linux-hardening - Re: [PATCH] gcc-plugins: randstruct: Only warn about true flexible arrays

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Date: Mon, 6 Nov 2023 09:53:36 -0600
From: "Gustavo A. R. Silva" <gustavo@...eddedor.com>
To: Kees Cook <keescook@...omium.org>, KP Singh <kpsingh@...nel.org>
Cc: linux-hardening@...r.kernel.org, kernel test robot <lkp@...el.com>,
 "Gustavo A. R. Silva" <gustavoars@...nel.org>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] gcc-plugins: randstruct: Only warn about true flexible
 arrays



On 11/4/23 14:43, Kees Cook wrote:
> The randstruct GCC plugin tried to discover "fake" flexible arrays
> to issue warnings about them in randomized structs. In the future
> LSM overhead reduction series, it would be legal to have a randomized
> struct with a 1-element array, and this should _not_ be treated as a
> flexible array, especially since commit df8fc4e934c1 ("kbuild: Enable
> -fstrict-flex-arrays=3"). Disable the 0-sized and 1-element array
> discovery logic in the plugin, but keep the "true" flexible array check.
> 
> Cc: KP Singh <kpsingh@...nel.org>
> Cc: linux-hardening@...r.kernel.org
> Reported-by: kernel test robot <lkp@...el.com>
> Closes: https://lore.kernel.org/oe-kbuild-all/202311021532.iBwuZUZ0-lkp@intel.com/
> Fixes: df8fc4e934c1 ("kbuild: Enable -fstrict-flex-arrays=3")
> Signed-off-by: Kees Cook <keescook@...omium.org>

Acked-by: Gustavo A. R. Silva <gustavoars@...nel.org>

Thanks!
--
Gustavo

> ---
>   scripts/gcc-plugins/randomize_layout_plugin.c | 10 ----------
>   1 file changed, 10 deletions(-)
> 
> diff --git a/scripts/gcc-plugins/randomize_layout_plugin.c b/scripts/gcc-plugins/randomize_layout_plugin.c
> index 366395cab490..910bd21d08f4 100644
> --- a/scripts/gcc-plugins/randomize_layout_plugin.c
> +++ b/scripts/gcc-plugins/randomize_layout_plugin.c
> @@ -278,8 +278,6 @@ static bool is_flexible_array(const_tree field)
>   {
>   	const_tree fieldtype;
>   	const_tree typesize;
> -	const_tree elemtype;
> -	const_tree elemsize;
>   
>   	fieldtype = TREE_TYPE(field);
>   	typesize = TYPE_SIZE(fieldtype);
> @@ -287,20 +285,12 @@ static bool is_flexible_array(const_tree field)
>   	if (TREE_CODE(fieldtype) != ARRAY_TYPE)
>   		return false;
>   
> -	elemtype = TREE_TYPE(fieldtype);
> -	elemsize = TYPE_SIZE(elemtype);
> -
>   	/* size of type is represented in bits */
>   
>   	if (typesize == NULL_TREE && TYPE_DOMAIN(fieldtype) != NULL_TREE &&
>   	    TYPE_MAX_VALUE(TYPE_DOMAIN(fieldtype)) == NULL_TREE)
>   		return true;
>   
> -	if (typesize != NULL_TREE &&
> -	    (TREE_CONSTANT(typesize) && (!tree_to_uhwi(typesize) ||
> -	     tree_to_uhwi(typesize) == tree_to_uhwi(elemsize))))
> -		return true;
> -
>   	return false;
>   }
>