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

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Message-ID: <20260116095318.46a149fb@pumpkin>
Date: Fri, 16 Jan 2026 09:53:18 +0000
From: David Laight <david.laight.linux@...il.com>
To: Bill Wendling <morbo@...gle.com>
Cc: Kees Cook <kees@...nel.org>, "Gustavo A. R. Silva"
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 linux-kbuild@...r.kernel.org
Subject: Re: [PATCH v4 1/2] Compiler Attributes: Add __counted_by_ptr macro

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?
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.

	David