linux-hardening - Re: [PATCH 1/2] stddef: Allow attributes to be used when creating flex arrays

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Date: Tue, 13 Feb 2024 08:22:00 +0100
From: Rasmus Villemoes <linux@...musvillemoes.dk>
To: Kees Cook <keescook@...omium.org>, Jakub Kicinski <kuba@...nel.org>
Cc: "Gustavo A . R . Silva" <gustavoars@...nel.org>,
 Dan Williams <dan.j.williams@...el.com>, Keith Packard <keithp@...thp.com>,
 Miguel Ojeda <ojeda@...nel.org>, Alexey Dobriyan <adobriyan@...il.com>,
 Dmitry Antipov <dmantipov@...dex.ru>, "David S. Miller"
 <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>,
 Paolo Abeni <pabeni@...hat.com>, Nathan Chancellor <nathan@...nel.org>,
 kernel test robot <lkp@...el.com>, linux-kernel@...r.kernel.org,
 netdev@...r.kernel.org, linux-hardening@...r.kernel.org
Subject: Re: [PATCH 1/2] stddef: Allow attributes to be used when creating
 flex arrays

On 10/02/2024 02.16, Kees Cook wrote:
> With the coming support for the __counted_by struct member attribute, we
> will need a way to add such annotations to the places where
> DECLARE_FLEX_ARRAY() is used. Introduce DECLARE_FLEX_ARRAY_ATTR() which
> takes a third argument: the attributes to apply to the flexible array.
> 

> - * __DECLARE_FLEX_ARRAY() - Declare a flexible array usable in a union
> - *
> + * __DECLARE_FLEX_ARRAY_ATTR() - Declare a flexible array usable in a union
>   * @TYPE: The type of each flexible array element
>   * @NAME: The name of the flexible array member
> + * @ATTRS: The list of member attributes to apply
>   *
>   * In order to have a flexible array member in a union or alone in a
>   * struct, it needs to be wrapped in an anonymous struct with at least 1
>   * named member, but that member can be empty.
>   */
> -#define __DECLARE_FLEX_ARRAY(TYPE, NAME)	\
> +#define __DECLARE_FLEX_ARRAY_ATTR(TYPE, NAME, ATTRS)	\
>  	struct { \
>  		struct { } __empty_ ## NAME; \
> -		TYPE NAME[]; \
> +		TYPE NAME[] ATTRS; \
>  	}

Is it too ugly to not introduce a separate _ATTR macro but instead just do

#define __DECLARE_FLEX_ARRAY(TYPE, NAME, ...) \
  ...
  TYPE NAME[] __VA_ARGS__;

?

Rasmus