randstruct: Enable function pointer struct detection

This enables the automatic structure selection logic in the randstruct
GCC plugin. The selection logic randomizes all structures that contain
only function pointers, unless marked with __no_randomize_layout.
Signed-off-by: Kees Cook <keescook@chromium.org>

arch/Kconfig

@@ -473,11 +473,13 @@ config GCC_PLUGIN_RANDSTRUCT
 	depends on GCC_PLUGINS
 	select MODVERSIONS if MODULES
 	help
-	  If you say Y here, the layouts of structures explicitly
-	  marked by __randomize_layout will be randomized at
-	  compile-time.  This can introduce the requirement of an
-	  additional information exposure vulnerability for exploits
-	  targeting these structure types.
+	  If you say Y here, the layouts of structures that are entirely
+	  function pointers (and have not been manually annotated with
+	  __no_randomize_layout), or structures that have been explicitly
+	  marked with __randomize_layout, will be randomized at compile-time.
+	  This can introduce the requirement of an additional information
+	  exposure vulnerability for exploits targeting these structure
+	  types.
 
 	  Enabling this feature will introduce some performance impact,
 	  slightly increase memory usage, and prevent the use of forensic

scripts/gcc-plugins/randomize_layout_plugin.c

@@ -436,9 +436,6 @@ static int is_pure_ops_struct(const_tree node)
 
 	gcc_assert(TREE_CODE(node) == RECORD_TYPE || TREE_CODE(node) == UNION_TYPE);
 
-	/* XXX: Do not apply randomization to all-ftpr structs yet. */
-	return 0;
-
 	for (field = TYPE_FIELDS(node); field; field = TREE_CHAIN(field)) {
 		const_tree fieldtype = get_field_type(field);
 		enum tree_code code = TREE_CODE(fieldtype);