi_startproc
>>  	PPC_STLU	r1, -PPC_MIN_STKFRM(r1)
>> +  .cfi_adjust_cfa_offset PPC_MIN_STKFRM
>>  	mflr		r0
>> -  .cfi_register lr, r0
>>  	PPC_STLU	r1, -PPC_MIN_STKFRM(r1)
>> +  .cfi_adjust_cfa_offset PPC_MIN_STKFRM
>>  	PPC_STL		r0, PPC_MIN_STKFRM + PPC_LR_STKOFF(r1)
>
> <snip>
>
>> @@ -46,6 +50,7 @@
>>  	mtlr		r0
>>    .cfi_restore lr
>>  	addi		r1, r1, 2 * PPC_MIN_STKFRM
>> +  .cfi_def_cfa_offset 0
>
> Should this be .cfi_adjust_cfa_offset, given that we used that at the
> start of the function?
 
AIUI "adjust x" is offset += x, whereas "def x" is offset = x.

So we could use adjust here, but we'd need to adjust by -(2 * PPC_MIN_STKFRM).

It seemed clearer to just set the offset back to 0, which is what it is
at the start of the function.

But I'm not a CFI expert at all, so I'll defer to anyone else who has an
opinion :)

cheers