Index: lib/libnv/Makefile
===================================================================
--- lib/libnv/Makefile
+++ lib/libnv/Makefile
@@ -9,7 +9,10 @@
 SHLIB_MAJOR= 0
 
 .PATH: ${SRCTOP}/sys/contrib/libnv ${SRCTOP}/sys/sys
-CFLAGS+=-I${SRCTOP}/sys -I${.CURDIR}
+.ifndef BOOTSTRAPPING
+CFLAGS+=-I${SRCTOP}/sys
+.endif
+CFLAGS+=-I${.CURDIR}
 
 SRCS=	cnvlist.c
 SRCS+=	dnvlist.c
Index: tools/build/mk/Makefile.boot
===================================================================
--- tools/build/mk/Makefile.boot
+++ tools/build/mk/Makefile.boot
@@ -7,3 +7,19 @@
 
 # we do not want to capture dependencies referring to the above
 UPDATE_DEPENDFILE= no
+
+# When building host tools we should never pull in headers from the source sys
+# directory to avoid any ABI issues that might cause the built binary to crash.
+# The only exception to this is sys/cddl which is needed by various dtrace tools.
+.if !empty(CFLAGS:N*${SRCTOP}/sys/cddl/compat*:M*${SRCTOP}/sys*)
+.error Do not include $${SRCTOP}/sys when building bootstrap tools. \
+    Copy the header to ${WORLDTMP}/legacy in tools/build/Makefile instead. \
+    Error was caused by Makefile in ${.CURDIR}
+.endif
+
+# ${SRCTOP}/include should also never be used to avoid ABI issues
+.if !empty(CFLAGS:M*${SRCTOP}/include*)
+.error Do not include $${SRCTOP}/include when building bootstrap tools. \
+    Copy the header to ${WORLDTMP}/legacy in tools/build/Makefile instead. \
+    Error was caused by Makefile in ${.CURDIR}
+.endif