linux-kernel - Re: [PATCH v2 1/2] Compiler Attributes: Add __counted_by

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Message-ID: <CANiq72=kqJApbOJ0hmC_mAYe6DO8YqXSFaLJOsbPhkXfxmvJHQ@mail.gmail.com>
Date: Fri, 21 Nov 2025 22:47:45 +0100
From: Miguel Ojeda <miguel.ojeda.sandonis@...il.com>
To: Bill Wendling <morbo@...gle.com>
Cc: linux-kernel@...r.kernel.org, Kees Cook <kees@...nel.org>, 
	Qing Zhao <qing.zhao@...cle.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-hardening@...r.kernel.org, 
	llvm@...ts.linux.dev, Jan Hendrik Farr <kernel@...rr.cc>
Subject: Re: [PATCH v2 1/2] Compiler Attributes: Add __counted_by_ptr macro

On Fri, Nov 21, 2025 at 8:55 PM Bill Wendling <morbo@...gle.com> 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

I guess there is a reason for this name, but it sounds to me a bit
like the thing between parenthesis is a pointer, i.e. that perhaps it
is the pointee that one that counts.

Hmm... what about `__ptr_counted_by`?

In addition, could we please provide a bit of context in the
documentation? i.e. links to the attribute docs in both Clang and GCC.

And perhaps explaining why this cannot use `__has_attribute`, i.e.
what the commit log mentions.

Thanks!

Cheers,
Miguel