Modifying the root image
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This article describes two methods for extending and modifying an existing tablet JFFS2 root image.
Background
The root filesystem (rootfs) of the tablets 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 device. Additionally, a pristine version of the rootfs can be obtained from the Nokia official firmware site.
There are two ways to mount the JFFS2 image:
- Have a block device emulate a Memory Technology Device (MTD) via block2mtd.
- 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 FIASCO image. After downloading the FIASCO image, run this on it to unpack it (this will give you all of the individual parts of the image):
sudo ./flasher-3.0 --unpack -F <FIASCO image>
Mounting the JFFS2 image
These commands will be executed with root privileges on the host machine and not on the device itself.
Block device emulating an MTD device
You must have the following kernel modules compiled and installed:
- 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 block2mtd echo "/dev/loop0" > /sys/module/block2mtd/parameters/block2mtd modprobe jffs2 mount -t jffs2 /tmp/mtdblock0 /media/jffs2
To unmount and cleanup:
umount /media/jffs2 modprobe -r block2mtd modprobe -r mtdblock losetup -d /dev/loop0
Or use the automated shell script.
Kernel memory emulating an MTD device
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
Or use the automated shell script.
Archiving and extracting the rootfs 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, which is why we use tar.
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 the image
Now the image is successfully archived and copied. The working directory $HOME/myRootImage can now be used for adding, changing or modifying packages.
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 on the tablet
When the new tarball is created, Using flasher and the reference root filesystem for creating JFFS2 image from the myNewRootImage.tar.gz.
Use flasher to install the image on the tablet.
./flasher-3.0 -r rootfs.jffs2 --f -R
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
Create a shell script (mount_jffs2.sh) from the following:
#!/bin/sh
JFFSIMG=$1 # jffs image
MP="/media/jffs2" # mount point
MTDBLOCK="/tmp/mtdblock0" # MTD device file
UMNT=""
echo "$0" | grep unmount_ >/dev/null 2>&1
[ $? -eq 0 ] && UMNT=1
if [ $# -gt 1 -a x"$2"x = x"unmount"x ]; then
UMNT=1
fi
if [ x"${UMNT}"x = x""x ]; then
if [ ! -b ${MTDBLOCK} ] ; then
mknod ${MTDBLOCK} b 31 0 || exit 1
fi
modprobe mtdblock
modprobe mtdram total_size=65536 erase_size=256
modprobe jffs2
dd if=${JFFSIMG} of=${MTDBLOCK}
[ ! -d ${MP} ] && mkdir -p ${MP}
mount -t jffs2 ${MTDBLOCK} ${MP}
else
umount ${MP}
if [ $? -ne 0 ]; then
echo "Cannot unmount JFFS2 at $MP" && exit 1
fi
modprobe -r jffs2
modprobe -r mtdram
modprobe -r mtdblock
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:
./mount_jffs2.sh rootfs.jffs2 unmount