[PATCH v2] random: vDSO: Avoid call to memset() when zeroing reserved in __cvdso_getrandom_data()

Fri Oct 2 11:21:47 PDT 2026

Le 02/10/2026 à 12:13, Jason A. Donenfeld a écrit :
> On Fri, Oct 02, 2026 at 11:49:32AM +0200, Nathan Chancellor wrote:
>> On Thu, Oct 01, 2026 at 01:03:10PM +0200, Nathan Chancellor wrote:
>>> On Thu, Oct 01, 2026 at 12:48:10PM +0200, Jason A. Donenfeld wrote:
>>>> Does this commit seem okay with you? I used the diff you sent below and
>>>> adjusted the commit message: https://eur01.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgit.zx2c4.com%2Flinux-rng%2Fcommit%2F%3Fid%3D56ff95ee85715047eb5b5220243778af657778c8&data=05%7C02%7Cchristophe.leroy%40csgroup.eu%7C4caf8956c2d94439c19a08df206dd40b%7C8b87af7d86474dc78df45f69a2011bb5%7C0%7C0%7C639265328227648970%7CUnknown%7CTWFpbGZsb3d8eyJFbXB0eU1hcGkiOnRydWUsIlYiOiIwLjAuMDAwMCIsIlAiOiJXaW4zMiIsIkFOIjoiTWFpbCIsIldUIjoyfQ%3D%3D%7C0%7C%7C%7C&sdata=H27VGYLdNkvQiA6l%2BJNnEModMbEySwTMXpLAt8iAFiI%3D&reserved=0
>>>
>>> Yeah, that seems fine to me, thanks for taking care of it!
>>
>> Can you adjust the LLVM value by one from 4294967295 to 4294967294? ~0U
>> is actually a special value, so we hit an assertion in the SystemZ
>> backend.
>>
>>    https://eur01.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgithub.com%2Fllvm%2Fllvm-project%2Fblob%2F3f48e22a1f321d5d3341bd812803694cea588785%2Fllvm%2Flib%2FCodeGen%2FSelectionDAG%2FSelectionDAG.cpp%23L9547&data=05%7C02%7Cchristophe.leroy%40csgroup.eu%7C4caf8956c2d94439c19a08df206dd40b%7C8b87af7d86474dc78df45f69a2011bb5%7C0%7C0%7C639265328227670583%7CUnknown%7CTWFpbGZsb3d8eyJFbXB0eU1hcGkiOnRydWUsIlYiOiIwLjAuMDAwMCIsIlAiOiJXaW4zMiIsIkFOIjoiTWFpbCIsIldUIjoyfQ%3D%3D%7C0%7C%7C%7C&sdata=u8petgY9wU9ur%2By4kaOrb6VFKL%2FNf4tcCIUL%2BcUcDfs%3D&reserved=0
>>    https://eur01.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgithub.com%2Fllvm%2Fllvm-project%2Fblob%2F3f48e22a1f321d5d3341bd812803694cea588785%2Fllvm%2Flib%2FTarget%2FSystemZ%2FSystemZISelLowering.cpp%23L1470-L1471&data=05%7C02%7Cchristophe.leroy%40csgroup.eu%7C4caf8956c2d94439c19a08df206dd40b%7C8b87af7d86474dc78df45f69a2011bb5%7C0%7C0%7C639265328227686571%7CUnknown%7CTWFpbGZsb3d8eyJFbXB0eU1hcGkiOnRydWUsIlYiOiIwLjAuMDAwMCIsIlAiOiJXaW4zMiIsIkFOIjoiTWFpbCIsIldUIjoyfQ%3D%3D%7C0%7C%7C%7C&sdata=cSwt%2BTJdYa%2Bbwe8HyO6FsIq6oU6YNplRlwCa0upif9Q%3D&reserved=0
>>
>> clang: llvm/lib/Target/SystemZ/SystemZISelLowering.cpp:1471: virtual bool llvm::SystemZTargetLowering::findOptimalMemOpLowering(LLVMContext &, std::vector<EVT> &, unsigned int, const MemOp &, unsigned int, unsigned int, const AttributeList &, EVT *) const: Assertion `Limit != ~0U && "Expected EmitTargetCodeForMemXXX() to handle AlwaysInline cases."' failed.
>> PLEASE submit a bug report to https://eur01.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgithub.com%2Fllvm%2Fllvm-project%2Fissues%2F&data=05%7C02%7Cchristophe.leroy%40csgroup.eu%7C4caf8956c2d94439c19a08df206dd40b%7C8b87af7d86474dc78df45f69a2011bb5%7C0%7C0%7C639265328227701575%7CUnknown%7CTWFpbGZsb3d8eyJFbXB0eU1hcGkiOnRydWUsIlYiOiIwLjAuMDAwMCIsIlAiOiJXaW4zMiIsIkFOIjoiTWFpbCIsIldUIjoyfQ%3D%3D%7C0%7C%7C%7C&sdata=%2F3q8laZb%2FDhCWp117N9EtyxLIc5lq%2FzEN2l0z%2BXwDHY%3D&reserved=0 and include the crash backtrace and dumped files.
>> Stack dump:
>> 0.      Program arguments: ...
>> 1.      <eof> parser at end of file
>> 2.      Code generation
>> 3.      Running pass 'Function Pass Manager' on module 'arch/s390/kernel/vdso/vgetrandom.c'.
>> 4.      Running pass 'SystemZ DAG->DAG Pattern Instruction Selection' on function '@__kernel_getrandom'
>> ...
> 
> Holy smokes. Sure, fixed:  https://eur01.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgit.zx2c4.com%2Flinux-rng%2Fcommit%2F%3Fid%3Deb13a1ff271b0d180687eebb30611b0b276d5193&data=05%7C02%7Cchristophe.leroy%40csgroup.eu%7C4caf8956c2d94439c19a08df206dd40b%7C8b87af7d86474dc78df45f69a2011bb5%7C0%7C0%7C639265328227716550%7CUnknown%7CTWFpbGZsb3d8eyJFbXB0eU1hcGkiOnRydWUsIlYiOiIwLjAuMDAwMCIsIlAiOiJXaW4zMiIsIkFOIjoiTWFpbCIsIldUIjoyfQ%3D%3D%7C0%7C%7C%7C&sdata=WcnFE6S72OrDTNNvU1T06BtUTyI1sDx%2F%2Btz%2FkpGslMA%3D&reserved=0
> 
> 
>  From eb13a1ff271b0d180687eebb30611b0b276d5193 Mon Sep 17 00:00:00 2001
> From: Nathan Chancellor <nathan at kernel.org>
> Date: Fri, 25 Sep 2026 22:46:32 +0100
> Subject: [PATCH] random: vDSO: avoid call to memset() when zeroing reserved
>   parameter
> 
> After a recent change in LLVM [1], builds with the random vDSO
> implementation, such as PowerPC and RISC-V, fail when checking the vDSO:
> 
>    arch/powerpc/kernel/vdso/vdso32.so.dbg: dynamic relocations are not supported
>    arch/riscv/kernel/vdso/vdso.so.dbg: dynamic relocations are not supported
> 
> memset() is now generated when zeroing params->reserved for some builds
> because LLVM has an optimization (now run in more instances) that can
> recognize at compile time when it is assigning a static value to a
> contiguous area of memory and turn that into a call to memset(). Both
> clang and GCC assume memset() is always available [2].
> 
> Clang has an internal fiddly hook, -max-store-memset, which we can set
> to a high number, to disable generating out of line memset calls [3].
> Similarly, GCC has -finline-stringops=memset to do the same [4], should
> this issue ever hit future version of GCC. While these options wouldn't
> make sense for normal kernel code, it is fine for the extremely limited
> and intentionally compact vDSO code.
> 
> Link: https://eur01.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgithub.com%2Fllvm%2Fllvm-project%2Fcommit%2F90cebef1411617fc3eedd359bdf00cb44b1c2439&data=05%7C02%7Cchristophe.leroy%40csgroup.eu%7C4caf8956c2d94439c19a08df206dd40b%7C8b87af7d86474dc78df45f69a2011bb5%7C0%7C0%7C639265328227732238%7CUnknown%7CTWFpbGZsb3d8eyJFbXB0eU1hcGkiOnRydWUsIlYiOiIwLjAuMDAwMCIsIlAiOiJXaW4zMiIsIkFOIjoiTWFpbCIsIldUIjoyfQ%3D%3D%7C0%7C%7C%7C&sdata=cYZ2nUalY%2F6yarydfMIlcGpDVlztXRBeg7UG7tpCO9g%3D&reserved=0 [1]
> Link: https://eur01.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgcc.gnu.org%2Fonlinedocs%2Fgcc-16.2.0%2Fgcc%2FStandards.html%23index-ffreestanding&data=05%7C02%7Cchristophe.leroy%40csgroup.eu%7C4caf8956c2d94439c19a08df206dd40b%7C8b87af7d86474dc78df45f69a2011bb5%7C0%7C0%7C639265328227748542%7CUnknown%7CTWFpbGZsb3d8eyJFbXB0eU1hcGkiOnRydWUsIlYiOiIwLjAuMDAwMCIsIlAiOiJXaW4zMiIsIkFOIjoiTWFpbCIsIldUIjoyfQ%3D%3D%7C0%7C%7C%7C&sdata=vRl%2BQgL5GJ0wowXAIlRIczC5XqUXyOFUvFVe5C8XWAc%3D&reserved=0 [2]
> Link: https://eur01.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgithub.com%2Fllvm%2Fllvm-project%2Fcommit%2Fb28eeb28bea39148738dc375e8a97072a1907e64&data=05%7C02%7Cchristophe.leroy%40csgroup.eu%7C4caf8956c2d94439c19a08df206dd40b%7C8b87af7d86474dc78df45f69a2011bb5%7C0%7C0%7C639265328227762721%7CUnknown%7CTWFpbGZsb3d8eyJFbXB0eU1hcGkiOnRydWUsIlYiOiIwLjAuMDAwMCIsIlAiOiJXaW4zMiIsIkFOIjoiTWFpbCIsIldUIjoyfQ%3D%3D%7C0%7C%7C%7C&sdata=nt59J3%2BOoiEKYGzfFfAq7WoAnEMAimt6vKDaJjhvHtg%3D&reserved=0 [3]
> Link: https://eur01.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgcc.gnu.org%2Fonlinedocs%2Fgcc%2FOptimize-Options.html%23index-finline-stringops&data=05%7C02%7Cchristophe.leroy%40csgroup.eu%7C4caf8956c2d94439c19a08df206dd40b%7C8b87af7d86474dc78df45f69a2011bb5%7C0%7C0%7C639265328227777215%7CUnknown%7CTWFpbGZsb3d8eyJFbXB0eU1hcGkiOnRydWUsIlYiOiIwLjAuMDAwMCIsIlAiOiJXaW4zMiIsIkFOIjoiTWFpbCIsIldUIjoyfQ%3D%3D%7C0%7C%7C%7C&sdata=fyZA9XC%2FwSAUc346VWT%2FmyeqddqeTo62dBKtiJilR9U%3D&reserved=0 [4]
> Closes: https://eur01.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgithub.com%2FClangBuiltLinux%2Flinux%2Fissues%2F2183&data=05%7C02%7Cchristophe.leroy%40csgroup.eu%7C4caf8956c2d94439c19a08df206dd40b%7C8b87af7d86474dc78df45f69a2011bb5%7C0%7C0%7C639265328227792263%7CUnknown%7CTWFpbGZsb3d8eyJFbXB0eU1hcGkiOnRydWUsIlYiOiIwLjAuMDAwMCIsIlAiOiJXaW4zMiIsIkFOIjoiTWFpbCIsIldUIjoyfQ%3D%3D%7C0%7C%7C%7C&sdata=49HfUDlzn%2FLUeISQ7%2BFR5jE7lZkqfORaqjauLn8cdm0%3D&reserved=0
> Cc: stable at vger.kernel.org # v6.12+
> Signed-off-by: Nathan Chancellor <nathan at kernel.org>
> Signed-off-by: Jason A. Donenfeld <Jason at zx2c4.com>

Ok, I checked the version that is in the rng tree, namely commit 
eb13a1ff271b ("random: vDSO: avoid call to memset() when zeroing 
reserved parameter"). Looks similar to this mail.

It looks ok, no change to generated loop on powerpc32 neither with gcc 
13 nor gcc 16.

Reviewed-by: Christophe Leroy (CS GROUP) <chleroy at kernel.org>

> ---
>   arch/arm64/kernel/vdso/Makefile     | 2 +-
>   arch/loongarch/vdso/Makefile        | 1 +
>   arch/powerpc/kernel/vdso/Makefile   | 1 +
>   arch/riscv/kernel/vdso/Makefile     | 1 +
>   arch/s390/kernel/vdso/Makefile      | 1 +
>   arch/x86/entry/vdso/vdso64/Makefile | 2 +-
>   init/Kconfig                        | 5 +++++
>   7 files changed, 11 insertions(+), 2 deletions(-)
> 