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

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Message-ID: <CAGG=3QWeYZqLA8FSMYZFRrJ24LtEKJdmRAq3awkMZj0+qy=4_w@mail.gmail.com>
Date: Tue, 20 Jan 2026 10:12:34 -0800
From: Bill Wendling <morbo@...gle.com>
To: Kees Cook <kees@...nel.org>
Cc: David Laight <david.laight.linux@...il.com>, 
	"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 Sat, Jan 17, 2026 at 11:07 AM Kees Cook <kees@...nel.org> wrote:
>
> 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@...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?
>
> 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.
>
Would it be good to 'fixup' a comment in the Makefile for that?

-bw