Re: [PATCH] compiler_types: Allow opting out of __counted_by and __counted_by_ptr

[Justin Stitt <justinstitt@google.com>]

Hi,

On Mon, Oct 05, 2026 at 10:25:18AM +0000, Bill Wendling wrote:
> Code that runs outside the kernel proper, such as the EFI stub, gets
> nothing out of the counted_by annotations: the bounds checks they feed
> (FORTIFY_SOURCE, UBSAN_BOUNDS) are already disabled there.
> 
> The annotations can also break the build. A __counted_by_ptr() that
> names a member declared after the pointer needs Clang's
> '-fexperimental-late-parse-attributes', which the top-level Makefile
> adds to 'KBUILD_CFLAGS'. The x86 EFI stub builds its own cflags and does
> not get that flag, so it fails as soon as such a struct is pulled in
> through a common header.
> 
> Overriding the macros from a Makefile doesn't work:
> 'compiler_types.h' is pulled in with '-include', which is processed
> after all -D/-U options, so it re-establishes the definitions. Follow
> the '__NO_FORTIFY' precedent instead: let a build define
> '__NO_COUNTED_BY' or '__NO_COUNTED_BY_PTR' to turn the corresponding
> annotation into a no-op.  The two are kept separate but parallel so they
> can be folded together once all supported compilers handle
> '__counted_by' on pointers.
> 
> Assisted-by: LLM
> Signed-off-by: Bill Wendling <morbo@google.com>
> ---
>  include/linux/compiler_types.h | 15 +++++++++++++--
>  1 file changed, 13 insertions(+), 2 deletions(-)
> 
> diff --git a/include/linux/compiler_types.h b/include/linux/compiler_types.h
> index c5921f139007..916a946f623c 100644
> --- a/include/linux/compiler_types.h
> +++ b/include/linux/compiler_types.h
> @@ -371,8 +371,14 @@ struct ftrace_likely_data {
>   *
>   * __bdos on clang < 19.1.3 can be off by 4:
>   * https://github.com/llvm/llvm-project/pull/112636
> + *
> + * Code that runs outside the kernel proper (e.g. the EFI stub) can define
> + * __NO_COUNTED_BY to drop the annotation, since it gains nothing from the
> + * bounds checks. Kept in step with __NO_COUNTED_BY_PTR below so the two can
> + * be folded together once __counted_by covers pointers on all supported
> + * compilers.
>   */
> -#ifdef CONFIG_CC_HAS_COUNTED_BY
> +#if defined(CONFIG_CC_HAS_COUNTED_BY) && !defined(__NO_COUNTED_BY)
>  # define __counted_by(member)		__attribute__((__counted_by__(member)))
>  #else
>  # define __counted_by(member)
> @@ -387,8 +393,13 @@ struct ftrace_likely_data {
>   *
>   *   gcc: https://gcc.gnu.org/pipermail/gcc-patches/2025-April/681727.html
>   * clang: https://clang.llvm.org/docs/AttributeReference.html#counted-by-counted-by-or-null-sized-by-sized-by-or-null
> + *
> + * Code that runs outside the kernel proper (e.g. the EFI stub) can define
> + * __NO_COUNTED_BY_PTR to drop the annotation, as with __NO_COUNTED_BY. Such
> + * code may also lack the flag Clang needs to parse a reference to a
> + * later-declared member (-fexperimental-late-parse-attributes).
>   */
> -#ifdef CONFIG_CC_HAS_COUNTED_BY_PTR
> +#if defined(CONFIG_CC_HAS_COUNTED_BY_PTR) && !defined(__NO_COUNTED_BY_PTR)
>  #define __counted_by_ptr(member)	__attribute__((__counted_by__(member)))
>  #else
>  #define __counted_by_ptr(member)
> -- 
> 2.56.0.rc1.315.gc6ed9934b7-goog
> 

Reviewed-by: Justin Stitt <justinstitt@google.com>

Thanks
Justin