On Fri, Sep 25, 2026 at 2:46 PM Nathan Chancellor <[email protected]> wrote: > > 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 provides a builtin, __builtin_memset_inline [3][4], that can be > used to ensure an external function call is not generated when zeroing > this memory. Use it when it is available. > > While GCC has no issues with the current code, it has generated memset() > before, as seen in commit b7bad082e113 ("random: vDSO: avoid call to out > of line memset()"). GCC 14 provides '-finline-stringops=memset' [5][6] > with a similar guarantee to the clang builtin, so use it and > __builtin_memset() when available. > > If no option is available, provide a simple memset_inline() like the one > from lib/string.c to avoid adding an ugly ifdef. > > 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] > Link: > https://github.com/llvm/llvm-project/commit/38637ee477541370a90b37f149069d8e5c0c2efd > [3] > Link: > https://clang.llvm.org/docs/LanguageExtensions.html#guaranteed-inlined-memset > [4] > Link: > https://gcc.gnu.org/git/?p=gcc.git;a=commit;h=1ff6d9f7428b0668cd8ab0b3e3ab94f1d733124d > [5] > Link: > https://gcc.gnu.org/onlinedocs/gcc/Optimize-Options.html#index-finline-stringops > [6] > Suggested-by: "Jason A. Donenfeld" <[email protected]> > Suggested-by: Nick Desaulniers <[email protected]> > Signed-off-by: Nathan Chancellor <[email protected]> > --- > Changes in v2: > - Switch approach entirely (Jason, Nick) > - clang: use __builtin_memset_inline() to avoid external call > - GCC 14+: use __builtin_memset + -finline-stringops=memset (hence the > massive CC list increase) > - GCC < 14: no issues currently but avoid ifdef with simple memset > implementation > - Link to v1: > https://patch.msgid.link/20260916-vdso-getrandom-avoid-memset-llvm-24-v1-1-80a92f2e2...@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 | 7 +++++++ > lib/vdso/getrandom.c | 19 +++++++++++++++++-- > 8 files changed, 30 insertions(+), 4 deletions(-) > > 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/powerpc/kernel/vdso/Makefile > b/arch/powerpc/kernel/vdso/Makefile > index 368759f81708..0d1a49529885 100644 > --- a/arch/powerpc/kernel/vdso/Makefile > +++ b/arch/powerpc/kernel/vdso/Makefile > @@ -43,6 +43,7 @@ ccflags-y := -fno-common -fno-builtin -DBUILD_VDSO > ccflags-y += $(DISABLE_LATENT_ENTROPY_PLUGIN) > ccflags-y += $(call cc-option, -fno-stack-protector) > ccflags-y += -DDISABLE_BRANCH_PROFILING > +ccflags-y += $(CONFIG_CC_OPT_INLINE_MEMSET) > ccflags-y += -ffreestanding -fasynchronous-unwind-tables > ccflags-remove-y := $(CC_FLAGS_FTRACE) > ldflags-y := -Wl,--hash-style=both -nostdlib -shared -z noexecstack > $(CLANG_FLAGS) > 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 8583d9f06c52..ff3f8475dd2f 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..f5cad5641985 100644 > --- a/lib/vdso/getrandom.c > +++ b/lib/vdso/getrandom.c > @@ -29,6 +29,22 @@ > } > \ > } 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) > +#else > +static inline void *memset_inline(void *dst, int value, size_t size) > +{ > + char *d = dst; > + > + while (size--) > + *d++ = value; > + > + return d; > +} > +#endif
Does this work? https://godbolt.org/z/Tx6EaGMfb (Sorry, I _should_ have replied to Jason on V1. I'm not ignoring him, just way behind on my code reviews. Sorry!) > + > static void memcpy_and_zero_src(void *dst, void *src, size_t len) > { > if (IS_ENABLED(CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS)) { > @@ -83,8 +99,7 @@ __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_inline(params->reserved, 0, sizeof(params->reserved)); > return 0; > } > > > --- > base-commit: cee9395acd8043be0644b25c34bfa86623f2b935 > change-id: 20260916-vdso-getrandom-avoid-memset-llvm-24-74d45fa6031e > > Best regards, > -- > Cheers, > Nathan > -- Thanks, ~Nick Desaulniers
