Re: [PATCH] powerpc/64: Avoid link stack corruption in kexec_wait()

Daniel Axtens
2021-08-31T12:43:12Z

Christophe Leroy <christophe.leroy@csgroup.eu> writes:

> Le 31/08/2021 à 08:17, Daniel Axtens a écrit :
>> Hi Christophe,
>>
>>> Use bcl 20,31,+4 instead of bl in order to preserve link stack.
>>>
>>> See commit c974809a26a1 ("powerpc/vdso: Avoid link stack corruption
>>> in __get_datapage()") for details.
>>
>> From my understanding of that commit message, the change helps to keep
>> the link stack correctly balanced which is helpful for performance,
>> rather than for correctness. If I understand correctly, kexec_wait is
>> not in a hot path - rather it is where CPUs spin while waiting for
>> kexec. Is there any benefit in using the more complicated opcode in this
>> situation?
>
> AFAICS the main benefit is to keep things consistent over the kernel and not have to wonder "is it a
> hot path or not ? If it is I use bcl 20,31, if it is not I use bl". The best way to keep things in
> order is to always use the right instruction.

Yeah, Nick Piggin convinced me of this offline as well.

>
>>
>>> Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
>>> ---
>>> arch/powerpc/kernel/misc_64.S | 2 +-
>>> 1 file changed, 1 insertion(+), 1 deletion(-)
>>>
>>> diff --git a/arch/powerpc/kernel/misc_64.S b/arch/powerpc/kernel/misc_64.S
>>> index 4b761a18a74d..613509907166 100644
>>> --- a/arch/powerpc/kernel/misc_64.S
>>> +++ b/arch/powerpc/kernel/misc_64.S
>>> @@ -255,7 +255,7 @@ _GLOBAL(scom970_write)
>>> * Physical (hardware) cpu id should be in r3.
>>> */
>>> _GLOBAL(kexec_wait)
>>> - bl 1f
>>> + bcl 20,31,1f
>>> 1: mflr r5
>>
>> Would it be better to create a macro of some sort to wrap this unusual
>> special form so that the meaning is more clear?
>
> Not sure, I think people working with assembly will easily recognise that form whereas an obscure
> macro is always puzzling.
>
> I like macros when they allow you to not repeat again and again the same sequence of several
> instructions, but here it is a single quite simple instruction which is not worth a macro in my mind.
>

Sure - I was mostly thinking specifically of the bcl; mflr situation but
I agree that for the single instruction it's not needed.

In short, I am convinced, and so:
Reviewed-by: Daniel Axtens <dja@axtens.net>

Kind regards,
Daniel

> Christophe