Re: [PATCH v3] compiler_types: Allow opting out of __counted_by and __counted_by_ptr

["Ard Biesheuvel" <ardb@kernel.org>]

On Tue, 6 Oct 2026, at 09:30, Justin Stitt wrote:
> Hi,
>
> On Mon, Oct 5, 2026 at 2:25 PM Ard Biesheuvel <ardb@kernel.org> wrote:
>>
>>
>>
>> On Mon, 5 Oct 2026, at 21:20, 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 __counted_by_ptr macro 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_PTR' to turn the corresponding annotation into a no-op.
>> >
>> > Assisted-by: LLM
>> > Signed-off-by: Bill Wendling <morbo@google.com>
>> > ---
>> > v2: Only specify __NO_COUNTED_BY_PTR. Leave __NO_COUNTED_BY until we need it.
>> > v3: Add the version to the Subject line.
>> > ---
>> >  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..8bde6798b4ec 100644
>> > --- a/include/linux/compiler_types.h
>> > +++ b/include/linux/compiler_types.h
>> > @@ -387,8 +387,13 @@ 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. 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)
>>
>> Apologies, I may have been unclear.
>>
>> What I would like to see here is something like
>>
>> #ifndef __NO_COUNTED_BY
>> #define __counted_by(member)  __attribute__((....))
>> #define __counted_by_ptr(member)  __attribute__((....))
>> #else
>> #define __counted_by(member)
>> #define __counted_by_ptr(member)
>> #endif
>
> FWIW, Bill's original patch had this explanation
>
> "The two are kept separate but parallel so they
> can be folded together once all supported compilers handle
> '__counted_by' on pointers."
>

When opting out of this, whether or not the compiler supports
all variants of counted_by() is irrelevant. The same #define
should just opt out of all of them.

If the need arises to be more granular here, we can always add
that later but I don't want to have to add

-D__NO_COUNTED_BY -D__NO_COUNTED_BY_PTR

everywhere today, and go back and remove the second part once
all compilers have caught up.