Re: [PATCH v3 1/2] kbuild: rust: add `CONFIG_RUSTC_CLANG_LLVM_COMPATIBLE`

[Tamir Duberstein]

On Wed, Mar 19, 2025 at 4:57 PM Gary Guo <gary@xxxxxxxxxxx> wrote:
>
> This config detects if Rust and Clang have matching LLVM major version.
> All IR or bitcode operations (e.g. LTO) rely on LLVM major version to be
> matching, otherwise it may generate errors, or worse, miscompile silently
> due to change of IR semantics.
>
> It's usually suggested to use the exact same LLVM version, but this can
> be difficult to guarantee. Rust's suggestion [1] is also major-version only,
> so I think this check is sufficient for the kernel.
>
> Link: https://doc.rust-lang.org/rustc/linker-plugin-lto.html [1]
> Reviewed-by: Andreas Hindborg <a.hindborg@xxxxxxxxxx>
> Signed-off-by: Gary Guo <gary@xxxxxxxxxxx>
> ---
>  init/Kconfig | 15 +++++++++++++++
>  1 file changed, 15 insertions(+)
>
> diff --git a/init/Kconfig b/init/Kconfig
> index d0d021b3fa3b..e76e4ad7d4b9 100644
> --- a/init/Kconfig
> +++ b/init/Kconfig
> @@ -82,6 +82,21 @@ config RUSTC_LLVM_VERSION
>         int
>         default $(rustc-llvm-version)
>
> +config RUSTC_LLVM_MAJOR_VERSION
> +       int
> +       default $(shell,expr $(rustc-llvm-version) / 10000)
> +
> +config RUSTC_CLANG_LLVM_COMPATIBLE
> +       bool
> +       default y if CC_IS_CLANG && RUSTC_LLVM_MAJOR_VERSION = $(shell,expr $(cc-version) / 10000)
> +       help
> +         This indicates whether Rust and Clang use LLVM of the same major
> +         version.
> +
> +         Operations involving handling LLVM IR or bitcode (e.g. cross-language
> +         LTO) requires the same LLVM major version to work properly. For best

s/requires/require/

> +         compatibility it is recommended that the exact same LLVM is used.

nit: s/LLVM/LLVM version/ unless you literally mean the same dylib.

> +
>  config CC_CAN_LINK
>         bool
>         default $(success,$(srctree)/scripts/cc-can-link.sh $(CC) $(CLANG_FLAGS) $(USERCFLAGS) $(USERLDFLAGS) $(m64-flag)) if 64BIT
> --
> 2.47.2
>
>

Reviewed-by: Tamir Duberstein <tamird@xxxxxxxxx>