diff --git a/sys/conf/kmod.mk b/sys/conf/kmod.mk
--- a/sys/conf/kmod.mk
+++ b/sys/conf/kmod.mk
@@ -540,7 +540,9 @@
 	-I${SYSDIR}/cddl/compat/opensolaris \
 	-I${SYSDIR}/cddl/contrib/opensolaris/uts/common \
 	-include ${ZINCDIR}/os/freebsd/spl/sys/ccompile.h
-
+OPENZFS_CWARNFLAGS= \
+	-Wno-nested-externs \
+	-Wno-redundant-decls
 
 .include <bsd.dep.mk>
 .include <bsd.clang-analyze.mk>
diff --git a/sys/modules/dtrace/dtaudit/Makefile b/sys/modules/dtrace/dtaudit/Makefile
--- a/sys/modules/dtrace/dtaudit/Makefile
+++ b/sys/modules/dtrace/dtaudit/Makefile
@@ -13,3 +13,4 @@
 .include <bsd.kmod.mk>
 
 CFLAGS+=	-include ${SYSDIR}/cddl/compat/opensolaris/sys/debug_compat.h
+CWARNFLAGS+=	${OPENZFS_CWARNFLAGS}
diff --git a/sys/modules/dtrace/dtmalloc/Makefile b/sys/modules/dtrace/dtmalloc/Makefile
--- a/sys/modules/dtrace/dtmalloc/Makefile
+++ b/sys/modules/dtrace/dtmalloc/Makefile
@@ -13,3 +13,4 @@
 .include <bsd.kmod.mk>
 
 CFLAGS+=	-include ${SYSDIR}/cddl/compat/opensolaris/sys/debug_compat.h
+CWARNFLAGS+=	${OPENZFS_CWARNFLAGS}
diff --git a/sys/modules/dtrace/dtnfscl/Makefile b/sys/modules/dtrace/dtnfscl/Makefile
--- a/sys/modules/dtrace/dtnfscl/Makefile
+++ b/sys/modules/dtrace/dtnfscl/Makefile
@@ -13,3 +13,4 @@
 .include <bsd.kmod.mk>
 
 CFLAGS+=	-include ${SYSDIR}/cddl/compat/opensolaris/sys/debug_compat.h
+CWARNFLAGS+=	${OPENZFS_CWARNFLAGS}
diff --git a/sys/modules/dtrace/dtrace/Makefile b/sys/modules/dtrace/dtrace/Makefile
--- a/sys/modules/dtrace/dtrace/Makefile
+++ b/sys/modules/dtrace/dtrace/Makefile
@@ -59,6 +59,7 @@
 
 CFLAGS+=	-include ${SYSDIR}/cddl/compat/opensolaris/sys/debug_compat.h
 CFLAGS.dtrace_asm.S+= -D_SYS_ERRNO_H_ -D_SYS_PARAM_H_ -DLOCORE
+CWARNFLAGS+=	${OPENZFS_CWARNFLAGS}
 CWARNFLAGS+=	-Wno-parentheses
 CWARNFLAGS+=	-Wno-uninitialized
 CWARNFLAGS+=	-Wno-cast-qual
diff --git a/sys/modules/dtrace/fasttrap/Makefile b/sys/modules/dtrace/fasttrap/Makefile
--- a/sys/modules/dtrace/fasttrap/Makefile
+++ b/sys/modules/dtrace/fasttrap/Makefile
@@ -26,5 +26,6 @@
 
 CFLAGS+=	-include ${SYSDIR}/cddl/compat/opensolaris/sys/debug_compat.h
 
+CWARNFLAGS+=	${OPENZFS_CWARNFLAGS}
 CWARNFLAGS+=	-Wno-cast-qual
 CWARNFLAGS+=	-Wno-unused
diff --git a/sys/modules/dtrace/fbt/Makefile b/sys/modules/dtrace/fbt/Makefile
--- a/sys/modules/dtrace/fbt/Makefile
+++ b/sys/modules/dtrace/fbt/Makefile
@@ -23,3 +23,4 @@
 .include <bsd.kmod.mk>
 
 CFLAGS+=	-include ${SYSDIR}/cddl/compat/opensolaris/sys/debug_compat.h
+CWARNFLAGS+=	${OPENZFS_CWARNFLAGS}
diff --git a/sys/modules/dtrace/profile/Makefile b/sys/modules/dtrace/profile/Makefile
--- a/sys/modules/dtrace/profile/Makefile
+++ b/sys/modules/dtrace/profile/Makefile
@@ -13,3 +13,4 @@
 .include <bsd.kmod.mk>
 
 CFLAGS+=	-include ${SYSDIR}/cddl/compat/opensolaris/sys/debug_compat.h
+CWARNFLAGS+=	${OPENZFS_CWARNFLAGS}
diff --git a/sys/modules/dtrace/prototype/Makefile b/sys/modules/dtrace/prototype/Makefile
--- a/sys/modules/dtrace/prototype/Makefile
+++ b/sys/modules/dtrace/prototype/Makefile
@@ -13,3 +13,4 @@
 .include <bsd.kmod.mk>
 
 CFLAGS+=	-include ${SYSDIR}/cddl/compat/opensolaris/sys/debug_compat.h
+CWARNFLAGS+=	${OPENZFS_CWARNFLAGS}
diff --git a/sys/modules/dtrace/sdt/Makefile b/sys/modules/dtrace/sdt/Makefile
--- a/sys/modules/dtrace/sdt/Makefile
+++ b/sys/modules/dtrace/sdt/Makefile
@@ -12,3 +12,4 @@
 
 .include <bsd.kmod.mk>
 CFLAGS+=	-include ${SYSDIR}/cddl/compat/opensolaris/sys/debug_compat.h
+CWARNFLAGS+=	${OPENZFS_CWARNFLAGS}
diff --git a/sys/modules/dtrace/systrace/Makefile b/sys/modules/dtrace/systrace/Makefile
--- a/sys/modules/dtrace/systrace/Makefile
+++ b/sys/modules/dtrace/systrace/Makefile
@@ -13,3 +13,4 @@
 .include <bsd.kmod.mk>
 
 CFLAGS+=	-include ${SYSDIR}/cddl/compat/opensolaris/sys/debug_compat.h
+CWARNFLAGS+=	${OPENZFS_CWARNFLAGS}
diff --git a/sys/modules/dtrace/systrace_freebsd32/Makefile b/sys/modules/dtrace/systrace_freebsd32/Makefile
--- a/sys/modules/dtrace/systrace_freebsd32/Makefile
+++ b/sys/modules/dtrace/systrace_freebsd32/Makefile
@@ -14,3 +14,4 @@
 .include <bsd.kmod.mk>
 
 CFLAGS+=	-include ${SYSDIR}/cddl/compat/opensolaris/sys/debug_compat.h
+CWARNFLAGS+=	${OPENZFS_CWARNFLAGS}
diff --git a/sys/modules/dtrace/systrace_linux/Makefile b/sys/modules/dtrace/systrace_linux/Makefile
--- a/sys/modules/dtrace/systrace_linux/Makefile
+++ b/sys/modules/dtrace/systrace_linux/Makefile
@@ -15,3 +15,4 @@
 .include <bsd.kmod.mk>
 
 CFLAGS+=	-include ${SYSDIR}/cddl/compat/opensolaris/sys/debug_compat.h
+CWARNFLAGS+=	${OPENZFS_CWARNFLAGS}
diff --git a/sys/modules/dtrace/systrace_linux32/Makefile b/sys/modules/dtrace/systrace_linux32/Makefile
--- a/sys/modules/dtrace/systrace_linux32/Makefile
+++ b/sys/modules/dtrace/systrace_linux32/Makefile
@@ -15,3 +15,4 @@
 .include <bsd.kmod.mk>
 
 CFLAGS+=	-include ${SYSDIR}/cddl/compat/opensolaris/sys/debug_compat.h
+CWARNFLAGS+=	${OPENZFS_CWARNFLAGS}
diff --git a/sys/modules/opensolaris/Makefile b/sys/modules/opensolaris/Makefile
--- a/sys/modules/opensolaris/Makefile
+++ b/sys/modules/opensolaris/Makefile
@@ -34,3 +34,4 @@
 .include <bsd.kmod.mk>
 
 CFLAGS+=	-include ${SYSDIR}/cddl/compat/opensolaris/sys/debug_compat.h
+CWARNFLAGS+=	${OPENZFS_CWARNFLAGS}
diff --git a/sys/modules/zfs/Makefile b/sys/modules/zfs/Makefile
--- a/sys/modules/zfs/Makefile
+++ b/sys/modules/zfs/Makefile
@@ -279,6 +279,7 @@
 
 .include <bsd.kmod.mk>
 
+CWARNFLAGS+=	${OPENZFS_CWARNFLAGS}
 
 CFLAGS.gcc+= -Wno-pointer-to-int-cast