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

[Bill Wendling]

On Tue, Oct 6, 2026 at 3:09 AM Ard Biesheuvel <ardb@xxxxxxxxxx> wrote:
>
>
>
> 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.
>
I realized that just now. *sigh* I'd take up basketweaving, but I'd
probably get splinters that would fester and I'd lose fingers.

Fixed in v5.

-bw