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

[Nathan Chancellor <nathan@kernel.org>]

After a recent change in LLVM [1], RISC-V builds fail when checking the
vDSO:

  arch/riscv/kernel/vdso/vdso.so.dbg: dynamic relocations are not supported
  make[4]: *** [arch/riscv/kernel/vdso/Makefile:78: arch/riscv/kernel/vdso/vdso.so.dbg] Error 1

memset() is now generated when zeroing params->reserved 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].

Hide the value of the iterator variable from the optimizer using
OPTIMIZER_HIDE_VAR to inhibit this optimization since it can no longer
assume that the zeroing is contiguous.

Link: https://github.com/llvm/llvm-project/commit/90cebef1411617fc3eedd359bdf00cb44b1c2439 [1]
Link: https://gcc.gnu.org/onlinedocs/gcc-16.2.0/gcc/Standards.html#index-ffreestanding [2]
Signed-off-by: Nathan Chancellor <nathan@kernel.org>
---
 lib/vdso/getrandom.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/lib/vdso/getrandom.c b/lib/vdso/getrandom.c
index 2851afa9154f..d48d1fdb3351 100644
--- a/lib/vdso/getrandom.c
+++ b/lib/vdso/getrandom.c
@@ -4,6 +4,7 @@
  */
 
 #include <linux/array_size.h>
+#include <linux/compiler.h>
 #include <linux/minmax.h>
 #include <vdso/datapage.h>
 #include <vdso/getrandom.h>
@@ -83,8 +84,11 @@ __cvdso_getrandom_data(const struct vdso_rng_data *rng_info, void *buffer, size_
 		params->size_of_opaque_state = sizeof(*state);
 		params->mmap_prot = PROT_READ | PROT_WRITE;
 		params->mmap_flags = MAP_DROPPABLE | MAP_ANONYMOUS;
-		for (size_t i = 0; i < ARRAY_SIZE(params->reserved); ++i)
+		for (size_t i = 0; i < ARRAY_SIZE(params->reserved); ++i) {
+			/* prevent compiler from turning loop into memset() */
+			OPTIMIZER_HIDE_VAR(i);
 			params->reserved[i] = 0;
+		}
 		return 0;
 	}
 

---
base-commit: cee9395acd8043be0644b25c34bfa86623f2b935
change-id: 20260916-vdso-getrandom-avoid-memset-llvm-24-74d45fa6031e

Best regards,
--  
Cheers,
Nathan