Re: [PATCH v4 1/2] Compiler Attributes: Add __counted_by_ptr macro

[Kees Cook]

On Fri, Jan 16, 2026 at 09:53:18AM +0000, David Laight wrote:
> On Fri, 16 Jan 2026 00:57:57 +0000
> Bill Wendling <morbo@xxxxxxxxxx> wrote:
> 
> > Introduce __counted_by_ptr(), which works like __counted_by(), but for
> > pointer struct members.
> > 
> > struct foo {
> > 	int a, b, c;
> > 	char *buffer __counted_by_ptr(bytes);
> > 	short nr_bars;
> > 	struct bar *bars __counted_by_ptr(nr_bars);
> > 	size_t bytes;
> > };
> > 
> > Because "counted_by" can only be applied to pointer members in very
> > recent compiler versions, its application ends up needing to be distinct
> > from flexibe array "counted_by" annotations, hence a separate macro.
> ...
> > diff --git a/Makefile b/Makefile
> > index 9d38125263fb..6b029f694bc2 100644
> > --- a/Makefile
> > +++ b/Makefile
> > @@ -952,6 +952,12 @@ KBUILD_CFLAGS	+= $(CC_AUTO_VAR_INIT_ZERO_ENABLER)
> >  endif
> >  endif
> >  
> > +ifdef CONFIG_CC_IS_CLANG
> > +ifdef CONFIG_CC_HAS_COUNTED_BY_PTR
> > +KBUILD_CFLAGS	+= -fexperimental-late-parse-attributes
> > +endif
> > +endif
> 
> Will that still be needed for clang 22?

AFAIK, yes. AIUI, this flag will remain while -fbounds-safety continues
to be upstreamed into LLVM.

> Looks a bit like a temporary flag to avoid regressions.
> Probably ought to at least have a comment that it won't be needed
> by some future clang version so that it gets tidied up.

Once it's no longer needed, yes, I will want it removed from the
Makefile.

-- 
Kees Cook