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Hacking an image file + Mounting devices with labels - LINUX

Evan Davies

Jun 12, 2013 • 4 min read

So I’ve been modifying the image file for almost a month and I would like to share with you how to customize an already built image (mostly stables ones).
Since I don’t have Windows, I will share how to do all these procedures with Linux, so here we go:

So, create a working directory and, once you have the image file, you can extract its content with unzip:

Code:: mkdir work && cd work unzip /path/to/your/update.img

Remove the certificates and manifest files as the signature procedure will create new ones later on:

Code:: rm META-INF/CERT.* && rm META-INF/*.MANIFEST.MF

I added tons of stuff to system/opt to have amule, a torrent client and some commands that I normally use on my desktop computer.
I also removed usbmount to create my own version of this script to use labels as mountpoints and also removed all the content of /media (if you leave empty directories like in my case, create a .empty file in that location). I wrote this version of usbmount which is simpler and does what I want:

Code:

#!/bin/sh

MOUNT_BASE=”/media”
MOUNTOPTS=”dirsync,nodev,noatime,nodiratime”
FILESYSTEMS=’ hfsplus vfat ext2 ext3 ext4 exfat ntfs xfs ‘

case $1 in
“add”)

# Get the device information
DEVINFO=$(/sbin/blkid -p $DEVNAME)
FSTYPE=$(echo “$DEVINFO” | sed ‘s/.*[[:blank:]]TYPE=”$[^”]*$”.*/\1/g; s/[[:blank:]]*//g;’)
USAGE=$(echo “$DEVINFO” | sed ‘s/.*[[:blank:]]USAGE=”$[^”]*$”.*/\1/g; s/[[:blank:]]*//g;’)
LABEL=$(echo “$DEVINFO” | sed ‘s/.*[[:blank:]]LABEL=”$[^”]*$”.*/\1/g; s/[[:blank:]]*//g;’)

# Check if it’s already mounted
if [ “`grep “^$DEVNAME” /proc/mounts`” != “” ]; then
logger info “$DEVNAME already mounted”
exit 1
fi

# Check if it’s actually a block device containing a filesystem
if ! echo $USAGE | egrep -q “(filesystem|disklabel)”; then
logger info “$DEVNAME does not contain a filesystem or disklabel”
exit 1
fi

# Check if it has a valid filesystem
if [ “`echo $FILESYSTEMS | grep ” ${FSTYPE} “`” = “” ]; then
logger info “$DEVNAME contains an unsuported filesystem: [${FSTYPE}]”
exit 1
fi

# If it has no label, use the device name as mountpoint
if [ -z “${LABEL}” ]; then
logger info “$DEVNAME has no label, using the device name as mountpoint”
LABEL=`basename ${DEVNAME}`
fi

# Create the mountpoint and mount the device in that location
mkdir -p ${MOUNT_BASE}/${LABEL}
logger info “executing command: mount -t${FSTYPE} “$DEVNAME” ${MOUNT_BASE}/${LABEL}”
mount -t${FSTYPE} -o${MOUNTOPTS} “$DEVNAME” ${MOUNT_BASE}/${LABEL}

;;
“remove”)

# If it is mounted, umount it
if [ “`grep “^$DEVNAME ${MOUNT_BASE}/” /proc/mounts`” != “” ]; then
logger info “executing command: umount -l ${MOUNT_BASE}/${LABEL}”
umount -l ${MOUNT_BASE}/${LABEL}
fi

;;
*)
logger err “Wrong option [$1]”
;;
esac

That way, directories in /media will be auto created when a new device will be added with the label of the device or, if it has no label, it will use the device name (so instead of /media/usb0 I have /media/Memory and if no labels where set I would have /media/sda1 instead of the devices moving around).

The /usr directory in the image is just a squash filesystem, so the best way to make modifications in it is to “unsquash it”, make the modifications/replacements and then re-create the filesystem again (you need squashfs-tools for this), this is how you can replace the usbmount file:

Code:: unsquashfs system/usr.sqsh -d usr

Now you can make all the modifications you want to that filesystem, in my case I replaced the usbmount script. If you want to, you can do it with this:

Code:

cat << _EOF_ > usr/share/usbmount/usbmount
#!/bin/sh

MOUNT_BASE=”/media”
MOUNTOPTS=”dirsync,nodev,noatime,nodiratime”
FILESYSTEMS=’ hfsplus vfat ext2 ext3 ext4 exfat ntfs xfs ‘

case $1 in
“add”)

# Check if it’s already mounted
if [ “`grep “^$DEVNAME” /proc/mounts`” != “” ]; then
logger info “$DEVNAME already mounted”
exit 1
fi

# Check if it has a valid filesystem
if [ “`echo $FILESYSTEMS | grep ” ${FSTYPE} “`” = “” ]; then
logger info “$DEVNAME contains an unsuported filesystem: [${FSTYPE}]”
exit 1
fi

# If it has no label, use the device name as mountpoint
if [ -z “${LABEL}” ]; then
logger info “$DEVNAME has no label, using the device name as mountpoint”
LABEL=`basename ${DEVNAME}`
fi

;;
“remove”)

;;
*)
logger err “Wrong option [$1]”
;;
esac
_EOF_

By doing this, you can remove some extra stuff in system/etc, like usbmount:

Code: Select all: rm -rf system/etc/usbmount*

Now, we have to recreate the squashfs:

Code:: mksquashfs usr system/usr.sqsh -comp gzip -noappend -all-root

Remember to remove the usr directory created when “unsquashing”:

Code:: rm -rf usr

If you added any new files (as I did in system/opt, I added irssi, amule, transmission, gnu-sed, vim, a su binary and disabled root ssh access in dropbear -I created a new user editing my hand system/etc/{passwd,shadow} and modified system/etc/init.d/S50dropbear adding — -w to the dropbear call-) you have to modify the updater-script, it is in

Code:: META-INF/com/google/android/updater-script

You’ll understand how that file is structured. Just take in note that special file permisions (with the set_perm instruction), like setuid are not 4 digits but 5, so permisions like 0644 are ok, but 4755 (for su) would be 04755 (I guess this is a some sort of bug in the android recovery system).

Finally you have to rebuild the image, to do this first compress the new image file with zip:

Code:: zip -r update-unsigned.img *

And then you have to sign that update-unsigned.img

Code:: java -Xmx1024m -jar signapk.jar -w testkey.x509.pem testkey.pk8 update-unsigned.img update.img