Commit eb13a1ff271b for kernel
commit eb13a1ff271b0d180687eebb30611b0b276d5193
Author: Nathan Chancellor <nathan@kernel.org>
Date: Fri Sep 25 22:46:32 2026 +0100
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://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/b28eeb28bea39148738dc375e8a97072a1907e64 [3]
Link: https://gcc.gnu.org/onlinedocs/gcc/Optimize-Options.html#index-finline-stringops [4]
Closes: https://github.com/ClangBuiltLinux/linux/issues/2183
Cc: stable@vger.kernel.org # v6.12+
Signed-off-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
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..ad592fdf29af 100644
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -173,6 +173,11 @@ 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_OPT_INLINE_MEMSET
+ string
+ default "-finline-stringops=memset" if $(cc-option,-finline-stringops=memset)
+ default "-mllvm -max-store-memset=4294967294" if $(cc-option,-mllvm -max-store-memset=4294967294)
+
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