diff --git a/sys/boot/efi/boot1/Makefile b/sys/boot/efi/boot1/Makefile
index c1b3714a5271..3642314eac4d 100644
--- a/sys/boot/efi/boot1/Makefile
+++ b/sys/boot/efi/boot1/Makefile
@@ -79,8 +79,15 @@ boot1.o: ${.CURDIR}/../../common/ufsread.c
 # created by generate-fat.sh
 
 .include "${.CURDIR}/Makefile.fat"
+BOOT1_MAXSIZE?=	131072
 
 boot1.efifat: boot1.efi
+	@set -- `ls -l boot1.efi`; \
+	x=$$(($$5-${BOOT1_MAXSIZE})); \
+	if [ $$x -ge 0 ]; then \
+	    echo "boot1 $$x bytes too large; regenerate FAT templates?" >&2 ;\
+	    exit 1; \
+	fi
 	echo ${.OBJDIR}
 	uudecode ${.CURDIR}/fat-${MACHINE}.tmpl.bz2.uu
 	mv fat-${MACHINE}.tmpl.bz2 ${.TARGET}.bz2
diff --git a/sys/boot/efi/boot1/generate-fat.sh b/sys/boot/efi/boot1/generate-fat.sh
index 7992b55086b3..826dd694d0a1 100755
--- a/sys/boot/efi/boot1/generate-fat.sh
+++ b/sys/boot/efi/boot1/generate-fat.sh
@@ -55,9 +55,20 @@ BOOT1_OFFSET=$(hd $OUTPUT_FILE | grep 'Boot1 START' | cut -f 1 -d ' ')
 # Convert to number of blocks
 BOOT1_OFFSET=$(echo 0x$BOOT1_OFFSET | awk '{printf("%x\n",$1/512);}')
 
+# Record maximum boot1 size in bytes
+case $BOOT1_SIZE in
+*k)
+	BOOT1_MAXSIZE=$(expr ${BOOT1_SIZE%k} '*' 1024)
+	;;
+*)
+	BOOT1_MAXSIZE=$BOOT1_SIZE
+	;;
+esac
+
 echo '# This file autogenerated by generate-fat.sh - DO NOT EDIT' > Makefile.fat
 echo '# $FreeBSD$' >> Makefile.fat
 echo "BOOT1_OFFSET=0x$BOOT1_OFFSET" >> Makefile.fat
+echo "BOOT1_MAXSIZE=$BOOT1_MAXSIZE" >> Makefile.fat
 
 bzip2 $OUTPUT_FILE
 echo 'FAT template boot filesystem created by generate-fat.sh' > $OUTPUT_FILE.bz2.uu