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

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Message-ID: <CAGG=3QXC7EH18mhJq6egqE6bvo8_KchPD_N4PeZ7Z7tLOv26-A@mail.gmail.com>
Date: Tue, 10 Feb 2026 03:00:02 -0800
From: Bill Wendling <morbo@...gle.com>
To: Arnd Bergmann <arnd@...db.de>
Cc: linux-kernel@...r.kernel.org, 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-hardening@...r.kernel.org, 
	llvm@...ts.linux.dev, Jan Hendrik Farr <kernel@...rr.cc>
Subject: Re: [PATCH 1/2] Compiler Attributes: Add __counted_by_ptr macro

On Tue, Feb 10, 2026 at 12:41 AM Arnd Bergmann <arnd@...db.de> wrote:
>
> On Fri, Nov 21, 2025, at 20:39, Bill Wendling wrote:
> >
> > +/*
> > + * Optional: only supported since clang >= 21
> > + *
> > + * clang: https://github.com/llvm/llvm-project/pull/137250
> > + */
> > +#ifdef CONFIG_CC_HAS_COUNTED_BY_FOR_POINTER
> > +#define __counted_by_ptr(member)     __attribute__((__counted_by__(member)))
> > +#else
> > +#define __counted_by_ptr(member)
> > +#endif
>
> Something changed in recent gcc versions. I had tested gcc-16.0.1
> successfully with CONFIG_CC_HAS_COUNTED_BY_PTR=y, but after upgrading
> to a recent git snapshot, I get this output:
>
> fs/coredump.c:95:15: error: 'counted_by' attribute is not allowed for a non-array field
>    95 |         char *corename __counted_by_ptr(size);
>       |               ^~~~~~~~
> drivers/misc/lkdtm/bugs.c:518:15: error: 'counted_by' attribute is not allowed for a non-array field
>   518 |         char *buf __counted_by_ptr(len);
>       |               ^~~
> drivers/misc/lkdtm/bugs.c:520:29: error: 'counted_by' attribute is not allowed for a non-array field
>   520 |         struct lkdtm_extra *extra __counted_by_ptr(nr_extra);
>       |                             ^~~~~
>
> This is every use of __counted_by_ptr() in linux-next at the moment.
>
I assume it's a git snapshot of GCC. :-) What does
`scripts/cc-version.sh` produce for it?

-bw