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

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:

quoted

On Thu, Oct 01, 2026 at 01:03:10PM +0200, Nathan Chancellor wrote:

quoted

On Thu, Oct 01, 2026 at 12:48:10PM +0200, Jason A. Donenfeld wrote:

quoted

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@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@vger.kernel.org # v6.12+
Signed-off-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Jason A. Donenfeld <Jason@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@kernel.org>