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, 156 insertions, 0 deletions

diff --git a/tools/objtool/Makefile b/tools/objtool/Makefile
new file mode 100644
index 000000000..a4484fd22
--- /dev/null
+++ b/tools/objtool/Makefile
@@ -0,0 +1,156 @@
+# SPDX-License-Identifier: GPL-2.0
+include ../scripts/Makefile.include
+include ../scripts/Makefile.arch
+
+ifeq ($(SRCARCH),x86)
+	BUILD_ORC    := y
+	ARCH_HAS_KLP := y
+endif
+
+ifeq ($(SRCARCH),loongarch)
+	BUILD_ORC	   := y
+endif
+
+ifeq ($(ARCH_HAS_KLP),y)
+	HAVE_XXHASH = $(shell printf "$(pound)include <xxhash.h>\nXXH3_state_t *state;int main() {}" | \
+		      $(HOSTCC) $(HOSTCFLAGS) -xc - -o /dev/null -lxxhash 2> /dev/null && echo y || echo n)
+	ifeq ($(HAVE_XXHASH),y)
+		BUILD_KLP	 := y
+		LIBXXHASH_CFLAGS := $(shell $(HOSTPKG_CONFIG) libxxhash --cflags 2>/dev/null) \
+				    -DBUILD_KLP
+		LIBXXHASH_LIBS   := $(shell $(HOSTPKG_CONFIG) libxxhash --libs 2>/dev/null || echo -lxxhash)
+	endif
+endif
+
+export BUILD_ORC BUILD_KLP
+
+ifeq ($(srctree),)
+srctree := $(patsubst %/,%,$(dir $(CURDIR)))
+srctree := $(patsubst %/,%,$(dir $(srctree)))
+endif
+
+RM ?= rm -f
+
+LIBSUBCMD_DIR = $(srctree)/tools/lib/subcmd/
+ifneq ($(OUTPUT),)
+  LIBSUBCMD_OUTPUT = $(abspath $(OUTPUT))/libsubcmd
+else
+  LIBSUBCMD_OUTPUT = $(CURDIR)/libsubcmd
+endif
+LIBSUBCMD = $(LIBSUBCMD_OUTPUT)/libsubcmd.a
+
+OBJTOOL    := $(OUTPUT)objtool
+OBJTOOL_IN := $(OBJTOOL)-in.o
+
+LIBELF_FLAGS := $(shell $(HOSTPKG_CONFIG) libelf --cflags 2>/dev/null)
+LIBELF_LIBS  := $(shell $(HOSTPKG_CONFIG) libelf --libs 2>/dev/null || echo -lelf)
+
+all: $(OBJTOOL)
+
+WARNINGS := -Werror -Wall -Wextra -Wmissing-prototypes			\
+	    -Wmissing-declarations -Wwrite-strings			\
+	    -Wno-implicit-fallthrough -Wno-sign-compare			\
+	    -Wno-unused-parameter
+
+INCLUDES := -I$(srctree)/tools/include \
+	    -I$(srctree)/tools/include/uapi \
+	    -I$(srctree)/tools/arch/$(HOSTARCH)/include/uapi \
+	    -I$(srctree)/tools/arch/$(SRCARCH)/include	\
+	    -I$(srctree)/tools/objtool/include \
+	    -I$(srctree)/tools/objtool/arch/$(SRCARCH)/include \
+	    -I$(srctree)/tools/lib
+
+OBJTOOL_CFLAGS  := -std=gnu11 -fomit-frame-pointer -O2 -g $(WARNINGS)	\
+		   $(INCLUDES) $(LIBELF_FLAGS) $(LIBXXHASH_CFLAGS) $(HOSTCFLAGS)
+
+OBJTOOL_LDFLAGS := $(LIBSUBCMD) $(LIBELF_LIBS) $(LIBXXHASH_LIBS) $(HOSTLDFLAGS)
+
+# Allow old libelf to be used:
+elfshdr := $(shell echo '$(pound)include <libelf.h>' | $(HOSTCC) $(OBJTOOL_CFLAGS) -x c -E - 2>/dev/null | grep elf_getshdr)
+OBJTOOL_CFLAGS += $(if $(elfshdr),,-DLIBELF_USE_DEPRECATED)
+
+# Always want host compilation.
+HOST_OVERRIDES := CC="$(HOSTCC)" LD="$(HOSTLD)" AR="$(HOSTAR)"
+
+#
+# To support disassembly, objtool needs libopcodes which is provided
+# with libbfd (binutils-dev or binutils-devel package).
+#
+# We check using HOSTCC directly rather than the shared feature framework
+# because objtool is a host tool that links against host libraries.
+#
+# When using shared libraries, -lopcodes is sufficient as dependencies are
+# resolved automatically. With static libraries, we must explicitly link
+# against libopcodes' dependencies: libbfd, libiberty, and sometimes libz.
+# Try each combination and use the first one that succeeds.
+#
+LIBOPCODES_LIBS := $(shell \
+	for libs in "-lopcodes" \
+		    "-lopcodes -lbfd" \
+		    "-lopcodes -lbfd -liberty" \
+		    "-lopcodes -lbfd -liberty -lz"; do \
+		echo 'extern void disassemble_init_for_target(void *);' \
+		     'int main(void) { disassemble_init_for_target(0); return 0; }' | \
+			$(HOSTCC) -xc - -o /dev/null $$libs 2>/dev/null && \
+			echo "$$libs" && break; \
+	done)
+
+# Styled disassembler support requires binutils >= 2.39
+HAVE_DISASM_STYLED := $(shell echo '$(pound)include <dis-asm.h>' | \
+			$(HOSTCC) -E -xc - 2>/dev/null | grep -q disassembler_style && echo y)
+
+BUILD_DISAS := n
+
+ifneq ($(LIBOPCODES_LIBS),)
+	BUILD_DISAS := y
+	OBJTOOL_CFLAGS += -DDISAS -DPACKAGE='"objtool"'
+	OBJTOOL_LDFLAGS += $(LIBOPCODES_LIBS)
+ifeq ($(HAVE_DISASM_STYLED),y)
+	OBJTOOL_CFLAGS += -DDISASM_INIT_STYLED
+endif
+endif
+
+export BUILD_DISAS
+
+AWK = awk
+MKDIR = mkdir
+
+export srctree OUTPUT CFLAGS SRCARCH AWK
+include $(srctree)/tools/build/Makefile.include
+
+$(OBJTOOL_IN): fixdep $(LIBSUBCMD) FORCE
+	$(Q)$(CONFIG_SHELL) ./sync-check.sh
+	$(Q)$(MAKE) $(build)=objtool $(HOST_OVERRIDES) CFLAGS="$(OBJTOOL_CFLAGS)" \
+		LDFLAGS="$(OBJTOOL_LDFLAGS)"
+
+
+$(OBJTOOL): $(LIBSUBCMD) $(OBJTOOL_IN)
+	$(QUIET_LINK)$(HOSTCC) $(OBJTOOL_IN) $(OBJTOOL_LDFLAGS) -o $@
+
+
+$(LIBSUBCMD_OUTPUT):
+	$(Q)$(MKDIR) -p $@
+
+$(LIBSUBCMD): fixdep $(LIBSUBCMD_OUTPUT) FORCE
+	$(Q)$(MAKE) -C $(LIBSUBCMD_DIR) O=$(LIBSUBCMD_OUTPUT) \
+		DESTDIR=$(LIBSUBCMD_OUTPUT) prefix= subdir= \
+		$(HOST_OVERRIDES) EXTRA_CFLAGS="$(OBJTOOL_CFLAGS)" \
+		$@
+
+$(LIBSUBCMD)-clean:
+	$(call QUIET_CLEAN, libsubcmd)
+	$(Q)$(RM) -r -- $(LIBSUBCMD_OUTPUT)
+
+clean: $(LIBSUBCMD)-clean
+	$(Q)find $(OUTPUT) \( -name '*.o' -o -name '\.*.cmd' -o -name '\.*.d' \) -type f -print | xargs $(RM)
+	$(Q)$(RM) $(OUTPUT)arch/x86/lib/cpu-feature-names.c $(OUTPUT)fixdep
+	$(Q)$(RM) $(OUTPUT)arch/x86/lib/inat-tables.c $(OUTPUT)fixdep
+	$(Q)$(RM) -- $(OUTPUT)FEATURE-DUMP.objtool
+	$(Q)$(RM) -r -- $(OUTPUT)feature
+
+mrproper: clean
+	$(call QUIET_CLEAN, objtool) $(RM) $(OBJTOOL)
+
+FORCE:
+
+.PHONY: clean mrproper FORCE