Modifying the root image

Revision as of 16:37, 24 May 2008

This HOWTO describes 2 methods for extending and modifying an existing JFFS2 root image of 770.

1 Background
2 Getting the JFFS2 Image
3 Mounting JFFS2 Image
- 3.1 Block Device Emulating a MTD
  - 3.1.1 Linux 2.4.x Kernel
  - 3.1.2 Linux 2.6.x Kernel
- 3.2 Kernel Memory Emulating a MTD
4 Archiving and Extracting the Root Image
5 Modifying the Copy of Image
6 Installing into Nokia 770
7 Final remarks
8 Shell script to mount/unmount JFFS2 using Block device Emulating MTD
9 Shell script to mount/unmount JFFS2 using Kernel Memory Emulating MTD
10 Maybe one could write a script removing unwanted stuff from that image (sample video, song), changing sudoers, gainroot script, adding sshd, ...

Background

The root filesystem (rootfs) of the Nokia 770 is stored in a Journal Flash File System version 2 (JFFS2) format. It resides on one of the partitions on the flash chip in the Nokia 770. Additionally, a pristine version of the rootfs can be obtained from the Nokia official firmware.

There are two ways to mount the JFFS2 image:

Have a block device emulate a Memory Technology Device (MTD) via blkmtd (v2.4.x Linux kernels) or block2mtd (v2.6.x Linux kernels)
Have kernel memory emulate a MTD via mtdram

Getting the JFFS2 Image

First of all, we need to get the rootfs.jffs2 from the official Nokia binary:

$ mkdir SE2005_image
$ cd SE2005_image
$ sudo ./flasher --unpack  -F /path_to_file/Nokia_770_SE2005_3_2005_51_13.bin

Found image 2nd (length 8576)
Found image secondary (length 79360)
Found image xloader (length 13824)
Found image initfs (length 1581824)
Found image kernel (length 1481856)
Found image rootfs (length 58851328)
Unpacking 2nd image to file '2nd.bin'...
Unpacking X-Loader image to file 'xloader.bin'...
Unpacking secondary image to file 'secondary.bin'...
Unpacking kernel image to file 'zImage'...
Unpacking initfs image to file 'initfs.jffs2'...
Unpacking rootfs image to file 'rootfs.jffs2'...

Mounting JFFS2 Image

It is assume that all these commands will be executed on the development/hacking host and not on the Nokia 770 itself. Also assumed is all these commands will be executed with root privileges (i.e. sudo /bin/sh).

Block Device Emulating a MTD

Linux 2.4.x Kernel

If you are using a Linux 2.4.x kernel, you must have the following kernel modules compiled:

CONFIG_MTD (mtdcore)
CONFIG_MTD_PARITIONS (mtdpart)
CONFIG_MTD_MTDRAM (mtdram)
CONFIG_MTD_BLKMTD (blkmtd)
CONFIG_BLK_DEV_LOOP (loop)

Linux 2.6.x Kernel

If you are using a Linux 2.6.x kernel, you must have the following kernel modules compiled:

CONFIG_MTD (mtdcore)
CONFIG_MTD_PARITIONS (mtdpart)
CONFIG_MTD_MTDRAM (mtdram)
CONFIG_MTD_BLOCK2MTD (block2mtd)
CONFIG_BLK_DEV_LOOP (loop)

I suggest you do not make these modules statically linked into the kernel. It is more practical to be able to unload these modules when not needed or when you want to start over from scratch.

You will use the loopback device (/dev/loop[0-15]) to simulate a block device whose contents are from the JFFS2 image. To mount the JFFS2 image, you perform the following steps:

mknod /tmp/mtdblock0 b 31 0
modprobe loop
losetup /dev/loop0 rootfs.jffs2
modprobe mtdblock
  modprobe blkmtd device=/dev/loop0  ### for Linux 2.4.x
    or
  modprobe block2mtd  ### for Linux 2.6.x
  echo "/dev/loop0" > /sys/module/block2mtd/parameters/block2mtd  ### Linux 2.6.x
modprobe jffs2
mount -t jffs2 /tmp/mtdblock0 /media/jffs2

To unmount and cleanup:

umount /media/jffs2
  modprobe -r blkmtd  ### Linux 2.4.x
    or
  modprobe -r block2mtd ## Linux 2.6.x
modprobe -r mtdblock
losetup -d /dev/loop0

You can use the following Shell script to automate the process.

Kernel Memory Emulating a MTD

(Idea originally from Michael Mlivoncic) To mount the JFFS2 image, you perform the following steps:

mknod /tmp/mtdblock0 b 31 0
modprobe mtdblock
modprobe mtdram total_size=65536 erase_size=256
modprobe jffs2
dd if=/pathtoimage/rootfs.jffs2 of=/tmp/mtdblock0
mkdir /media/jffs2
mount -t jffs2 /tmp/mtdblock0 /media/jffs2

To unmount and cleanup:

umount /media/jffs2
modprobe -r jffs2
modprobe -r mtdram
modprobe -r mtdblock

You can use the following Shell script to automate the process.

Archiving and Extracting the Root Image

The image is now accessible under /media/jffs2. Copy the whole image to another directory. This new directory will be used for modifying the image. Extending the currently mounted JFFS2 image is not suggested. Using cp for copying the image to a working directory won't work due to special files in /media/jffs2/dev, for example. This is the reason we use tar. proceed as above, i.e.:

$ cd /media/jffs2
$ tar cvzf /my_path/myRootImage.tar.gz .
$ cd $HOME
$ mkdir myRootImage
$ cd myRootImage
$ tar xvpzf /my_path/myRootImage.tar.gz

Modifying the Copy of Image

Now the image is successfully archived and copied. The working directory $HOME/myRootImage can now be used for adding packages etc, for example.

$ cd $HOME/myRootImage
$ dpkg -x $HOME/arm_debs/mypackage_arm.deb .

Now we create a new tarball from the working directory of the image.

$ cd $HOME/myRootImage
$ tar cvzf $HOME/myNewRootImage.tar.gz .

Installing into Nokia 770

When the new tarball is created, refer to HOWTO: Using flasher and the reference root filesystem for creating JFFS2 image from the myNewRootImage.tar.gz.

Use flasher to install the image to Nokia 770.

$ ./flasher --flash-only rootfs -F image.bin -f

$ ./flasher --rootfs rootfs.jffs2 --flash-only rootfs --flash

$ ./flasher --enable-rd-mode --reboot

The flash-only parameter is used just to make sure ;-)

Final remarks

Tonight, I had to re-flash the first time, as I screwed up a script in /etc/init.d/. For the future, I will try to make a full backup of my productive rootfs, to avoid starting all over again

Shell script to mount/unmount JFFS2 using Block device Emulating MTD

Create a shell script (mount_jffs2.sh) from the following:

#!/bin/sh
JFFSIMG=$1 # jffs image
LOOP="/dev/loop1" # loop device
MP="/media/jffs2" # mount point
MTDBLOCK="/tmp/mtdblock0" # MTD device file
KVER="2.6"
BLKMTD="block2mtd"
UMNT=""

echo "$0" | grep unmount_ >/dev/null 2>&1
[ $? -eq 0 ] && UMNT=1
if [ $# -gt 1 -a x"$2"x = x"unmount"x ]; then
  UMNT=1
fi

uname -r | egrep '^2\.6' >/dev/null 2>&1
if [ $? -ne 0 ]; then
  KVER="2.4"
  BLKMTD=blkmtd
fi

if [ x"${UMNT}"x = x""x ]; then
  if [ ! -b ${MTDBLOCK} ] ; then
    mknod ${MTDBLOCK} b 31 0 || exit 1
  fi
  lsmod | grep loop >/dev/null 2>&1
  if [ $? -ne 0 [; then
    modprobe loop
    [ $? -ne 0 ] && echo "loopback loading failed" && exit 1
    sleep 1
  fi
  losetup ${LOOP} ${JFFSIMG} || exit 1
  sleep 1
  modprobe mtdblock
  if [ x"${KVER}"x = x"2.4"x [; then
    modprobe ${BLKMTD} device=${LOOP} || exit 1
  else
    modprobe ${BLKMTD} || exit 1
    echo "${LOOP}" > /sys/module/block2mtd/parameters/block2mtd
  fi
  sleep 1
  modprobe jffs2
  [ ! -d ${MP} ] && mkdir -p ${MP}
  mount -t jffs2 ${MTDBLOCK} ${MP} || exit 1
else
  umount ${MP}
  if [ $? -ne 0 ]; then
    echo "Cannot unmount JFFS2 at $MP" && exit 1
  fi
  modprobe -r jffs2
  modprobe -r ${BLKMTD}
  modprobe -r mtdblock
  sleep 1
  losetup -d ${LOOP}
fi

Make sure you chmod a+x mount_jffs2.sh to make the shell script executable.

Usage: $ ./mount_jffs2.sh rootfs.jffs2

You can also use this script to unmount and unload the non-utilized kernel modules and loopback reference: $ ./mount_jffs2.sh rootfs.jffs2 unmount

Shell script to mount/unmount JFFS2 using Kernel Memory Emulating MTD