linux-kernel - Re: [PATCH 1/3] compiler_types: Introduce __counted_by

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Message-ID: <202510221746.7C09BBE@keescook>
Date: Wed, 22 Oct 2025 17:47:43 -0700
From: Kees Cook <kees@...nel.org>
To: Peter Zijlstra <peterz@...radead.org>
Cc: Miguel Ojeda <ojeda@...nel.org>, Nathan Chancellor <nathan@...nel.org>,
	Nick Desaulniers <nick.desaulniers+lkml@...il.com>,
	Bill Wendling <morbo@...gle.com>,
	Justin Stitt <justinstitt@...gle.com>,
	Marco Elver <elver@...gle.com>,
	Przemek Kitszel <przemyslaw.kitszel@...el.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Masahiro Yamada <masahiroy@...nel.org>,
	Christophe Leroy <christophe.leroy@...roup.eu>,
	Johannes Weiner <hannes@...xchg.org>, llvm@...ts.linux.dev,
	Al Viro <viro@...iv.linux.org.uk>, Arnd Bergmann <arnd@...db.de>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Christian Brauner <brauner@...nel.org>, Jan Kara <jack@...e.cz>,
	Nicolas Schier <nicolas.schier@...ux.dev>,
	Shuah Khan <shuah@...nel.org>,
	"Gustavo A. R. Silva" <gustavoars@...nel.org>,
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	kernel test robot <lkp@...el.com>,
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	Jan Hendrik Farr <kernel@...rr.cc>,
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	Jeff Xu <jeffxu@...omium.org>,
	Michal Koutný <mkoutny@...e.com>,
	Shakeel Butt <shakeel.butt@...ux.dev>,
	Randy Dunlap <rdunlap@...radead.org>,
	Brian Gerst <brgerst@...il.com>, linux-kernel@...r.kernel.org,
	linux-fsdevel@...r.kernel.org, linux-kbuild@...r.kernel.org,
	linux-kselftest@...r.kernel.org, linux-hardening@...r.kernel.org
Subject: Re: [PATCH 1/3] compiler_types: Introduce __counted_by_ptr()

On Tue, Oct 21, 2025 at 12:24:05PM -0700, Kees Cook wrote:
> On Tue, Oct 21, 2025 at 11:54:47AM +0200, Peter Zijlstra wrote:
> > > [...]
> > > Unfortunately, this annotation cannot be used for "void *" members
> > > (since such a member is considered a pointer to an incomplete type,
> > > and neither Clang nor GCC developers could be convinced otherwise[1],
> > > even in the face of the GNU extension that "void *" has size "1 byte"
> > > for pointer arithmetic). For "void *" members, we must use the coming
> > > "sized_by" attribute.
> > 
> > So why do we need both __counted_by_ptr() and this __sized_by(), won't
> > one be good enough?
> [...]
> Let me take another stab at it...

It seems this will be acceptable as long as it is gated by GNU
extensions. GCC patch in progress. Clang PR here:
https://github.com/llvm/llvm-project/pull/163698

-- 
Kees Cook