Add a KASAN option to the kernel build

LLVM support for enabling KASAN has not yet landed so the option is not
yet usable, but hopefully this will change soon.

Reviewed by:	imp, andrew
MFC after:	2 weeks
Sponsored by:	The FreeBSD Foundation
Differential Revision:	https://reviews.freebsd.org/D29454

sys/conf/kern.pre.mk

@@ -92,6 +92,16 @@ CFLAGS+=	-fno-common
 # XXX LOCORE means "don't declare C stuff" not "for locore.s".
 ASM_CFLAGS= -x assembler-with-cpp -DLOCORE ${CFLAGS} ${ASM_CFLAGS.${.IMPSRC:T}} 
 
+KASAN_ENABLED!=	grep KASAN opt_global.h || true ; echo
+.if !empty(KASAN_ENABLED)
+SAN_CFLAGS+=	-fsanitize=kernel-address \
+		-mllvm -asan-stack=true \
+		-mllvm -asan-instrument-dynamic-allocas=true \
+		-mllvm -asan-globals=true \
+		-mllvm -asan-use-after-scope=true \
+		-mllvm -asan-instrumentation-with-call-threshold=0
+.endif
+
 KCSAN_ENABLED!=	grep KCSAN opt_global.h || true ; echo
 .if !empty(KCSAN_ENABLED)
 SAN_CFLAGS+=	-fsanitize=thread

sys/conf/options

@@ -229,6 +229,7 @@ ZSTDIO		opt_zstdio.h
 
 # Sanitizers
 COVERAGE	opt_global.h
+KASAN		opt_global.h
 KCOV
 KCSAN		opt_global.h
 KUBSAN		opt_global.h