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

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

[Kees Cook]

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@kernel.org>
Cc: linux-hardening@vger.kernel.org
Reported-by: kernel test robot <lkp@intel.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@chromium.org>
---
 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;
 }
 
-- 
2.34.1

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

[Bill Wendling]

On Sat, Nov 4, 2023 at 1:43 PM Kees Cook <keescook@chromium.org> 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@kernel.org>
> Cc: linux-hardening@vger.kernel.org
> Reported-by: kernel test robot <lkp@intel.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@chromium.org>

Reviewed-by: Bill Wendling <morbo@google.com>

> ---
>  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;
>  }
>
> --
> 2.34.1
>
>

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

[Gustavo A. R. Silva]

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@kernel.org>
> Cc: linux-hardening@vger.kernel.org
> Reported-by: kernel test robot <lkp@intel.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@chromium.org>

Acked-by: Gustavo A. R. Silva <gustavoars@kernel.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;
>   }
>   

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

[Kees Cook]

On Sat, 04 Nov 2023 13:43:37 -0700, 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.
> 
> [...]

Applied to for-next/hardening, thanks!

[1/1] gcc-plugins: randstruct: Only warn about true flexible arrays
      https://git.kernel.org/kees/c/1ee60356c2dc

Take care,

-- 
Kees Cook