Re: [PATCH v4] compiler_types: Allow opting out of __counted_by and __counted_by_ptr

[Ard Biesheuvel]

On Tue, 6 Oct 2026, at 11:43, 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' to turn the corresponding annotation into a no-op.
>
> Assisted-by: LLM
> Signed-off-by: Bill Wendling <morbo@xxxxxxxxxx>
> ---
> v2: Only specify __NO_COUNTED_BY_PTR. Leave __NO_COUNTED_BY until we need it.
> v3: Add the version to the Subject line.
> v4: Use __NO_COUNTED_BY to reduce churn later on.
> ---
>  include/linux/compiler_types.h | 7 ++++++-
>  1 file changed, 6 insertions(+), 1 deletion(-)
>
> diff --git a/include/linux/compiler_types.h b/include/linux/compiler_types.h
> index c5921f139007..05e2b637f1cd 100644
> --- a/include/linux/compiler_types.h
> +++ b/include/linux/compiler_types.h
> @@ -371,8 +371,13 @@ 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. __NO_COUNTED_BY should be used sparingly, because there are
> + * better options for opting out of bounds checking.
>   */
> -#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)


What happened to counted_by_ptr() now? __NO_COUNTED_BY should disable that too.