lkdtm: Add Control Flow Integrity test

This adds a simple test for forward CFI (indirect function calls) with
function prototype granularity (as implemented by Clang's CFI).

Signed-off-by: Kees Cook <keescook@chromium.org>
diff --git a/drivers/misc/lkdtm/lkdtm.h b/drivers/misc/lkdtm/lkdtm.h
index 6a284a8..8a25afb 100644
--- a/drivers/misc/lkdtm/lkdtm.h
+++ b/drivers/misc/lkdtm/lkdtm.h
@@ -95,4 +95,7 @@ void lkdtm_USERCOPY_KERNEL_DS(void);
 /* lkdtm_stackleak.c */
 void lkdtm_STACKLEAK_ERASING(void);
 
+/* cfi.c */
+void lkdtm_CFI_FORWARD_PROTO(void);
+
 #endif