[PATCH 6/6] randstruct: Enable Clang support

[Kees Cook]

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@xxxxxxxxxx>
Cc: linux-kbuild@xxxxxxxxxxxxxxx
Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
---
 scripts/Makefile.randstruct |  3 +++
 security/Kconfig.hardening  | 14 ++++++++++++--
 2 files changed, 15 insertions(+), 2 deletions(-)

diff --git a/scripts/Makefile.randstruct b/scripts/Makefile.randstruct
index 4d741e6db554..24e283e89893 100644
--- a/scripts/Makefile.randstruct
+++ b/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)
diff --git a/security/Kconfig.hardening b/security/Kconfig.hardening
index 0277ba578779..bd2aabb2c60f 100644
--- a/security/Kconfig.hardening
+++ b/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
-- 
2.32.0