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kbuild: rust: preserve unreachable traps with inline helpers

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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: 3a2486cc1d ("kbuild: rust: provide an option to inline C helpers into Rust")
Acked-by: Gary Guo <gary@garyguo.net>
Link: https://patch.msgid.link/20260816133233.197500-1-ojeda@kernel.org
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>
This commit is contained in:
Miguel Ojeda 2026-08-16 15:32:32 +02:00
parent 29b03d1de9
commit 0bff7711c1
3 changed files with 14 additions and 2 deletions
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10
Makefile
View File

@ -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

3
rust/Makefile
View File

@ -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)

3
scripts/Makefile.build
View File

@ -347,7 +347,8 @@ 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)