randstruct: Enable Clang support

Clang 15 will support randstruct via the -frandomize-layout-seed-file=...
option. Update the Kconfig and Makefile to recognize this feature.

Cc: Masahiro Yamada <masahiroy@kernel.org>
Cc: linux-kbuild@vger.kernel.org
Signed-off-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20220503205503.3054173-7-keescook@chromium.org

scripts/Makefile.randstruct

@@ -7,6 +7,9 @@ randstruct-cflags-y	\
 	+= -fplugin=$(objtree)/scripts/gcc-plugins/randomize_layout_plugin.so
 randstruct-cflags-$(CONFIG_RANDSTRUCT_PERFORMANCE)		\
 	+= -fplugin-arg-randomize_layout_plugin-performance-mode
+else
+randstruct-cflags-y	\
+	+= -frandomize-layout-seed-file=$(objtree)/scripts/basic/randstruct.seed
 endif
 
 export RANDSTRUCT_CFLAGS := $(randstruct-cflags-y)

security/Kconfig.hardening

@@ -266,9 +266,12 @@ config ZERO_CALL_USED_REGS
 
 endmenu
 
+config CC_HAS_RANDSTRUCT
+	def_bool $(cc-option,-frandomize-layout-seed-file=/dev/null)
+
 choice
 	prompt "Randomize layout of sensitive kernel structures"
-	default RANDSTRUCT_FULL if COMPILE_TEST && GCC_PLUGINS
+	default RANDSTRUCT_FULL if COMPILE_TEST && (GCC_PLUGINS || CC_HAS_RANDSTRUCT)
 	default RANDSTRUCT_NONE
 	help
 	  If you enable this, the layouts of structures that are entirely
@@ -297,13 +300,20 @@ choice
 
 	config RANDSTRUCT_FULL
 		bool "Fully randomize structure layout"
-		depends on GCC_PLUGINS
+		depends on CC_HAS_RANDSTRUCT || GCC_PLUGINS
 		select MODVERSIONS if MODULES
 		help
 		  Fully randomize the member layout of sensitive
 		  structures as much as possible, which may have both a
 		  memory size and performance impact.
 
+		  One difference between the Clang and GCC plugin
+		  implementations is the handling of bitfields. The GCC
+		  plugin treats them as fully separate variables,
+		  introducing sometimes significant padding. Clang tries
+		  to keep adjacent bitfields together, but with their bit
+		  ordering randomized.
+
 	config RANDSTRUCT_PERFORMANCE
 		bool "Limit randomization of structure layout to cache-lines"
 		depends on GCC_PLUGINS