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

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Message-ID: <20260116083604.GJ830755@noisy.programming.kicks-ass.net>
Date: Fri, 16 Jan 2026 09:36:04 +0100
From: Peter Zijlstra <peterz@...radead.org>
To: Kees Cook <kees@...nel.org>
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>,
	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>,
	Eric Dumazet <edumazet@...gle.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 1/2] Compiler Attributes: Add __counted_by_ptr macro

On Wed, Jan 14, 2026 at 08:00:54PM -0800, Kees Cook wrote:
> On Wed, Jan 14, 2026 at 07:36:47PM +0000, Bill Wendling 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.
> > 
> > Note that Clang's support for "void *" members will be in version 22.
> > So, when using Clang, you'll need to wait until its release before using
> > the feature with "void *". No such restriction applies to GCC's version
> > 16.
> 
> I think to keep operational parity, we should limit counted_ptr on Clang
> to version 22 then, otherwise we'll have problems using it on void *.

Ooh, you got that fixed! Nice!