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, 74 insertions, 0 deletions

diff --git a/scripts/Makefile.modfinal b/scripts/Makefile.modfinal
new file mode 100644
index 000000000..01a37ec87
--- /dev/null
+++ b/scripts/Makefile.modfinal
@@ -0,0 +1,74 @@
+# SPDX-License-Identifier: GPL-2.0-only
+# ===========================================================================
+# Module final link
+# ===========================================================================
+
+PHONY := __modfinal
+__modfinal:
+
+include $(objtree)/include/config/auto.conf
+include $(srctree)/scripts/Kbuild.include
+include $(srctree)/scripts/Makefile.lib
+
+# find all modules listed in modules.order
+modules := $(call read-file, modules.order)
+
+__modfinal: $(modules:%.o=%.ko)
+	@:
+
+# modname and part-of-module are set to make c_flags define proper module flags
+modname = $(notdir $(@:.mod.o=))
+part-of-module = y
+GCOV_PROFILE := n
+ccflags-remove-y := $(CC_FLAGS_CFI)
+
+%.mod.o: %.mod.c FORCE
+	$(call if_changed_rule,cc_o_c)
+
+.module-common.o: $(srctree)/scripts/module-common.c FORCE
+	$(call if_changed_rule,cc_o_c)
+
+ifneq ($(WARN_ON_UNUSED_TRACEPOINTS),)
+cmd_check_tracepoint = $(objtree)/scripts/tracepoint-update --module $<;
+endif
+
+quiet_cmd_ld_ko_o = LD [M]  $@
+      cmd_ld_ko_o =							\
+	$(LD) -r $(KBUILD_LDFLAGS)					\
+		$(KBUILD_LDFLAGS_MODULE) $(LDFLAGS_MODULE)		\
+		-T $(objtree)/scripts/module.lds -o $@ $(filter %.o, $^)
+
+quiet_cmd_btf_ko = BTF [M] $@
+      cmd_btf_ko = 							\
+	if [ ! -f $(objtree)/vmlinux ]; then				\
+		printf "Skipping BTF generation for %s due to unavailability of vmlinux\n" $@ 1>&2; \
+	else	\
+		$(CONFIG_SHELL) $(srctree)/scripts/gen-btf.sh --btf_base $(objtree)/vmlinux $@; \
+	fi;
+
+# Re-generate module BTFs if either module's .ko or vmlinux changed
+%.ko: %.o %.mod.o .module-common.o $(objtree)/scripts/module.lds $(and $(CONFIG_DEBUG_INFO_BTF_MODULES),$(KBUILD_BUILTIN),$(objtree)/vmlinux) FORCE
+	+$(call if_changed,ld_ko_o)
+ifdef CONFIG_DEBUG_INFO_BTF_MODULES
+	+$(if $(newer-prereqs),$(call cmd,btf_ko))
+endif
+	+$(call cmd,check_tracepoint)
+
+targets += $(modules:%.o=%.ko) $(modules:%.o=%.mod.o) .module-common.o
+
+# Add FORCE to the prerequisites of a target to force it to be always rebuilt.
+# ---------------------------------------------------------------------------
+
+PHONY += FORCE
+FORCE:
+
+# Read all saved command lines and dependencies for the $(targets) we
+# may be building above, using $(if_changed{,_dep}). As an
+# optimization, we don't need to read them if the target does not
+# exist, we will rebuild anyway in that case.
+
+existing-targets := $(wildcard $(sort $(targets)))
+
+-include $(foreach f,$(existing-targets),$(dir $(f)).$(notdir $(f)).cmd)
+
+.PHONY: $(PHONY)