diff --git a/share/mk/bsd.sys.mk b/share/mk/bsd.sys.mk
index 5f2007906f5..04e7b9de07b 100644
--- a/share/mk/bsd.sys.mk
+++ b/share/mk/bsd.sys.mk
@@ -109,6 +109,13 @@ CWARNFLAGS+=	-Werror
 CWARNFLAGS+=	-Wno-format
 .endif # NO_WFORMAT || NO_WFORMAT.${COMPILER_TYPE}
 
+# How to handle FreeBSD custom printf format specifiers.
+.if ${COMPILER_TYPE} == "clang" && ${COMPILER_VERSION} >= 30600
+FORMAT_EXTENSIONS=	-D__printf__=__freebsd_kprintf__
+.else
+FORMAT_EXTENSIONS=	-fformat-extensions
+.endif
+
 .if defined(IGNORE_PRAGMA)
 CWARNFLAGS+=	-Wno-unknown-pragmas
 .endif # IGNORE_PRAGMA
diff --git a/sys/boot/efi/libefi/Makefile b/sys/boot/efi/libefi/Makefile
index 3edeb22ea5a..55abffc3079 100644
--- a/sys/boot/efi/libefi/Makefile
+++ b/sys/boot/efi/libefi/Makefile
@@ -20,8 +20,7 @@ CFLAGS+= -I${.CURDIR}/../../common
 
 CFLAGS+= -DNO_PCI
 
- 
-# Suppress warning from clang for FreeBSD %b and %D formats
-CFLAGS+= -fformat-extensions
+# Handle FreeBSD specific %b and %D printf format specifiers
+CFLAGS+= ${FORMAT_EXTENSIONS}
 
 .include <bsd.lib.mk>
diff --git a/sys/boot/i386/libi386/Makefile b/sys/boot/i386/libi386/Makefile
index 0f360e99ab8..3e61a1b9686 100644
--- a/sys/boot/i386/libi386/Makefile
+++ b/sys/boot/i386/libi386/Makefile
@@ -55,8 +55,8 @@ CFLAGS+=	-I${.CURDIR}/../../common -I${.CURDIR}/../common \
 # the location of libstand
 CFLAGS+=	-I${.CURDIR}/../../../../lib/libstand/
 
-# Suppress warning from clang for FreeBSD %b and %D formats
-CFLAGS+= -fformat-extensions
+# Handle FreeBSD specific %b and %D printf format specifiers
+CFLAGS+= ${FORMAT_EXTENSIONS}
 
 .if ${MACHINE_CPUARCH} == "amd64"
 CLEANFILES+=	machine
diff --git a/sys/boot/pc98/libpc98/Makefile b/sys/boot/pc98/libpc98/Makefile
index e612205de46..6b11de9f699 100644
--- a/sys/boot/pc98/libpc98/Makefile
+++ b/sys/boot/pc98/libpc98/Makefile
@@ -44,7 +44,7 @@ CFLAGS+=	-I${.CURDIR}/../../common \
 # the location of libstand
 CFLAGS+=	-I${.CURDIR}/../../../../lib/libstand/
 
-# Suppress warning from clang for FreeBSD %b and %D formats
-CFLAGS+= -fformat-extensions
+# Handle FreeBSD specific %b and %D printf format specifiers
+CFLAGS+= ${FORMAT_EXTENSIONS}
 
 .include <bsd.lib.mk>
diff --git a/sys/conf/kern.mk b/sys/conf/kern.mk
index b964151437e..bfc447379a2 100644
--- a/sys/conf/kern.mk
+++ b/sys/conf/kern.mk
@@ -58,6 +58,8 @@ CWARNEXTRA?=	-Wno-uninitialized
 # to be disabled.  WARNING: format checking is disabled in this case.
 .if ${MK_FORMAT_EXTENSIONS} == "no"
 FORMAT_EXTENSIONS=	-Wno-format
+.elif ${COMPILER_TYPE} == "clang" && ${COMPILER_VERSION} >= 30600
+FORMAT_EXTENSIONS=	-D__printf__=__freebsd_kprintf__
 .else
 FORMAT_EXTENSIONS=	-fformat-extensions
 .endif