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, 240 insertions, 0 deletions

diff --git a/tools/testing/selftests/lib.mk b/tools/testing/selftests/lib.mk
new file mode 100644
index 000000000..f02cc8a2e
--- /dev/null
+++ b/tools/testing/selftests/lib.mk
@@ -0,0 +1,240 @@
+# This mimics the top-level Makefile. We do it explicitly here so that this
+# Makefile can operate with or without the kbuild infrastructure.
+ifneq ($(LLVM),)
+ifneq ($(filter %/,$(LLVM)),)
+LLVM_PREFIX := $(LLVM)
+else ifneq ($(filter -%,$(LLVM)),)
+LLVM_SUFFIX := $(LLVM)
+endif
+
+CLANG := $(LLVM_PREFIX)clang$(LLVM_SUFFIX)
+
+CLANG_TARGET_FLAGS_arm          := arm-linux-gnueabi
+CLANG_TARGET_FLAGS_arm64        := aarch64-linux-gnu
+CLANG_TARGET_FLAGS_hexagon      := hexagon-linux-musl
+CLANG_TARGET_FLAGS_i386         := i386-linux-gnu
+CLANG_TARGET_FLAGS_m68k         := m68k-linux-gnu
+CLANG_TARGET_FLAGS_mips         := mipsel-linux-gnu
+CLANG_TARGET_FLAGS_powerpc      := powerpc64le-linux-gnu
+CLANG_TARGET_FLAGS_riscv        := riscv64-linux-gnu
+CLANG_TARGET_FLAGS_s390         := s390x-linux-gnu
+CLANG_TARGET_FLAGS_x86          := x86_64-linux-gnu
+CLANG_TARGET_FLAGS_x86_64       := x86_64-linux-gnu
+
+# Default to host architecture if ARCH is not explicitly given.
+ifeq ($(ARCH),)
+CLANG_TARGET_FLAGS := $(shell $(CLANG) -print-target-triple)
+else
+CLANG_TARGET_FLAGS := $(CLANG_TARGET_FLAGS_$(ARCH))
+endif
+
+ifeq ($(CROSS_COMPILE),)
+ifeq ($(CLANG_TARGET_FLAGS),)
+$(error Specify CROSS_COMPILE or add '--target=' option to lib.mk)
+else
+CLANG_FLAGS     += --target=$(CLANG_TARGET_FLAGS)
+endif # CLANG_TARGET_FLAGS
+else
+CLANG_FLAGS     += --target=$(notdir $(CROSS_COMPILE:%-=%))
+endif # CROSS_COMPILE
+
+# gcc defaults to silence (off) for the following warnings, but clang defaults
+# to the opposite. The warnings are not useful for the kernel itself, which is
+# why they have remained disabled in gcc for the main kernel build. And it is
+# only due to including kernel data structures in the selftests, that we get the
+# warnings from clang. Therefore, disable the warnings for clang builds.
+CFLAGS += -Wno-address-of-packed-member
+CFLAGS += -Wno-gnu-variable-sized-type-not-at-end
+
+CC := $(CLANG) $(CLANG_FLAGS) -fintegrated-as
+else
+CC := $(CROSS_COMPILE)gcc
+endif # LLVM
+
+ifeq (0,$(MAKELEVEL))
+    ifeq ($(OUTPUT),)
+	OUTPUT := $(shell pwd)
+	DEFAULT_INSTALL_HDR_PATH := 1
+    endif
+endif
+selfdir = $(realpath $(dir $(filter %/lib.mk,$(MAKEFILE_LIST))))
+top_srcdir = $(selfdir)/../../..
+
+# msg: emit succinct information message describing current building step
+# $1 - generic step name (e.g., CC, LINK, etc);
+# $2 - optional "flavor" specifier; if provided, will be emitted as [flavor];
+# $3 - target (assumed to be file); only file name will be emitted;
+# $4 - optional extra arg, emitted as-is, if provided.
+ifeq ($(V),1)
+Q =
+msg =
+else
+Q = @
+msg = @printf '  %-8s%s %s%s\n' "$(1)" "$(if $(2), [$(2)])" "$(notdir $(3))" "$(if $(4), $(4))";
+MAKEFLAGS += --no-print-directory
+endif
+
+ifeq ($(KHDR_INCLUDES),)
+KHDR_INCLUDES := -isystem $(top_srcdir)/usr/include
+endif
+
+# In order to use newer items that haven't yet been added to the user's system
+# header files, add $(TOOLS_INCLUDES) to the compiler invocation in each
+# each selftest.
+# You may need to add files to that location, or to refresh an existing file. In
+# order to do that, run "make headers" from $(top_srcdir), then copy the
+# header file that you want from $(top_srcdir)/usr/include/... , to the matching
+# subdir in $(TOOLS_INCLUDE).
+TOOLS_INCLUDES := -isystem $(top_srcdir)/tools/include/uapi
+
+# The following are built by lib.mk common compile rules.
+# TEST_CUSTOM_PROGS should be used by tests that require
+# custom build rule and prevent common build rule use.
+# TEST_PROGS are for test shell scripts.
+# TEST_CUSTOM_PROGS and TEST_PROGS will be run by common run_tests
+# and install targets. Common clean doesn't touch them.
+TEST_GEN_PROGS := $(patsubst %,$(OUTPUT)/%,$(TEST_GEN_PROGS))
+TEST_GEN_PROGS_EXTENDED := $(patsubst %,$(OUTPUT)/%,$(TEST_GEN_PROGS_EXTENDED))
+TEST_GEN_FILES := $(patsubst %,$(OUTPUT)/%,$(TEST_GEN_FILES))
+
+all: $(TEST_GEN_PROGS) $(TEST_GEN_PROGS_EXTENDED) $(TEST_GEN_FILES) \
+	$(if $(TEST_GEN_MODS_DIR),gen_mods_dir)
+
+define RUN_TESTS
+	BASE_DIR="$(selfdir)";			\
+	. $(selfdir)/kselftest/runner.sh;	\
+	if [ "X$(summary)" != "X" ]; then       \
+		per_test_logging=1;		\
+	fi;                                     \
+	run_many $(1)
+endef
+
+define INSTALL_INCLUDES
+	$(if $(TEST_INCLUDES), \
+		relative_files=""; \
+		for entry in $(TEST_INCLUDES); do \
+			entry_dir=$$(readlink -e "$$(dirname "$$entry")"); \
+			entry_name=$$(basename "$$entry"); \
+			relative_dir=$${entry_dir#"$$SRC_PATH"/}; \
+			if [ "$$relative_dir" = "$$entry_dir" ]; then \
+				echo "Error: TEST_INCLUDES entry \"$$entry\" not located inside selftests directory ($$SRC_PATH)" >&2; \
+				exit 1; \
+			fi; \
+			relative_files="$$relative_files $$relative_dir/$$entry_name"; \
+		done; \
+		cd $(SRC_PATH) && rsync -aR $$relative_files $(OBJ_PATH)/ \
+	)
+endef
+
+run_tests: all
+ifdef building_out_of_srctree
+	@if [ "X$(TEST_PROGS)$(TEST_PROGS_EXTENDED)$(TEST_FILES)$(TEST_GEN_MODS_DIR)" != "X" ]; then \
+		rsync -aq --copy-unsafe-links $(TEST_PROGS) $(TEST_PROGS_EXTENDED) $(TEST_FILES) $(TEST_GEN_MODS_DIR) $(OUTPUT); \
+	fi
+	@$(INSTALL_INCLUDES)
+	@if [ "X$(TEST_PROGS)" != "X" ]; then \
+		$(call RUN_TESTS, $(TEST_GEN_PROGS) $(TEST_CUSTOM_PROGS) \
+				  $(addprefix $(OUTPUT)/,$(TEST_PROGS))) ; \
+	else \
+		$(call RUN_TESTS, $(TEST_GEN_PROGS) $(TEST_CUSTOM_PROGS)); \
+	fi
+else
+	@$(call RUN_TESTS, $(TEST_GEN_PROGS) $(TEST_CUSTOM_PROGS) $(TEST_PROGS))
+endif
+
+gen_mods_dir:
+	$(Q)$(MAKE) -C $(TEST_GEN_MODS_DIR)
+
+clean_mods_dir:
+	$(Q)$(MAKE) -C $(TEST_GEN_MODS_DIR) clean
+
+define INSTALL_SINGLE_RULE
+	$(if $(INSTALL_LIST),@mkdir -p $(INSTALL_PATH))
+	$(if $(INSTALL_LIST),rsync -a --copy-unsafe-links $(INSTALL_LIST) $(INSTALL_PATH)/)
+endef
+
+define INSTALL_MODS_RULE
+	$(if $(INSTALL_LIST),@mkdir -p $(INSTALL_PATH)/$(INSTALL_LIST))
+	$(if $(INSTALL_LIST),rsync -a --copy-unsafe-links $(INSTALL_LIST)/*.ko $(INSTALL_PATH)/$(INSTALL_LIST))
+endef
+
+define INSTALL_RULE
+	$(eval INSTALL_LIST = $(TEST_PROGS)) $(INSTALL_SINGLE_RULE)
+	$(eval INSTALL_LIST = $(TEST_PROGS_EXTENDED)) $(INSTALL_SINGLE_RULE)
+	$(eval INSTALL_LIST = $(TEST_FILES)) $(INSTALL_SINGLE_RULE)
+	$(eval INSTALL_LIST = $(TEST_GEN_PROGS)) $(INSTALL_SINGLE_RULE)
+	$(eval INSTALL_LIST = $(TEST_CUSTOM_PROGS)) $(INSTALL_SINGLE_RULE)
+	$(eval INSTALL_LIST = $(TEST_GEN_PROGS_EXTENDED)) $(INSTALL_SINGLE_RULE)
+	$(eval INSTALL_LIST = $(TEST_GEN_FILES)) $(INSTALL_SINGLE_RULE)
+	$(eval INSTALL_LIST = $(notdir $(TEST_GEN_MODS_DIR))) $(INSTALL_MODS_RULE)
+	$(eval INSTALL_LIST = $(wildcard config settings)) $(INSTALL_SINGLE_RULE)
+endef
+
+install: all
+ifdef INSTALL_PATH
+	$(INSTALL_RULE)
+	$(INSTALL_INCLUDES)
+else
+	$(error Error: set INSTALL_PATH to use install)
+endif
+
+emit_tests:
+	for TEST in $(TEST_GEN_PROGS) $(TEST_CUSTOM_PROGS) $(TEST_PROGS); do \
+		BASENAME_TEST=`basename $$TEST`;	\
+		echo "$(COLLECTION):$$BASENAME_TEST";	\
+	done
+
+# define if isn't already. It is undefined in make O= case.
+ifeq ($(RM),)
+RM := rm -f
+endif
+
+define CLEAN
+	$(RM) -r $(TEST_GEN_PROGS) $(TEST_GEN_PROGS_EXTENDED) $(TEST_GEN_FILES) $(EXTRA_CLEAN)
+endef
+
+clean: $(if $(TEST_GEN_MODS_DIR),clean_mods_dir)
+	$(CLEAN)
+
+# Build with _GNU_SOURCE by default
+CFLAGS += -D_GNU_SOURCE=
+
+# Additional include paths needed by kselftest.h and local headers
+CFLAGS += -I${top_srcdir}/tools/testing/selftests
+
+# Enables to extend CFLAGS and LDFLAGS from command line, e.g.
+# make USERCFLAGS=-Werror USERLDFLAGS=-static
+CFLAGS += $(USERCFLAGS)
+LDFLAGS += $(USERLDFLAGS)
+
+# When make O= with kselftest target from main level
+# the following aren't defined.
+#
+ifdef building_out_of_srctree
+LINK.c = $(CC) $(CFLAGS) $(CPPFLAGS) $(LDFLAGS) $(TARGET_ARCH)
+COMPILE.S = $(CC) $(ASFLAGS) $(CPPFLAGS) $(TARGET_ARCH) -c
+LINK.S = $(CC) $(ASFLAGS) $(CPPFLAGS) $(LDFLAGS) $(TARGET_ARCH)
+endif
+
+# Selftest makefiles can override those targets by setting
+# OVERRIDE_TARGETS = 1.
+ifeq ($(OVERRIDE_TARGETS),)
+LOCAL_HDRS += $(selfdir)/kselftest_harness.h $(selfdir)/kselftest.h
+$(OUTPUT)/%:%.c $(LOCAL_HDRS)
+	$(call msg,CC,,$@)
+	$(Q)$(LINK.c) $(filter-out $(LOCAL_HDRS),$^) $(LDLIBS) -o $@
+
+$(OUTPUT)/%.o:%.S
+	$(COMPILE.S) $^ -o $@
+
+$(OUTPUT)/%:%.S
+	$(LINK.S) $^ $(LDLIBS) -o $@
+endif
+
+# Extract the expected header directory
+khdr_output := $(patsubst %/usr/include,%,$(filter %/usr/include,$(KHDR_INCLUDES)))
+
+headers:
+	$(Q)$(MAKE) -f $(top_srcdir)/Makefile -C $(khdr_output) headers
+
+.PHONY: run_tests all clean install emit_tests gen_mods_dir clean_mods_dir headers