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, 236 insertions, 0 deletions

diff --git a/arch/arm64/Makefile b/arch/arm64/Makefile
new file mode 100644
index 000000000..6b005c8fe
--- /dev/null
+++ b/arch/arm64/Makefile
@@ -0,0 +1,236 @@
+#
+# arch/arm64/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
+
+ifeq ($(CONFIG_RELOCATABLE), y)
+# Pass --no-apply-dynamic-relocs to restore pre-binutils-2.27 behaviour
+# for relative relocs, since this leads to better Image compression
+# with the relocation offsets always being zero.
+LDFLAGS_vmlinux	+= -shared -Bsymbolic -z notext --no-apply-dynamic-relocs
+endif
+
+ifeq ($(CONFIG_ARM64_ERRATUM_843419),y)
+LDFLAGS_vmlinux	+= --fix-cortex-a53-843419
+endif
+
+cc_has_k_constraint := $(call try-run,echo				\
+	'int main(void) {						\
+		asm volatile("and w0, w0, %w0" :: "K" (4294967295));	\
+		return 0;						\
+	}' | $(CC) -S -x c -o "$$TMP" -,,-DCONFIG_CC_HAS_K_CONSTRAINT=1)
+
+ifeq ($(CONFIG_BROKEN_GAS_INST),y)
+$(warning Detected assembler with broken .inst; disassembly will be unreliable)
+endif
+
+# The GCC option -ffreestanding is required in order to compile code containing
+# ARM/NEON intrinsics in a non C99-compliant environment (such as the kernel)
+CC_FLAGS_FPU	:= -ffreestanding
+# Enable <arm_neon.h>
+CC_FLAGS_FPU	+= -isystem $(shell $(CC) -print-file-name=include)
+CC_FLAGS_NO_FPU	:= -mgeneral-regs-only
+
+KBUILD_CFLAGS	+= $(CC_FLAGS_NO_FPU) \
+		   $(compat_vdso) $(cc_has_k_constraint)
+KBUILD_CFLAGS	+= $(call cc-disable-warning, psabi)
+KBUILD_AFLAGS	+= $(compat_vdso)
+
+ifeq ($(call rustc-min-version, 108500),y)
+KBUILD_RUSTFLAGS += --target=aarch64-unknown-none-softfloat
+else
+KBUILD_RUSTFLAGS += --target=aarch64-unknown-none -Ctarget-feature="-neon"
+endif
+
+KBUILD_CFLAGS	+= $(call cc-option,-mabi=lp64)
+KBUILD_AFLAGS	+= $(call cc-option,-mabi=lp64)
+
+# Avoid generating .eh_frame* sections.
+ifneq ($(CONFIG_UNWIND_TABLES),y)
+KBUILD_CFLAGS	+= -fno-asynchronous-unwind-tables -fno-unwind-tables
+KBUILD_AFLAGS	+= -fno-asynchronous-unwind-tables -fno-unwind-tables
+KBUILD_RUSTFLAGS += -Cforce-unwind-tables=n
+else
+KBUILD_CFLAGS	+= -fasynchronous-unwind-tables
+KBUILD_AFLAGS	+= -fasynchronous-unwind-tables
+KBUILD_RUSTFLAGS += -Cforce-unwind-tables=y -Zuse-sync-unwind=n
+# Work around rustc bug on compilers without
+# https://github.com/rust-lang/rust/pull/156973.
+KBUILD_RUSTFLAGS += $(if $(call rustc-min-version,109800),,-Zllvm_module_flag=uwtable:u32:2:max)
+endif
+
+ifeq ($(CONFIG_STACKPROTECTOR_PER_TASK),y)
+prepare: stack_protector_prepare
+stack_protector_prepare: prepare0
+	$(eval KBUILD_CFLAGS += -mstack-protector-guard=sysreg		  \
+				-mstack-protector-guard-reg=sp_el0	  \
+				-mstack-protector-guard-offset=$(shell	  \
+			awk '{if ($$2 == "TSK_STACK_CANARY") print $$3;}' \
+				$(objtree)/include/generated/asm-offsets.h))
+endif
+
+ifeq ($(CONFIG_ARM64_BTI_KERNEL),y)
+  KBUILD_CFLAGS += -mbranch-protection=pac-ret+bti
+  KBUILD_RUSTFLAGS += -Zbranch-protection=bti,pac-ret
+else ifeq ($(CONFIG_ARM64_PTR_AUTH_KERNEL),y)
+  KBUILD_RUSTFLAGS += -Zbranch-protection=pac-ret
+  ifeq ($(CONFIG_CC_HAS_BRANCH_PROT_PAC_RET),y)
+    KBUILD_CFLAGS += -mbranch-protection=pac-ret
+  else
+    KBUILD_CFLAGS += -msign-return-address=non-leaf
+  endif
+else
+  KBUILD_CFLAGS += $(call cc-option,-mbranch-protection=none)
+endif
+
+# Tell the assembler to support instructions from the latest target
+# architecture.
+#
+# For non-integrated assemblers we'll pass this on the command line, and for
+# integrated assemblers we'll define ARM64_ASM_ARCH and ARM64_ASM_PREAMBLE for
+# inline usage.
+#
+# We cannot pass the same arch flag to the compiler as this would allow it to
+# freely generate instructions which are not supported by earlier architecture
+# versions, which would prevent a single kernel image from working on earlier
+# hardware.
+ifeq ($(CONFIG_AS_HAS_ARMV8_5), y)
+  asm-arch := armv8.5-a
+else
+  asm-arch := armv8.4-a
+endif
+
+ifdef asm-arch
+KBUILD_CFLAGS	+= -Wa,-march=$(asm-arch) \
+		   -DARM64_ASM_ARCH='"$(asm-arch)"'
+endif
+
+ifeq ($(CONFIG_SHADOW_CALL_STACK), y)
+KBUILD_CFLAGS	+= -ffixed-x18
+KBUILD_RUSTFLAGS += -Zfixed-x18
+endif
+
+ifeq ($(CONFIG_CPU_BIG_ENDIAN), y)
+KBUILD_CPPFLAGS	+= -mbig-endian
+CHECKFLAGS	+= -D__AARCH64EB__
+# Prefer the baremetal ELF build target, but not all toolchains include
+# it so fall back to the standard linux version if needed.
+KBUILD_LDFLAGS	+= -EB $(call ld-option, -maarch64elfb, -maarch64linuxb -z norelro)
+UTS_MACHINE	:= aarch64_be
+else
+KBUILD_CPPFLAGS	+= -mlittle-endian
+CHECKFLAGS	+= -D__AARCH64EL__
+# Same as above, prefer ELF but fall back to linux target if needed.
+KBUILD_LDFLAGS	+= -EL $(call ld-option, -maarch64elf, -maarch64linux -z norelro)
+UTS_MACHINE	:= aarch64
+endif
+
+ifeq ($(CONFIG_LD_IS_LLD), y)
+KBUILD_LDFLAGS	+= -z norelro
+endif
+
+CHECKFLAGS	+= -D__aarch64__
+
+ifeq ($(CONFIG_DYNAMIC_FTRACE_WITH_CALL_OPS),y)
+  KBUILD_CPPFLAGS += -DCC_USING_PATCHABLE_FUNCTION_ENTRY
+  CC_FLAGS_FTRACE := -fpatchable-function-entry=4,2
+else ifeq ($(CONFIG_DYNAMIC_FTRACE_WITH_ARGS),y)
+  KBUILD_CPPFLAGS += -DCC_USING_PATCHABLE_FUNCTION_ENTRY
+  CC_FLAGS_FTRACE := -fpatchable-function-entry=2
+endif
+
+ifeq ($(CONFIG_KASAN_SW_TAGS), y)
+KASAN_SHADOW_SCALE_SHIFT := 4
+else ifeq ($(CONFIG_KASAN_GENERIC), y)
+KASAN_SHADOW_SCALE_SHIFT := 3
+endif
+
+KBUILD_CFLAGS += -DKASAN_SHADOW_SCALE_SHIFT=$(KASAN_SHADOW_SCALE_SHIFT)
+KBUILD_CPPFLAGS += -DKASAN_SHADOW_SCALE_SHIFT=$(KASAN_SHADOW_SCALE_SHIFT)
+KBUILD_AFLAGS += -DKASAN_SHADOW_SCALE_SHIFT=$(KASAN_SHADOW_SCALE_SHIFT)
+
+libs-y		:= arch/arm64/lib/ $(libs-y)
+libs-$(CONFIG_EFI_STUB) += $(objtree)/drivers/firmware/efi/libstub/lib.a
+
+# Default target when executing plain make
+boot		:= arch/arm64/boot
+
+BOOT_TARGETS	:= Image vmlinuz.efi image.fit
+
+PHONY += $(BOOT_TARGETS)
+
+ifeq ($(CONFIG_EFI_ZBOOT),)
+KBUILD_IMAGE	:= $(boot)/Image.gz
+else
+KBUILD_IMAGE	:= $(boot)/vmlinuz.efi
+endif
+
+all:	$(notdir $(KBUILD_IMAGE))
+
+image.fit: dtbs
+
+vmlinuz.efi image.fit: Image
+$(BOOT_TARGETS): vmlinux
+	$(Q)$(MAKE) $(build)=$(boot) $(boot)/$@
+
+Image.%: Image
+	$(Q)$(MAKE) $(build)=$(boot) $(boot)/$@
+
+ifeq ($(CONFIG_COMPRESSED_INSTALL),y)
+ DEFAULT_KBUILD_IMAGE = $(KBUILD_IMAGE)
+else
+ DEFAULT_KBUILD_IMAGE = $(boot)/Image
+endif
+
+install: KBUILD_IMAGE := $(DEFAULT_KBUILD_IMAGE)
+install zinstall:
+	$(call cmd,install)
+
+archprepare:
+	$(Q)$(MAKE) $(build)=arch/arm64/tools kapi
+
+ifeq ($(KBUILD_EXTMOD),)
+# We need to generate vdso-offsets.h before compiling certain files in kernel/.
+# In order to do that, we should use the archprepare target, but we can't since
+# asm-offsets.h is included in some files used to generate vdso-offsets.h, and
+# asm-offsets.h is built in prepare0, for which archprepare is a dependency.
+# Therefore we need to generate the header after prepare0 has been made, hence
+# this hack.
+prepare: vdso_prepare
+vdso_prepare: prepare0
+	$(Q)$(MAKE) $(build)=arch/arm64/kernel/vdso \
+	include/generated/vdso-offsets.h arch/arm64/kernel/vdso/vdso.so
+ifdef CONFIG_COMPAT_VDSO
+	$(Q)$(MAKE) $(build)=arch/arm64/kernel/vdso32 \
+	arch/arm64/kernel/vdso32/vdso.so
+endif
+endif
+
+vdso-install-y				+= arch/arm64/kernel/vdso/vdso.so.dbg
+vdso-install-$(CONFIG_COMPAT_VDSO)	+= arch/arm64/kernel/vdso32/vdso32.so.dbg
+
+include $(srctree)/scripts/Makefile.defconf
+
+PHONY += virtconfig
+virtconfig:
+	$(call merge_into_defconfig_override,defconfig,virt)
+
+define archhelp
+  echo  '* Image.gz      - Compressed kernel image (arch/$(ARCH)/boot/Image.gz)'
+  echo  '  Image         - Uncompressed kernel image (arch/$(ARCH)/boot/Image)'
+  echo  '  image.fit     - Flat Image Tree (arch/$(ARCH)/boot/image.fit)'
+  echo  '  install       - Install kernel (compressed if COMPRESSED_INSTALL set)'
+  echo  '  zinstall      - Install compressed kernel'
+  echo  '                  Install using (your) ~/bin/installkernel or'
+  echo  '                  (distribution) /sbin/installkernel or'
+  echo  '                  install to $$(INSTALL_PATH) and run lilo'
+endef