Custom Debian Root Filesystem Image

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This page details how a Debian root file system for the odroid can be created. Following the steps will not give you exactly the same results as in the prepared images because packages are periodically updated.

Debian is Debian. If you have any Debian system, it can be updated or upgraded -- without having to go through your own bootstrap process. It can be difficult to go through a bootstrap for a number of reasons including:

  • Crossing processor architectures is always an issue. If you are building for x86, no problem. Most utilities work. But, most people are not faced with building custom x86 distros
  • Debian has been around a long time. Tools and approaches are always changing. Googling is helpful, but you get a lot of information that is dated.
  • If you are bringing up a new platform, you don't have a known-good starting point. For example, to do a network install, you need to have a root file system of some sort and you need a working network connection.
  • I did not invent any of this stuff, I just put it together and tested it specifically for use on an odroid target.

    The bootstrap approach I show was tested on a clean Debian 6 machine, a VMWare virtual machine. You can find your own copy here: Debian6t VM VMWare Player is free as well: VMWare Player.

    Setup sudo on the host

    You might want to install the sudo package and add yourself to the sudoers list if you have not already done it. This section is optional, but if you object to using sudo, there are commands you need to run as root.

    As root (on the host):

    apt-get install sudo
    # You need to add yourself to the "sudoers list", and how exactly that is done depends on the distribution.
    # This hack works for Debian Wheezy
    # as root, add normal user (named user) to adm group
    adduser user adm
    # configure the adm group to have no restrictions
    echo "%adm ALL=(ALL) ALL" >> /etc/sudoers
    # sudo will complain if it cannot resolve the hostname
    echo "  $(hostname )" >> /etc/hosts

    You need to logout/login after making the change to the sudoers list in order for it to take effect

    Install Prerequisites

    sudo apt-get update
    sudo apt-get install binfmt-support qemu qemu-user-static debootstrap

    We must have the qemu-arm-static binary to bring up the system and qemu version 0.12.5 is known to fail. As of this writing, qemu 1.4.0 is current and is known to work. Interim versions might also work.

    If it helps, you might be able to use one of the binaries I built, posted here:

    md5sum -c debian-armhf.tgz.md5sum
    # if the checksum is OK, then extract the files:
    tar -xvzf debian-armhf.tgz

    The files are:

    launch-wheezy-no-bridge  A script to launch the root file system on the host using qemu-system-arm
    qemu-arm-static-i386     Statically built, current version of qemu-arm for 32-bit Linux host
    qemu-arm-static-x86_64   Statically built, current version of qemu-arm for 64-bit Linux host 
    zImage-vexpress-a9       A kernel built for a cortex-a9 machine, vexpress-a9 that is a good stand-in for 

    The -i386 version will probably run on and x86 Linux host, but only because it is statically linked. Just rename it to qemu-arm-static. The x86_64 will not run on a 32-bit host.

    Check the "Building QEMU" section of this tutorial if you need to build an updated qemu-arm-static binary: Building QEMU

    Use debootstrap to Create Debian Root File System

    To get started, we need a root file system that has all the components needed to boot Debian. Once we have that, we can configure the system and add more packages. This technique uses qemu-system-arm, so qemu (quick emulator) needs to be installed.

    # Go to a convenient directory
    cd $HOME
    # make a place to work
    mkdir debian-bootstrap
    # and go there
    cd debian-bootstrap
    # Create the root file system, this can take some time...
    sudo debootstrap --verbose --arch armhf --foreign wheezy $PWD/armhf
    # Now the directory $HOME/debian-bootstrap/armhf holds a wheezy root file system, built from binary packages

    We are going to chroot into the the armhf directory and we need qemu-arm-static available to be able to run the bootstrap script while hiding the rest of the host files from the process. qemu-arm-static knows how to run armhf binaries on our x86 machine -- cool when you think about it. Quite a nice piece of work.

    Find qemu-arm-static:

    $(which qemu-arm-static)


    Make a local copy, for clarity: (copy from your downloads folder, if you plan to use the pre-built binary)

    sudo cp /usr/local/bin/qemu-arm-static .

    copy qemu-arm-static into what will be the chroot jail:

    sudo cp qemu-arm-static armhf/usr/bin

    Run debian second stage: (this is the place where a downlevel qemu-arm-static will fail)

    LC_ALL=C LANGUAGE=C LANG=C sudo chroot armhf /debootstrap/debootstrap --second-stage

    Trigger post-install scripts:

    LC_ALL=C LANGUAGE=C LANG=C sudo chroot armhf dpkg --configure -a

    Fixup Virgin Root File System

    Edit inittab to put a getty on the serial console -- and also to boot to the root shell since I don't know the default username/password. (maybe there is a better, more general way to do this)

    Make sure to notice: if you want to boot with qemu, then you must change ttySAC1 below to ttyAMA0. If you do not do this, you will not get an initial console and the problem will not be obvious...

    As root, in armhf/etc/inittab, change:

    #T0:23:respawn:/sbin/getty -L ttyS0 9600 vt100


    # To use with qemu, in next line, replace ttySAC1 with ttyAMA0
    T0:23:respawn:/sbin/getty -L ttySAC1 115200 vt102

    As root, in armhf/etc/inittab, change:




    Drop in Kernel Modules

    Kernel modules must always match the kernel in use. If you don't care about bring up the network, you can come back to this step later.

    See the main page for a list of kernel building tutorials: [1]

    You need to either build a custom kernel and the kernel modules (drivers) that go with it or you need to extract the kernel modules from an existing root file system that you know has a set matching the kernel you want to use. Kernel modules would be stored at /lib/modules/3.0.68 for example, if you have a set for 3.0.68.

    [TODO: add a link to the HardKernel kernel update repository ]

    Add Device Nodes

    Drop in some device nodes to let us run the root file system with qemu or real hardware

    sudo mknod  armhf/dev/ttyAMA0 c 204 64
    sudo mknod  armhf/dev/ttySAC0 c 204 64

    Create a Root File System Tarball

    It is convenient to make a tarball of the root file system.

    cd $HOME/debian-bootstrap/armhf
    sudo tar -cvzf ../rootfs-wheezy-custom.tgz .
    cd ..

    Mount the SD-Card on your host and follow the tutorial: Updating from Root File System Images to update your SD-Card.

    Create a File System Image to use with QEMU (optional)

    Convert the root file system to a filesystem image. If you host has enough memory, you could increase the 500M to some larger value.

    qemu-img create rootfs-wheezy.ext4 500M
    sudo mkfs.ext4 rootfs-wheezy.ext4

    And the second step:

    # We are going to need a mount point
    mkdir mnt
    sudo mount -o loop rootfs-wheezy.ext4 mnt
    sudo cp -a armhf/* mnt
    sudo umount mnt
    # Now rootfs-wheezy.ext4 has the same contents as the bootstrapped Debian

    Boot with QEMU

    TBD show how to use the qemu example to boot the RFS

    Boot the Target

    The system will come up in a root shell. We should change the passwords. If you want to match the Hardkernel Ubuntu image, set the root password to root and the user password to password.

    passwd root
    # answer with the root password you will remember -- consider making a note
    # Add a user to login with
    adduser user

    As root, in armhf/etc/inittab, change back to a password-protected shell, and make sure the serial console name and terminal type are correct:

    T0:23:respawn:/sbin/getty -L ttyAMA0 9600 vt100


    T0:23:respawn:/sbin/getty -L ttySAC1 115200 vt102

    You probably want to change the hostname too. For the debootstrap build, the hostname will be the same as the host you built it on. To set the hostname to odroidu2, do this: (as root)

    echo odroidu2 > /etc/hostname

    No need to keep the binary packages (about 50MiB) so clean them up like this: (as root)

    apt-get clean

    And reboot

    Now, we can (as root) set the apt sources, bring up the network and update. You should choose a nearby mirror (respository)

    echo "deb wheezy main" > /etc/apt/sources.list
    dhclient eth0
    apt-get update
    The system is complete.