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

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

[Miguel Ojeda]

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

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

[Miguel Ojeda]

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: stable@vger.kernel.org
Cc: Josh Poimboeuf <jpoimboe@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Fixes: 3a2486cc1da5 ("kbuild: rust: provide an option to inline C helpers into Rust")
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>
---
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

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

[Gary Guo]

On Sun Aug 16, 2026 at 2:32 PM BST, Miguel Ojeda wrote:
> 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>

Acked-by: Gary Guo <gary@garyguo.net>

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

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

[Gary Guo]

On Sun Aug 16, 2026 at 2:32 PM BST, Miguel Ojeda wrote:
> 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: stable@vger.kernel.org
> Cc: Josh Poimboeuf <jpoimboe@kernel.org>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Fixes: 3a2486cc1da5 ("kbuild: rust: provide an option to inline C helpers into Rust")
> Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

Acked-by: Gary Guo <gary@garyguo.net>

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