Re: [PATCH v2] random: vDSO: Avoid call to memset() when zeroing reserved in __cvdso_getrandom_data()
From: Christophe Leroy (CS GROUP)
Date: Thu Oct 01 2026 - 00:43:08 EST
Hi,
Le 30/09/2026 à 17:16, Jason A. Donenfeld a écrit :
On Wed, Sep 30, 2026 at 5:13 PM Nathan Chancellor <nathan@xxxxxxxxxx> wrote:
On Wed, Sep 30, 2026 at 04:44:29PM +0200, Jason A. Donenfeld wrote:
Nathan, would this be okay with you?
https://eur01.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgit.zx2c4.com%2Flinux-rng%2Fcommit%2F%3Fid%3Dd216701724b7d8209ff42150658ad5c712bdb503&data=05%7C02%7Cchristophe.leroy2%40cs-soprasteria.com%7C8f69935bffb344bf0ddc08df1f05e358%7C8b87af7d86474dc78df45f69a2011bb5%7C0%7C0%7C639263782324080837%7CUnknown%7CTWFpbGZsb3d8eyJFbXB0eU1hcGkiOnRydWUsIlYiOiIwLjAuMDAwMCIsIlAiOiJXaW4zMiIsIkFOIjoiTWFpbCIsIldUIjoyfQ%3D%3D%7C0%7C%7C%7C&sdata=BA7Fyr0%2FynggvBnR%2FpiCvNCQHtNBagqvSb0i11AvoLU%3D&reserved=0
Can you stick
Cc: stable@xxxxxxxxxxxxxxx # v6.12+
Closes: https://eur01.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgithub.com%2FClangBuiltLinux%2Flinux%2Fissues%2F2183&data=05%7C02%7Cchristophe.leroy2%40cs-soprasteria.com%7C8f69935bffb344bf0ddc08df1f05e358%7C8b87af7d86474dc78df45f69a2011bb5%7C0%7C0%7C639263782324110679%7CUnknown%7CTWFpbGZsb3d8eyJFbXB0eU1hcGkiOnRydWUsIlYiOiIwLjAuMDAwMCIsIlAiOiJXaW4zMiIsIkFOIjoiTWFpbCIsIldUIjoyfQ%3D%3D%7C0%7C%7C%7C&sdata=6C8%2FAEQoh%2B8pfgfYQkL7qf5Ue%2F5sAGtfqRptzsSG0CM%3D&reserved=0
on that? Otherwise, looks good to me, that's basically what I had for my
v3 locally.
Sure, done. Also removed the now-unused array_size.h include.
I'm still very sceptic with this patch. You are degrading the behaviour with GCC for a problem with CLANG. Why ?
Before the patch, with both GCC 13 and GCC 16 on powerpc32 I get a pretty standard optimised loop that clears words 4 by 4 (with auto increment of pointer) which is the most optimal on powerpc:
3f0: 39 00 00 0c li r8,12
3f4: 35 08 ff fc addic. r8,r8,-4
3f8: 91 49 00 04 stw r10,4(r9)
3fc: 91 49 00 08 stw r10,8(r9)
400: 91 49 00 0c stw r10,12(r9)
404: 95 49 00 10 stwu r10,16(r9)
408: 40 82 ff ec bne 3f4 <__c_kernel_getrandom+0x3f4>
With the patch,
With GCC 13 I get a very suboptimal loop copying bytes one by one
3d8: 39 40 00 34 li r10,52
...
3e4: 39 20 00 00 li r9,0
3e8: 7d 49 03 a6 mtctr r10
3ec: 9d 3e 00 01 stbu r9,1(r30)
3f0: 42 00 ff fc bdnz 3ec <__c_kernel_getrandom+0x3ec>
With GCC 16 I get something a bit better but not as good as before, it is a loop clearing words only one by one and incrementing pointer with an additional insn instead of using auto-increment instruction stwu.
3e0: 39 40 00 0d li r10,13
...
3f0: 7d 49 03 a6 mtctr r10
3f4: 91 3f 00 00 stw r9,0(r31)
3f8: 3b ff 00 04 addi r31,r31,4
3fc: 42 00 ff f8 bdnz 3f4 <__c_kernel_getrandom+0x3f4>
Please restrict the patch to clang builds.
Christophe