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

["Jason A. Donenfeld" <Jason@zx2c4.com>]

On Fri, Sep 25, 2026 at 03:00:46PM -0700, Nick Desaulniers wrote:
> On Fri, Sep 25, 2026 at 2:56 PM Nick Desaulniers
> <ndesaulniers@google.com> wrote:
> >
> > On Fri, Sep 25, 2026 at 2:53 PM Nick Desaulniers
> > <ndesaulniers@google.com> wrote:
> > >
> > > On Fri, Sep 25, 2026 at 2:46 PM Nathan Chancellor <nathan@kernel.org> 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" <Jason@zx2c4.com>
> > > > Suggested-by: Nick Desaulniers <ndesaulniers@google.com>
> > > > Signed-off-by: Nathan Chancellor <nathan@kernel.org>
> > > > ---
> > > > 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-v1-1-80a92f2e225a@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
> >
> > Hmmm...possibly.
> > https://godbolt.org/z/ec9rzd9Yf
> > I don't get it...
> 
> Just keep it under 2 pages, it will be fine:
> https://godbolt.org/z/qqoMqfxev

https://godbolt.org/z/9W9f5a7Wx

Apparently not on RISCV, which is what prompted this patch in the first
place. So I suspect Nathan's v2 here is the way to go.

Jason