kbuild: rust: keep Rust objects out of Clang LTO with inline helpersgrafted

Under `CONFIG_LTO_CLANG` + `CONFIG_RUST_INLINE_HELPERS`, one may hit
`objtool` errors such as:

    vmlinux.o: error: objtool: _R..._3Gsp4boot+0xd6a:
    can't find jump dest instruction at .text._R..._3Gsp4boot+0x1dfd

The reason is that in such builds, the Clang invocation that compiles
the combined Rust plus helpers bitcode emits LLVM bitcode (again) --
the final code generation happens in the linker's LTO step, which the
`-mllvm` trap options passed to Clang do not reach.

This, in turn, means that unreachable traps are missing, and the
impossible paths do not merely fallthrough to the next symbol, but past
the end of their own section, since LTO builds place each function in
its own section.

Thus filter `CC_FLAGS_LTO` out of the Clang invocation, so that it always
emits machine code directly, with the traps in place.

Assisted-by: LLM
Cc: Gary Guo <gary@garyguo.net>
Cc: Boqun Feng <boqun@kernel.org>
Cc: Alice Ryhl <aliceryhl@google.com>
Cc: Matthew Maurer <mmaurer@google.com>
Cc: Josh Poimboeuf <jpoimboe@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: stable@vger.kernel.org
Fixes: 3a2486cc1da5 ("kbuild: rust: provide an option to inline C helpers into Rust")
Acked-by: Gary Guo <gary@garyguo.net>
Link: https://patch.msgid.link/20260816133233.197500-2-ojeda@kernel.org
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

1 files changed, 333 insertions, 0 deletions

diff --git a/arch/arm/Makefile b/arch/arm/Makefile
new file mode 100644
index 000000000..573813ef5
--- /dev/null
+++ b/arch/arm/Makefile
@@ -0,0 +1,333 @@
+#
+# arch/arm/Makefile
+#
+# This file is included by the global makefile so that you can add your own
+# architecture-specific flags and dependencies.
+#
+# This file is subject to the terms and conditions of the GNU General Public
+# License.  See the file "COPYING" in the main directory of this archive
+# for more details.
+#
+# Copyright (C) 1995-2001 by Russell King
+
+LDFLAGS_vmlinux	:= --no-undefined -X --pic-veneer -z norelro
+ifeq ($(CONFIG_CPU_ENDIAN_BE8),y)
+LDFLAGS_vmlinux	+= --be8
+KBUILD_LDFLAGS_MODULE	+= --be8
+endif
+
+GZFLAGS		:=-9
+#KBUILD_CFLAGS	+=-pipe
+
+# Never generate .eh_frame
+KBUILD_CFLAGS	+= $(call cc-option,-fno-dwarf2-cfi-asm)
+
+# Disable FDPIC ABI
+KBUILD_CFLAGS	+= $(call cc-option,-mno-fdpic)
+
+# This should work on most of the modern platforms
+KBUILD_DEFCONFIG := multi_v7_defconfig
+
+# defines filename extension depending memory management type.
+ifeq ($(CONFIG_MMU),)
+MMUEXT		:= -nommu
+KBUILD_CFLAGS	+= $(call cc-option,-mno-unaligned-access)
+endif
+
+ifeq ($(CONFIG_FRAME_POINTER),y)
+KBUILD_CFLAGS	+=-fno-omit-frame-pointer
+ifeq ($(CONFIG_CC_IS_GCC),y)
+KBUILD_CFLAGS += -mapcs -mno-sched-prolog
+endif
+endif
+
+ifeq ($(CONFIG_CPU_BIG_ENDIAN),y)
+KBUILD_CPPFLAGS	+= -mbig-endian
+CHECKFLAGS	+= -D__ARMEB__
+KBUILD_LDFLAGS	+= -EB
+else
+KBUILD_CPPFLAGS	+= -mlittle-endian
+CHECKFLAGS	+= -D__ARMEL__
+KBUILD_LDFLAGS	+= -EL
+endif
+
+#
+# The Scalar Replacement of Aggregates (SRA) optimization pass in GCC 4.9 and
+# later may result in code being generated that handles signed short and signed
+# char struct members incorrectly. So disable it.
+# (https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65932)
+#
+KBUILD_CFLAGS	+= $(call cc-option,-fno-ipa-sra)
+
+# This selects which instruction set is used.
+arch-$(CONFIG_CPU_32v7M)	:=-march=armv7-m
+arch-$(CONFIG_CPU_32v7)		:=-march=armv7-a
+arch-$(CONFIG_CPU_32v6)		:=-march=armv6
+# Only override the compiler option if ARMv6. The ARMv6K extensions are
+# always available in ARMv7
+ifeq ($(CONFIG_CPU_32v6),y)
+arch-$(CONFIG_CPU_32v6K)	:=-march=armv6k
+endif
+arch-$(CONFIG_CPU_32v5)		:=-march=armv5te
+arch-$(CONFIG_CPU_32v4T)	:=-march=armv4t
+arch-$(CONFIG_CPU_32v4)		:=-march=armv4
+arch-$(CONFIG_CPU_32v3)		:=-march=armv3m
+
+# Note that GCC does not numerically define an architecture version
+# macro, but instead defines a whole series of macros which makes
+# testing for a specific architecture or later rather impossible.
+cpp-$(CONFIG_CPU_32v7M)		:=-D__LINUX_ARM_ARCH__=7
+cpp-$(CONFIG_CPU_32v7)		:=-D__LINUX_ARM_ARCH__=7
+cpp-$(CONFIG_CPU_32v6)		:=-D__LINUX_ARM_ARCH__=6
+# Only override the compiler option if ARMv6. The ARMv6K extensions are
+# always available in ARMv7
+ifeq ($(CONFIG_CPU_32v6),y)
+cpp-$(CONFIG_CPU_32v6K)		:=-D__LINUX_ARM_ARCH__=6
+endif
+cpp-$(CONFIG_CPU_32v5)		:=-D__LINUX_ARM_ARCH__=5
+cpp-$(CONFIG_CPU_32v4T)		:=-D__LINUX_ARM_ARCH__=4
+cpp-$(CONFIG_CPU_32v4)		:=-D__LINUX_ARM_ARCH__=4
+cpp-$(CONFIG_CPU_32v3)		:=-D__LINUX_ARM_ARCH__=3
+
+# This selects how we optimise for the processor.
+tune-$(CONFIG_CPU_ARM7TDMI)	:=-mtune=arm7tdmi
+tune-$(CONFIG_CPU_ARM720T)	:=-mtune=arm7tdmi
+tune-$(CONFIG_CPU_ARM740T)	:=-mtune=arm7tdmi
+tune-$(CONFIG_CPU_ARM9TDMI)	:=-mtune=arm9tdmi
+tune-$(CONFIG_CPU_ARM940T)	:=-mtune=arm9tdmi
+tune-$(CONFIG_CPU_ARM946E)	:=-mtune=arm9e
+tune-$(CONFIG_CPU_ARM920T)	:=-mtune=arm9tdmi
+tune-$(CONFIG_CPU_ARM922T)	:=-mtune=arm9tdmi
+tune-$(CONFIG_CPU_ARM925T)	:=-mtune=arm9tdmi
+tune-$(CONFIG_CPU_ARM926T)	:=-mtune=arm9tdmi
+tune-$(CONFIG_CPU_FA526)	:=-mtune=arm9tdmi
+tune-$(CONFIG_CPU_SA110)	:=-mtune=strongarm110
+tune-$(CONFIG_CPU_SA1100)	:=-mtune=strongarm1100
+tune-$(CONFIG_CPU_XSCALE)	:=-mtune=xscale
+tune-$(CONFIG_CPU_XSC3)		:=-mtune=xscale
+tune-$(CONFIG_CPU_FEROCEON)	:=-mtune=xscale
+tune-$(CONFIG_CPU_V6)		:=-mtune=arm1136j-s
+tune-$(CONFIG_CPU_V6K)		:=-mtune=arm1136j-s
+
+ifeq ($(CONFIG_AEABI),y)
+CFLAGS_ABI	:=-mabi=aapcs-linux -mfpu=vfp
+else
+CFLAGS_ABI	:=$(call cc-option,-mapcs-32,-mabi=apcs-gnu) $(call cc-option,-mno-thumb-interwork,)
+endif
+
+ifeq ($(CONFIG_ARM_UNWIND),y)
+CFLAGS_ABI	+=-funwind-tables
+endif
+
+ifeq ($(CONFIG_CC_IS_CLANG),y)
+CFLAGS_ABI	+= -meabi gnu
+endif
+
+ifeq ($(CONFIG_CURRENT_POINTER_IN_TPIDRURO),y)
+KBUILD_CFLAGS	+= -mtp=cp15
+endif
+
+# Accept old syntax despite ".syntax unified"
+AFLAGS_NOWARN	:=$(call as-option,-Wa$(comma)-mno-warn-deprecated,-Wa$(comma)-W)
+
+# The GCC option -ffreestanding is required in order to compile code containing
+# ARM/NEON intrinsics in a non C99-compliant environment (such as the kernel)
+CC_FLAGS_FPU	:= -ffreestanding
+# Enable <arm_neon.h>
+CC_FLAGS_FPU	+= -isystem $(shell $(CC) -print-file-name=include)
+CC_FLAGS_FPU	+= -march=armv7-a -mfloat-abi=softfp -mfpu=neon
+
+ifeq ($(CONFIG_THUMB2_KERNEL),y)
+CFLAGS_ISA	:=-Wa,-mimplicit-it=always $(AFLAGS_NOWARN)
+AFLAGS_ISA	:=$(CFLAGS_ISA) -Wa$(comma)-mthumb
+CFLAGS_ISA	+=-mthumb
+else
+CFLAGS_ISA	:=$(call cc-option,-marm,) $(AFLAGS_NOWARN)
+AFLAGS_ISA	:=$(CFLAGS_ISA)
+endif
+
+# Need -Uarm for gcc < 3.x
+KBUILD_CPPFLAGS	+=$(cpp-y)
+KBUILD_CFLAGS	+=$(CFLAGS_ABI) $(CFLAGS_ISA) $(arch-y) $(tune-y) $(call cc-option,-mshort-load-bytes,$(call cc-option,-malignment-traps,)) -msoft-float -Uarm
+KBUILD_AFLAGS	+=$(CFLAGS_ABI) $(AFLAGS_ISA) -Wa,$(arch-y) $(tune-y) -include $(srctree)/arch/arm/include/asm/unified.h -msoft-float
+KBUILD_RUSTFLAGS += --target=arm-unknown-linux-gnueabi
+
+CHECKFLAGS	+= -D__arm__
+
+# Text offset. This list is sorted numerically by address in order to
+# provide a means to avoid/resolve conflicts in multi-arch kernels.
+# Note: the 32kB below this value is reserved for use by the kernel
+# during boot, and this offset is critical to the functioning of
+# kexec-tools.
+textofs-y	:= 0x00008000
+# RTD1195 has Boot ROM at start of address space
+textofs-$(CONFIG_ARCH_REALTEK)  := 0x00108000
+# SA1111 DMA bug: we don't want the kernel to live in precious DMA-able memory
+ifeq ($(CONFIG_ARCH_SA1100),y)
+textofs-$(CONFIG_SA1111) := 0x00208000
+endif
+textofs-$(CONFIG_ARCH_QCOM_RESERVE_SMEM) := 0x00208000
+textofs-$(CONFIG_ARCH_MESON) := 0x00208000
+textofs-$(CONFIG_ARCH_AXXIA) := 0x00308000
+
+# Machine directory name.  This list is sorted alphanumerically
+# by CONFIG_* macro name.
+machine-$(CONFIG_ARCH_ACTIONS)		+= actions
+machine-$(CONFIG_ARCH_ALPINE)		+= alpine
+machine-$(CONFIG_ARCH_ARTPEC)		+= artpec
+machine-$(CONFIG_ARCH_ASPEED)           += aspeed
+machine-$(CONFIG_ARCH_AT91)		+= at91
+machine-$(CONFIG_ARCH_AXXIA)		+= axxia
+machine-$(CONFIG_ARCH_BCM)		+= bcm
+machine-$(CONFIG_ARCH_BERLIN)		+= berlin
+machine-$(CONFIG_ARCH_CLPS711X)		+= clps711x
+machine-$(CONFIG_ARCH_DAVINCI)		+= davinci
+machine-$(CONFIG_ARCH_DIGICOLOR)	+= digicolor
+machine-$(CONFIG_ARCH_DOVE)		+= dove
+machine-$(CONFIG_ARCH_EXYNOS)		+= exynos
+machine-$(CONFIG_ARCH_FOOTBRIDGE)	+= footbridge
+machine-$(CONFIG_ARCH_GEMINI)		+= gemini
+machine-$(CONFIG_ARCH_HIGHBANK)		+= highbank
+machine-$(CONFIG_ARCH_HISI)		+= hisi
+machine-$(CONFIG_ARCH_IXP4XX)		+= ixp4xx
+machine-$(CONFIG_ARCH_KEYSTONE)		+= keystone
+machine-$(CONFIG_ARCH_LPC18XX)		+= lpc18xx
+machine-$(CONFIG_ARCH_LPC32XX)		+= lpc32xx
+machine-$(CONFIG_ARCH_MESON)		+= meson
+machine-$(CONFIG_ARCH_MMP)		+= mmp
+machine-$(CONFIG_ARCH_MV78XX0)		+= mv78xx0
+machine-$(CONFIG_ARCH_MVEBU)		+= mvebu
+machine-$(CONFIG_ARCH_MXC)		+= imx
+machine-$(CONFIG_ARCH_MEDIATEK)		+= mediatek
+machine-$(CONFIG_ARCH_MILBEAUT)		+= milbeaut
+machine-$(CONFIG_ARCH_MXS)		+= mxs
+machine-$(CONFIG_ARCH_MSTARV7)		+= mstar
+machine-$(CONFIG_ARCH_NOMADIK)		+= nomadik
+machine-$(CONFIG_ARCH_NPCM)		+= npcm
+machine-$(CONFIG_ARCH_OMAP1)		+= omap1
+machine-$(CONFIG_ARCH_OMAP2PLUS)	+= omap2
+machine-$(CONFIG_ARCH_ORION5X)		+= orion5x
+machine-$(CONFIG_ARCH_PXA)		+= pxa
+machine-$(CONFIG_ARCH_QCOM)		+= qcom
+machine-$(CONFIG_ARCH_REALTEK)		+= realtek
+machine-$(CONFIG_ARCH_ROCKCHIP)		+= rockchip
+machine-$(CONFIG_ARCH_RPC)		+= rpc
+machine-$(CONFIG_PLAT_SAMSUNG)		+= s3c
+machine-$(CONFIG_ARCH_S5PV210)		+= s5pv210
+machine-$(CONFIG_ARCH_SA1100)		+= sa1100
+machine-$(CONFIG_ARCH_RENESAS)	 	+= shmobile
+machine-$(CONFIG_ARCH_INTEL_SOCFPGA)	+= socfpga
+machine-$(CONFIG_ARCH_STI)		+= sti
+machine-$(CONFIG_ARCH_STM32)		+= stm32
+machine-$(CONFIG_ARCH_SUNXI)		+= sunxi
+machine-$(CONFIG_ARCH_TEGRA)		+= tegra
+machine-$(CONFIG_ARCH_U8500)		+= ux500
+machine-$(CONFIG_ARCH_VT8500)		+= vt8500
+machine-$(CONFIG_ARCH_ZTE)		+= zte
+machine-$(CONFIG_ARCH_ZYNQ)		+= zynq
+machine-$(CONFIG_PLAT_VERSATILE)	+= versatile
+machine-$(CONFIG_PLAT_SPEAR)		+= spear
+
+# legacy platforms provide their own mach/*.h headers globally,
+# these three are mutually exclusive
+machdirs-$(CONFIG_ARCH_FOOTBRIDGE)	+= arch/arm/mach-footbridge
+machdirs-$(CONFIG_ARCH_RPC)		+= arch/arm/mach-rpc
+machdirs-$(CONFIG_ARCH_SA1100)		+= arch/arm/mach-sa1100
+KBUILD_CPPFLAGS += $(patsubst %,-I$(srctree)/%/include,$(machdirs-y))
+
+# The byte offset of the kernel image in RAM from the start of RAM.
+TEXT_OFFSET := $(textofs-y)
+
+export	TEXT_OFFSET GZFLAGS MMUEXT
+
+# If we have a machine-specific directory, then include it in the build.
+core-y				+= $(patsubst %,arch/arm/mach-%/,$(machine-y))
+# For cleaning
+core-				+= $(patsubst %,arch/arm/mach-%/,$(machine-))
+
+core-$(CONFIG_PLAT_ORION)	+= arch/arm/plat-orion/
+
+libs-y				:= arch/arm/lib/ $(libs-y)
+
+# Default target when executing plain make
+boot := arch/arm/boot
+ifeq ($(CONFIG_XIP_KERNEL),y)
+KBUILD_IMAGE := $(boot)/xipImage
+else
+KBUILD_IMAGE := $(boot)/zImage
+endif
+
+ifeq ($(CONFIG_STACKPROTECTOR_PER_TASK),y)
+prepare: stack_protector_prepare
+ifeq ($(CONFIG_CC_HAVE_STACKPROTECTOR_TLS),y)
+stack_protector_prepare: prepare0
+	$(eval KBUILD_CFLAGS += \
+		-mstack-protector-guard=tls \
+		-mstack-protector-guard-offset=$(shell	\
+			awk '{if ($$2 == "TSK_STACK_CANARY") print $$3;}'\
+				$(objtree)/include/generated/asm-offsets.h))
+else
+stack_protector_prepare: prepare0
+	$(eval SSP_PLUGIN_CFLAGS := \
+		-fplugin-arg-arm_ssp_per_task_plugin-offset=$(shell	\
+			awk '{if ($$2 == "TSK_STACK_CANARY") print $$3;}'\
+				$(objtree)/include/generated/asm-offsets.h))
+	$(eval KBUILD_CFLAGS += $(SSP_PLUGIN_CFLAGS))
+	$(eval GCC_PLUGINS_CFLAGS += $(SSP_PLUGIN_CFLAGS))
+endif
+endif
+
+all:	$(notdir $(KBUILD_IMAGE))
+
+
+archheaders:
+	$(Q)$(MAKE) $(build)=arch/arm/tools uapi
+
+archprepare:
+	$(Q)$(MAKE) $(build)=arch/arm/tools kapi
+
+# Convert bzImage to zImage
+bzImage: zImage
+
+BOOT_TARGETS	= zImage Image xipImage bootpImage uImage
+INSTALL_TARGETS	= zinstall uinstall install
+
+PHONY += bzImage $(BOOT_TARGETS) $(INSTALL_TARGETS)
+
+bootpImage uImage: zImage
+zImage: Image
+
+$(BOOT_TARGETS): vmlinux
+	$(Q)$(MAKE) $(build)=$(boot) $(boot)/$@
+	@$(kecho) '  Kernel: $(boot)/$@ is ready'
+
+$(INSTALL_TARGETS): KBUILD_IMAGE = $(boot)/$(patsubst %install,%Image,$@)
+$(INSTALL_TARGETS):
+	$(call cmd,install)
+
+vdso-install-$(CONFIG_VDSO) += arch/arm/vdso/vdso.so.dbg
+
+# My testing targets (bypasses dependencies)
+bp:;	$(Q)$(MAKE) $(build)=$(boot) $(boot)/bootpImage
+
+include $(srctree)/scripts/Makefile.defconf
+PHONY += multi_v7_lpae_defconfig
+multi_v7_lpae_defconfig:
+	$(call merge_into_defconfig,multi_v7_defconfig,lpae)
+
+define archhelp
+  echo  '* zImage        - Compressed kernel image (arch/$(ARCH)/boot/zImage)'
+  echo  '  Image         - Uncompressed kernel image (arch/$(ARCH)/boot/Image)'
+  echo  '* xipImage      - XIP kernel image, if configured (arch/$(ARCH)/boot/xipImage)'
+  echo  '  uImage        - U-Boot wrapped zImage'
+  echo  '  bootpImage    - Combined zImage and initial RAM disk'
+  echo  '                  (supply initrd image via make variable INITRD=<path>)'
+  echo  '  install       - Install uncompressed kernel'
+  echo  '  zinstall      - Install compressed kernel'
+  echo  '  uinstall      - Install U-Boot wrapped compressed kernel'
+  echo  '                  Install using (your) ~/bin/$(INSTALLKERNEL) or'
+  echo  '                  (distribution) /sbin/$(INSTALLKERNEL) or'
+  echo  '                  install to $$(INSTALL_PATH) and run lilo'
+  echo
+  echo  '  multi_v7_lpae_defconfig     - multi_v7_defconfig with CONFIG_ARM_LPAE enabled'
+endef