set_lsb_release.sh: Make it mount rootfs r/w only if necessary.

scripts/image_signing/common.sh

 #!/bin/bash
 
 # Copyright (c) 2010 The Chromium OS Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 # Determine script directory
 SCRIPT_DIR=$(dirname $0)
 PROG=$(basename $0)
 GPT=cgpt
 
 # List of Temporary files and mount points.
 TEMP_FILE_LIST=$(mktemp)
 TEMP_DIR_LIST=$(mktemp)
 
 # Read GPT table to find the starting location of a specific partition.
 # Args: DEVICE PARTNUM
 # Returns: offset (in sectors) of partition PARTNUM
 partoffset() {
   sudo $GPT show -b -i $2 $1
 }
 
 # Read GPT table to find the size of a specific partition.
 # Args: DEVICE PARTNUM
 # Returns: size (in sectors) of partition PARTNUM
 partsize() {
   sudo $GPT show -s -i $2 $1
 }
 
+# Mount a partition read-only from an image into a local directory
+# Args: IMAGE PARTNUM MOUNTDIRECTORY
+mount_image_partition_ro() {
+  local image=$1
+  local partnum=$2
+  local mount_dir=$3
+  local offset=$(partoffset "$image" "$partnum")
+  sudo mount -o loop,ro,offset=$((offset * 512)) "$image" "$mount_dir"
+}
+
 # Mount a partition from an image into a local directory
 # Args: IMAGE PARTNUM MOUNTDIRECTORY
 mount_image_partition() {
   local image=$1
   local partnum=$2
   local mount_dir=$3
   local offset=$(partoffset "$image" "$partnum")
   sudo mount -o loop,offset=$((offset * 512)) "$image" "$mount_dir"
 }
 
@@ skipping 46 matching lines @@
       sudo umount -d $i 2>/dev/null
       rm -rf $i
     fi
   done
   set -e
   rm -rf $TEMP_DIR_LIST $TEMP_FILE_LIST
 }
 
 trap "cleanup_temps_and_mounts" EXIT