Re: [PATCH] random: vDSO: Avoid call to memset() when zeroing reserved in __cvdso_getrandom_data()
From: Nathan Chancellor
Date: Fri Sep 18 2026 - 17:08:26 EST
On Fri, Sep 18, 2026 at 09:19:00PM +0200, Jason A. Donenfeld wrote:
> Untested, but putting this all together I suppose looks like this, if
> you want to play around and see:
Thanks!
> diff --git a/lib/vdso/getrandom.c b/lib/vdso/getrandom.c
> index 2851afa9154f..713e95dfdedc 100644
> --- a/lib/vdso/getrandom.c
> +++ b/lib/vdso/getrandom.c
> @@ -29,6 +29,12 @@
> } \
> } while (0)
>
> +#if __has_builtin(__builtin_memset_inline)
> +#define memset(dst, value, size) __builtin_memset_inline(dst, value, size)
> +#else
> +#define memset(dst, value, size) __builtin_memset(dst, value, size)
> +#endif
> +
> static void memcpy_and_zero_src(void *dst, void *src, size_t len)
> {
> if (IS_ENABLED(CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS)) {
> @@ -83,8 +89,7 @@ __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)
> - params->reserved[i] = 0;
> + memset(params->reserved, 0, sizeof(params->reserved));
> return 0;
> }
This will regress GCC < 14 because '-finline-stringops=memset' does not
exist yet, meaning __builtin_memset() may still generated a call to
memset(). Testing GCC 13 with ARCH=riscv defconfig, I get the same error
as originally reported up thread. I am not sure we can get rid of the
loop for that reason. I tested the following diff that keeps
clang (__builtin_memset_inline)
GCC 14+ (-finline-stringops=memset + __builtin_memset)
GCC <14 (simple loop)
happy (and moves the checks into Kconfig, which is being discussed at a
wider level [1]) but it is admittedly slightly uglier... That said, it
certainly seems more robust.
[1]: https://lore.kernel.org/linux-kbuild/20260917-build-speedup-v3-11-9ecf4163ff36@xxxxxxxxxx
diff --git a/arch/arm64/kernel/vdso/Makefile b/arch/arm64/kernel/vdso/Makefile
index 7dec05dd33b7..f6619e1cb2ce 100644
--- a/arch/arm64/kernel/vdso/Makefile
+++ b/arch/arm64/kernel/vdso/Makefile
@@ -41,7 +41,7 @@ CC_FLAGS_REMOVE_VDSO := $(CC_FLAGS_FTRACE) -Os $(CC_FLAGS_SCS) \
$(CC_FLAGS_LTO) $(CC_FLAGS_CFI) \
-Wmissing-prototypes -Wmissing-declarations
-CC_FLAGS_ADD_VDSO := -O2 -mcmodel=tiny -fasynchronous-unwind-tables
+CC_FLAGS_ADD_VDSO := -O2 -mcmodel=tiny -fasynchronous-unwind-tables $(CONFIG_CC_OPT_INLINE_MEMSET)
CFLAGS_REMOVE_vgettimeofday.o = $(CC_FLAGS_REMOVE_VDSO)
CFLAGS_REMOVE_vgetrandom.o = $(CC_FLAGS_REMOVE_VDSO)
diff --git a/arch/loongarch/vdso/Makefile b/arch/loongarch/vdso/Makefile
index 9c9181bb4071..0b84892d084b 100644
--- a/arch/loongarch/vdso/Makefile
+++ b/arch/loongarch/vdso/Makefile
@@ -16,6 +16,7 @@ ccflags-vdso := \
$(filter -m64,$(KBUILD_CFLAGS)) \
$(filter -march=%,$(KBUILD_CFLAGS)) \
$(filter -m%-float,$(KBUILD_CFLAGS)) \
+ $(CONFIG_CC_OPT_INLINE_MEMSET) \
$(CLANG_FLAGS) \
-D__VDSO__
diff --git a/arch/riscv/kernel/vdso/Makefile b/arch/riscv/kernel/vdso/Makefile
index 8dbf2532a573..c023046a3fd7 100644
--- a/arch/riscv/kernel/vdso/Makefile
+++ b/arch/riscv/kernel/vdso/Makefile
@@ -36,6 +36,7 @@ endif
ccflags-y := -fno-stack-protector
ccflags-y += -DDISABLE_BRANCH_PROFILING
ccflags-y += -fno-builtin
+ccflags-y += $(CONFIG_CC_OPT_INLINE_MEMSET)
ccflags-y += $(KBUILD_BASE_ISA)$(CFI_MARCH)
ccflags-y += $(CFI_FULL)
asflags-y += $(KBUILD_BASE_ISA)$(CFI_MARCH)
diff --git a/arch/s390/kernel/vdso/Makefile b/arch/s390/kernel/vdso/Makefile
index 35c834b895ec..54bcf2984ca1 100644
--- a/arch/s390/kernel/vdso/Makefile
+++ b/arch/s390/kernel/vdso/Makefile
@@ -29,6 +29,7 @@ KBUILD_CFLAGS_VDSO := $(filter-out -munaligned-symbols,$(KBUILD_CFLAGS_VDSO))
KBUILD_CFLAGS_VDSO := $(filter-out -fno-asynchronous-unwind-tables,$(KBUILD_CFLAGS_VDSO))
KBUILD_CFLAGS_VDSO += -fPIC -fno-common -fno-builtin -fasynchronous-unwind-tables
KBUILD_CFLAGS_VDSO += -fno-stack-protector $(DISABLE_KSTACK_ERASE)
+KBUILD_CFLAGS_VDSO += $(CONFIG_CC_OPT_INLINE_MEMSET)
ldflags-y := -shared -soname=linux-vdso.so.1 \
--hash-style=both --build-id=sha1 \
$(call ld-option, --eh-frame-hdr) -T
diff --git a/arch/x86/entry/vdso/vdso64/Makefile b/arch/x86/entry/vdso/vdso64/Makefile
index 7c0790065b5e..c2353a065279 100644
--- a/arch/x86/entry/vdso/vdso64/Makefile
+++ b/arch/x86/entry/vdso/vdso64/Makefile
@@ -14,7 +14,7 @@ vobjs-$(CONFIG_X86_SGX) += vsgx.o
vobjs-$(CONFIG_FUTEX_ROBUST_UNLOCK) += vfutex.o
# Compilation flags
-flags-y := -DBUILD_VDSO64 -m64 -mcmodel=small
+flags-y := -DBUILD_VDSO64 -m64 -mcmodel=small $(CONFIG_CC_OPT_INLINE_MEMSET)
# The location of this include matters!
include $(src)/../common/Makefile.include
diff --git a/init/Kconfig b/init/Kconfig
index 6e82b4957a5d..09251b5f79d4 100644
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -173,6 +173,13 @@ config CC_HAS_ALLOC_TOKEN
config CC_HAS_MULTIDIMENSIONAL_NONSTRING
def_bool $(success,echo 'char tag[][4] __attribute__((__nonstring__)) = { };' | $(CC) $(CLANG_FLAGS) -x c - -c -o /dev/null -Werror)
+config CC_HAS_OPT_INLINE_MEMSET
+ def_bool $(cc-option,-finline-stringops=memset)
+
+config CC_OPT_INLINE_MEMSET
+ string
+ default "-finline-stringops=memset" if CC_HAS_OPT_INLINE_MEMSET
+
config LD_CAN_USE_KEEP_IN_OVERLAY
# ld.lld prior to 21.0.0 did not support KEEP within an overlay description
# https://github.com/llvm/llvm-project/pull/130661
diff --git a/lib/vdso/getrandom.c b/lib/vdso/getrandom.c
index 2851afa9154f..83831dab08b2 100644
--- a/lib/vdso/getrandom.c
+++ b/lib/vdso/getrandom.c
@@ -29,6 +29,12 @@
} \
} while (0)
+#if __has_builtin(__builtin_memset_inline)
+#define memset_inline(dst, value, size) __builtin_memset_inline(dst, value, size)
+#elif IS_ENABLED(CONFIG_CC_HAS_OPT_INLINE_MEMSET)
+#define memset_inline(dst, value, size) __builtin_memset(dst, value, size)
+#endif
+
static void memcpy_and_zero_src(void *dst, void *src, size_t len)
{
if (IS_ENABLED(CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS)) {
@@ -83,8 +89,12 @@ __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;
+#ifdef memset_inline
+ memset_inline(params->reserved, 0, sizeof(params->reserved));
+#else
for (size_t i = 0; i < ARRAY_SIZE(params->reserved); ++i)
params->reserved[i] = 0;
+#endif
return 0;
}
--
Cheers,
Nathan