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

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 <[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]>

Acked-by: Gary Guo <[email protected]>

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