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

Christophe Leroy
2024-09-02T13:12:50Z

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