[PATCH 1/2] kbuild: rust: preserve unreachable traps with inline helpers

[Miguel Ojeda <ojeda@kernel.org>]

When `CONFIG_RUST_INLINE_HELPERS` is enabled, it is possible to hit
`objtool` warnings like:

    vmlinux.o: warning: objtool: _R..._4cmdq12CommandToGsp4init()
    falls through to next function _R..._4core5array4iter8IntoIterRShKj3_EEEBa_()

`rustc` normally emits traps for unreachable paths. However, under
`CONFIG_RUST_INLINE_HELPERS=y`, `rustc` emits LLVM bitcode and Clang
performs final code generation after the helper bitcode is linked,
but Clang does not trap unreachable IR by default.

In turn, this means `objtool` follows compiler-generated impossible Rust
`enum` paths through alignment padding into the next function, resulting
in fallthrough warnings.

Thus pass the LLVM `trap-unreachable` option to the final Clang invocation
and suppress traps immediately after `noreturn` calls, which `objtool`
already recognizes as dead ends. The combination of both flags makes it
match `rustc`'s behavior.

Rust 1.85.0 (the minimum supported one) supports LLVM >= 18, and both
flags are available in LLVM 18.

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")
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>
---
I pinged Gary offline to ask if he had to deal with these `unreachable`
cases back when he sent the patch, and he didn't. So it looks like we
simply did not hit the case until now.

 Makefile               | 10 ++++++++++
 rust/Makefile          |  3 ++-
 scripts/Makefile.build |  3 ++-
 3 files changed, 14 insertions(+), 2 deletions(-)

diff --git a/Makefile b/Makefile
index 902f3f3d54b7..85b84d895781 100644
--- a/Makefile
+++ b/Makefile
@@ -1083,6 +1083,16 @@ endif
 export CC_FLAGS_SCS
 endif

+ifdef CONFIG_RUST_INLINE_HELPERS
+# `rustc` normally emits traps for unreachable paths during code generation.
+# With inline helpers, Clang performs code generation from the linked bitcode
+# instead, so request the same behavior explicitly. Otherwise `objtool` may
+# follow an impossible Rust path into the next function.
+CC_FLAGS_RUST_INLINE_HELPERS := -mllvm -trap-unreachable \
+				-mllvm -no-trap-after-noreturn
+export CC_FLAGS_RUST_INLINE_HELPERS
+endif
+
 ifdef CONFIG_LTO_CLANG
 ifdef CONFIG_LTO_CLANG_FULL
 CC_FLAGS_LTO	:= -flto
diff --git a/rust/Makefile b/rust/Makefile
index fbe0accc51a3..f871d94f6af2 100644
--- a/rust/Makefile
+++ b/rust/Makefile
@@ -644,7 +644,8 @@ 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) -Wno-override-module -c $(patsubst %.o,%.m.bc,$@) -o $@ \
+		$(CC) $(CLANG_FLAGS) $(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) $@) \
 	$(cmd_objtool)
diff --git a/scripts/Makefile.build b/scripts/Makefile.build
index 911745743246..0b3b81f4a652 100644
--- a/scripts/Makefile.build
+++ b/scripts/Makefile.build
@@ -346,7 +346,8 @@ 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) -Wno-override-module -c $(patsubst %.o,%.m.bc,$@) -o $@ \
+		$(CC) $(CLANG_FLAGS) $(KBUILD_CFLAGS) \
+		$(CC_FLAGS_RUST_INLINE_HELPERS) -Wno-override-module -c $(patsubst %.o,%.m.bc,$@) -o $@ \
 		$(cmd_ld_single)) \
 	$(cmd_objtool)


base-commit: 47f27155f17498fccb1f222f79089642337498a9
--
2.55.0