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

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Message-ID: <202601171106.90E508A9D4@keescook>
Date: Sat, 17 Jan 2026 11:07:24 -0800
From: Kees Cook <kees@...nel.org>
To: David Laight <david.laight.linux@...il.com>
Cc: Bill Wendling <morbo@...gle.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 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.

-- 
Kees Cook