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

[Christophe Leroy]

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