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

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Date: Sat,  4 Nov 2023 13:43:37 -0700
From: Kees Cook <keescook@...omium.org>
To: KP Singh <kpsingh@...nel.org>
Cc: Kees Cook <keescook@...omium.org>,
	linux-hardening@...r.kernel.org,
	kernel test robot <lkp@...el.com>,
	"Gustavo A. R. Silva" <gustavoars@...nel.org>,
	linux-kernel@...r.kernel.org
Subject: [PATCH] gcc-plugins: randstruct: Only warn about true flexible arrays

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>
---
 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