Re: [PATCH v4 5/5] powerpc/vdso: Wire up getrandom() vDSO implementation on PPC64

Christophe Leroy
2024-09-02T14:16:51Z

Le 02/09/2024 à 16:00, Jason A. Donenfeld a écrit :
> On Mon, Sep 02, 2024 at 03:12:47PM +0200, Christophe Leroy wrote:
>>
>>
>> Le 02/09/2024 à 14:41, Jason A. Donenfeld a écrit :
>>> On Mon, Sep 02, 2024 at 02:04:42PM +0200, Christophe Leroy wrote:
>>>> SYM_FUNC_START(__arch_chacha20_blocks_nostack)
>>>> #ifdef __powerpc64__
>>>> - blr
>>>> + std r5, -216(r1)
>>>> +
>>>> + std r14, -144(r1)
>>>> + std r15, -136(r1)
>>>> + std r16, -128(r1)
>>>> + std r17, -120(r1)
>>>> + std r18, -112(r1)
>>>> + std r19, -104(r1)
>>>> + std r20, -96(r1)
>>>> + std r21, -88(r1)
>>>> + std r22, -80(r1)
>>>> + std r23, -72(r1)
>>>> + std r24, -64(r1)
>>>> + std r25, -56(r1)
>>>> + std r26, -48(r1)
>>>> + std r27, -40(r1)
>>>> + std r28, -32(r1)
>>>> + std r29, -24(r1)
>>>> + std r30, -16(r1)
>>>> + std r31, -8(r1)
>>>> #else
>>>> stwu r1, -96(r1)
>>>> stw r5, 20(r1)
>>>> +#ifdef __BIG_ENDIAN__
>>>> stmw r14, 24(r1)
>>>> +#else
>>>> + stw r14, 24(r1)
>>>> + stw r15, 28(r1)
>>>> + stw r16, 32(r1)
>>>> + stw r17, 36(r1)
>>>> + stw r18, 40(r1)
>>>> + stw r19, 44(r1)
>>>> + stw r20, 48(r1)
>>>> + stw r21, 52(r1)
>>>> + stw r22, 56(r1)
>>>> + stw r23, 60(r1)
>>>> + stw r24, 64(r1)
>>>> + stw r25, 68(r1)
>>>> + stw r26, 72(r1)
>>>> + stw r27, 76(r1)
>>>> + stw r28, 80(r1)
>>>> + stw r29, 84(r1)
>>>> + stw r30, 88(r1)
>>>> + stw r31, 92(r1)
>>>> +#endif
>>>> +#endif
>>>
>>> This confuses me. Why are you adding code to the !__powerpc64__ branch
>>> in this commit? (Also, why does stmw not work on LE?)
>>
>> That's for the VDSO32 ie running 32 bits binaries on a 64 bits kernel.
>>
>> "Programming Environments Manual for 32-Bit Implementations of the
>> PowerPC™ Architecture" say: In some implementations operating with
>> little-endian byte order, execution of an lmw or stmw instruction
>> causes the system alignment error handler to be invoked
>>
>> And GCC doesn't like it either:
>>
>> tools/arch/powerpc/vdso/vgetrandom-chacha.S:84: Error: `stmw' invalid
>> when little-endian
>
> Does it make sense to do all the 32-bit stuff in the PPC32 commit (and
> then you can introduce the selftests there without the error you
> mentioned), and then add the 64-bit stuff in this commit?

Can do that, but there will still be a problem with chacha selftests if
I don't opt-out the entire function content when it is ppc64. It will
build properly but if someone runs it on a ppc64 it will likely crash
because only the low 32 bits of registers will be saved.

That's the reason why I really prefered the approach where I set
something in vdso_config.h so that the assembly is used only for
powerpc32 and when building powerpc64 the assembly part is kept out and
vdso_test_chacha simply tells it is not supported.

Christophe