Re: [PATCH v2 2/3] rust: helpers: #define __rust_helper

[Nathan Chancellor <nathan@kernel.org>]

On Tue, Feb 03, 2026 at 11:34:09AM +0000, Alice Ryhl wrote:
> From: Gary Guo <gary@garyguo.net>
> 
> Because of LLVM inling checks, it's generally not possible to inline a C
> helper into Rust code, even with LTO:
> 
> * LLVM doesn't want to inline functions compiled with
>   `-fno-delete-null-pointer-checks` with code compiled without. The C
>   CGUs all have this enabled and Rust CGUs don't. Inlining is okay since
>   this is one of the hardening features that does not change the ABI,
>   and we shouldn't have null pointer dereferences in these helpers.
> 
> * LLVM doesn't want to inline functions with different list of builtins. C
>   side has `-fno-builtin-wcslen`; `wcslen` is not a Rust builtin, so
>   they should be compatible, but LLVM does not perform inlining due to
>   attributes mismatch.
> 
> * clang and Rust doesn't have the exact target string. Clang generates
>   `+cmov,+cx8,+fxsr` but Rust doesn't enable them (in fact, Rust will
>   complain if `-Ctarget-feature=+cmov,+cx8,+fxsr` is used). x86-64
>   always enable these features, so they are in fact the same target
>   string, but LLVM doesn't understand this and so inlining is inhibited.
>   This can be bypassed with `--ignore-tti-inline-compatible`, but this
>   is a hidden option.
> 
> To fix this, we can add __always_inline on every helper, which skips
> these LLVM inlining checks. For this purpose, introduce a new
> __rust_helper macro that needs to be added to every helper.
> 
> Most helpers already have __rust_helper specified, but there are a few
> missing. The only consequence of this is that those specific helpers do
> not get inlined.
> 
> Signed-off-by: Gary Guo <gary@garyguo.net>
> Signed-off-by: Alice Ryhl <aliceryhl@google.com>
> ---
Reviewed-by: Nathan Chancellor <nathan@kernel.org>
Tested-by: Nathan Chancellor <nathan@kernel.org>

>  rust/helpers/helpers.c | 29 +++++++++++++++++++++++++++++
>  1 file changed, 29 insertions(+)
> 
> diff --git a/rust/helpers/helpers.c b/rust/helpers/helpers.c
> index a3c42e51f00a0990bea81ebce6e99bb397ce7533..e05c6e7e4abb7e6a4c4a3a417e35022dac1d9c58 100644
> --- a/rust/helpers/helpers.c
> +++ b/rust/helpers/helpers.c
> @@ -7,7 +7,36 @@
>   * Sorted alphabetically.
>   */
>  
> +#include <linux/compiler_types.h>
> +
> +#ifdef __BINDGEN__
> +// Omit `inline` for bindgen as it ignores inline functions.
>  #define __rust_helper
> +#else
> +// The helper functions are all inline functions.
> +//
> +// We use `__always_inline` here to bypass LLVM inlining checks, in case the
> +// helpers are inlined directly into Rust CGUs.
> +//
> +// The LLVM inlining checks are false positives:
> +// * LLVM doesn't want to inline functions compiled with
> +//   `-fno-delete-null-pointer-checks` with code compiled without.
> +//   The C CGUs all have this enabled and Rust CGUs don't. Inlining is okay
> +//   since this is one of the hardening features that does not change the ABI,
> +//   and we shouldn't have null pointer dereferences in these helpers.
> +// * LLVM doesn't want to inline functions with different list of builtins. C
> +//   side has `-fno-builtin-wcslen`; `wcslen` is not a Rust builtin, so they
> +//   should be compatible, but LLVM does not perform inlining due to attributes
> +//   mismatch.
> +// * clang and Rust doesn't have the exact target string. Clang generates
> +//   `+cmov,+cx8,+fxsr` but Rust doesn't enable them (in fact, Rust will
> +//   complain if `-Ctarget-feature=+cmov,+cx8,+fxsr` is used). x86-64 always
> +//   enable these features, so they are in fact the same target string, but
> +//   LLVM doesn't understand this and so inlining is inhibited. This can be
> +//   bypassed with `--ignore-tti-inline-compatible`, but this is a hidden
> +//   option.
> +#define __rust_helper __always_inline
> +#endif
>  
>  #include "atomic.c"
>  #include "atomic_ext.c"
> 
> -- 
> 2.53.0.rc1.225.gd81095ad13-goog
>