[PATCH 2/2] kbuild: rust: keep Rust objects out of Clang LTO with inline helpers

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 <[email protected]>
Cc: Boqun Feng <[email protected]>
Cc: Alice Ryhl <[email protected]>
Cc: Matthew Maurer <[email protected]>
Cc: [email protected]
Cc: Josh Poimboeuf <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Fixes: 3a2486cc1da5 ("kbuild: rust: provide an option to inline C helpers into Rust")
Signed-off-by: Miguel Ojeda <[email protected]>
---
This is like the previous patch, but for the LTO case.

Another possible alternative that Gary suggested is to turn the flags on
globally for C, whether only when inline helpers are enabled or in all
cases, but it may be intended that in C we expect not to generate the
`unreachable`s.

Even if we do that later, we may want to still land this as the minimal
fix for backporting, since it only affects the experimental option and
only the Rust side.

 rust/Makefile          | 2 +-
 scripts/Makefile.build | 2 +-
 2 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/rust/Makefile b/rust/Makefile
index f871d94f6af2..3afaad4a4a3a 100644
--- a/rust/Makefile
+++ b/rust/Makefile
@@ -644,7 +644,7 @@ quiet_cmd_rustc_library = $(if $(skip_clippy),RUSTC,$(RUSTC_OR_CLIPPY_QUIET)) L
 		-Zunstable-options \
 	$(if $(link_helper),;$(LLVM_LINK) --internalize --suppress-warnings $(patsubst %.o,%.bc,$@) \
 		$(obj)/helpers/helpers$(if $(part-of-module),_module).bc -o $(patsubst %.o,%.m.bc,$@); \
-		$(CC) $(CLANG_FLAGS) $(KBUILD_CFLAGS) \
+		$(CC) $(CLANG_FLAGS) $(filter-out $(CC_FLAGS_LTO),$(KBUILD_CFLAGS)) \
 		$(CC_FLAGS_RUST_INLINE_HELPERS) -Wno-override-module -c $(patsubst %.o,%.m.bc,$@) -o $@ \
 		$(cmd_ld_single)) \
 	$(if $(rustc_objcopy),;$(OBJCOPY) $(rustc_objcopy) $@) \
diff --git a/scripts/Makefile.build b/scripts/Makefile.build
index 0b3b81f4a652..0b71f759eb5c 100644
--- a/scripts/Makefile.build
+++ b/scripts/Makefile.build
@@ -346,7 +346,7 @@ quiet_cmd_rustc_o_rs = $(RUSTC_OR_CLIPPY_QUIET) $(quiet_modtag) $@
       cmd_rustc_o_rs = $(rust_common_cmd) --emit=$(if $(CONFIG_RUST_INLINE_HELPERS),llvm-bc=$(patsubst %.o,%.bc,$@),obj=$@) $< \
 	$(if $(CONFIG_RUST_INLINE_HELPERS),;$(LLVM_LINK) --internalize --suppress-warnings $(patsubst %.o,%.bc,$@) \
 		$(objtree)/rust/helpers/helpers$(if $(part-of-module),_module).bc -o $(patsubst %.o,%.m.bc,$@); \
-		$(CC) $(CLANG_FLAGS) $(KBUILD_CFLAGS) \
+		$(CC) $(CLANG_FLAGS) $(filter-out $(CC_FLAGS_LTO),$(KBUILD_CFLAGS)) \
 		$(CC_FLAGS_RUST_INLINE_HELPERS) -Wno-override-module -c $(patsubst %.o,%.m.bc,$@) -o $@ \
 		$(cmd_ld_single)) \
 	$(cmd_objtool)
--
2.55.0