adding support for diskfiles

diff --git a/overlay-diskfile b/overlay-diskfile
new file mode 100755 (executable)
index 0000000..2cefcd7
--- /dev/null
+++ b/overlay-diskfile
@@ -0,0 +1,45 @@
+#!/bin/sh
+#
+# Helper script for using a diskfile for the LOWER or UPPER+WORK
+# filesystems.
+#
+# Example use a whole diskfile filesystem as LOWER:
+# LOWER=!overlay-diskfile mnt/ disk.img
+#
+# Example use a diskfile partition 2 as LOWER:
+# LOWER=!overlay-diskfile mnt/ disk.img 2
+#
+# Example use a subtree of a diskfile partition 2 as LOWER:
+# LOWER=!overlay-diskfile mnt/some/path disk.img 2
+#
+# Example use a while diskfile as UPPER and WORK.
+# The diskfile is mounted on "mnt".
+# UPPER=!overlay-diskfile mnt/some/upper/path disk.img 2
+# WORK= mnt/some/work/path
+
+# Helper function to determine the mount offset for the partition, if any
+partoffset() {
+    local X
+    if [ -z "$2" ] ; then
+       echo 0
+    else
+       fdisk -lo Start "$1" | grep -A$2 '^ Start' | \
+           sed '${s|^|512*|;b};d' | bc -l
+    fi
+}
+
+# Only do something when invoked by overlay-boot
+if [ "$ACTION" = "overlay-boot" ] ; then
+    DISKFILE="$2"
+    # .. and a diskfile is given
+    if [ -n "$DISKFILE" ] ; then
+       MOUNTPOINT="$(realpath "${1%/*}")"
+       PARTITIONINDEX="$3"
+       # .. and it's not mounted yet (for this subhost)
+       if ! grep -qE "^[^ ]* $MOUNTPOINT" /proc/mounts ; then
+           OFFSET=$(partoffset "$DISKFILE" "$PARTITIONINDEX")
+           mount -ooffset=$OFFSET "$DISKFILE" "$MOUNTPOINT" || exit 1
+       fi
+    fi
+fi
+echo "${1#/}"