linux-kernel - Re: [PATCH v4 1/2] Compiler Attributes: Add __counted_by

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Message-ID: <CAGG=3QWLn1uRih-F0mVfEjXNjw47Tb=-RWsoqBJUHH=7e_qwPQ@mail.gmail.com>
Date: Tue, 20 Jan 2026 10:11:24 -0800
From: Bill Wendling <morbo@...gle.com>
To: David Laight <david.laight.linux@...il.com>
Cc: Kees Cook <kees@...nel.org>, "Gustavo A. R. Silva" <gustavoars@...nel.org>, 
	Nathan Chancellor <nathan@...nel.org>, Nick Desaulniers <nick.desaulniers+lkml@...il.com>, 
	Justin Stitt <justinstitt@...gle.com>, Miguel Ojeda <ojeda@...nel.org>, 
	Peter Zijlstra <peterz@...radead.org>, Andrew Morton <akpm@...ux-foundation.org>, 
	Heiko Carstens <hca@...ux.ibm.com>, Marc Herbert <Marc.Herbert@...ux.intel.com>, 
	Uros Bizjak <ubizjak@...il.com>, Tejun Heo <tj@...nel.org>, Jeff Xu <jeffxu@...omium.org>, 
	Michal Koutný <mkoutny@...e.com>, 
	Shakeel Butt <shakeel.butt@...ux.dev>, 
	Thomas Weißschuh <thomas.weissschuh@...utronix.de>, 
	John Stultz <jstultz@...gle.com>, Christian Brauner <brauner@...nel.org>, 
	Randy Dunlap <rdunlap@...radead.org>, Brian Gerst <brgerst@...il.com>, 
	Masahiro Yamada <masahiroy@...nel.org>, linux-kernel@...r.kernel.org, 
	linux-hardening@...r.kernel.org, llvm@...ts.linux.dev, 
	Nicolas Schier <nsc@...nel.org>, Tamir Duberstein <tamird@...il.com>, Steven Rostedt <rostedt@...dmis.org>, 
	"Jason A. Donenfeld" <Jason@...c4.com>, "H. Peter Anvin" <hpa@...or.com>, 
	Naman Jain <namjain@...ux.microsoft.com>, Simon Horman <horms@...nel.org>, 
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>, Ingo Molnar <mingo@...nel.org>, 
	Thomas Gleixner <tglx@...nel.org>, Douglas Anderson <dianders@...omium.org>, linux-kbuild@...r.kernel.org
Subject: Re: [PATCH v4 1/2] Compiler Attributes: Add __counted_by_ptr macro

On Fri, Jan 16, 2026 at 1:53 AM David Laight
<david.laight.linux@...il.com> wrote:
>
> On Fri, 16 Jan 2026 00:57:57 +0000
> Bill Wendling <morbo@...gle.com> 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?
> 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.

I don't believe that there's a timeline for removing this flag, but I
agree that it should go at some point.

-bw