Debian initramfs

Because of our firmware security model, we regularly use signed initramfsen such as olpcrd/olpcrd-rootskel to handle deployment and security related tasks on laptops which may be unactivated, activated but not individuated, or fully indivduated (i.e. configured for a specific user). This article describes the method we use for constructing these initramfsen.

Our initramfsen are current constructed with debian-installer on a lenny or sid. Since I happen to be working from an F-7 machine located at MIT, I built an appropriate Debian chroot by running

sudo su -
yum install debootstrap
mkdir sid-root
debootstrap --arch i386 sid sid-root/ http://debian.lcs.mit.edu/debian/

as root. NB: debootstrap requires that lots of things from /sbin and /usr/sbin be accessible on $PATH. Be careful if you're using sudo to exercise root privilege.

(If you're making your own chroot, please choose a suitable Debian mirror)

Once we've got the chroot up, we need to do some configuration inside the chroot:

chroot sid-root /bin/su -
mount -t proc proc /proc
mount -t sysfs sys /sys
mount -t devpts devpts /dev/pts
echo 'deb-src http://debian.lcs.mit.edu/debian sid main' >> /etc/apt/sources.list
apt-get update

Then we'll install the build-dependencies of the initramfs:

apt-get install git-core pbuilder yaird debhelper python-pyrex netpbm
apt-get build-dep debian-installer

Next, we'll check out the source code of the initramfs:

git clone git://dev.laptop.org/users/cscott/olpcrd
git clone git://dev.laptop.org/users/cscott/olpcrd-rootskel
cd olpcrd-rootskel
git submodule init
git submodule update

Finally, we'll fill in appropriate paths and run make:

cd ../olpcrd
$EDITOR Makefile    # patch up the paths in the first three environment variables. All we need are the paths to /root/olpcrd and /root/olpcrd-rootskel
                    # In particular, set OLPC=$(HOME), ROOTSKEL=$(HOME)/olpcrd-rootskel, and DI=$(HOME)/olpcrd
make di

If the build is successful, your new initramfs will be available at

ls build/dest/initrd.gz

To change the initramfs, modify the source files in ~/olpcrd-rootskel/olpc-src/ then re-run make di from ~/olpcrd.

Simple initramfs modification

To modify an existing initramfs it is often simplest to unpack it into a directory, modify it as suited, and pack it back up:

mkdir initramfs   # make and enter work directory to unpack the initramfs
cd initramfs
gunzip -c ../olpcrd.img | cpio -i  # unpack the image
### make your changes here ###
find . -print | cpio -H newc -o | gzip -9 >../olpcrd.img  # and repack it

Kernel modules and firmware

Due to some peculiarities of the OLPC build system, initramfsen are created and only later combined with appropriate kernel modules and firmware by the build compose-tools. This means that when generating initramfsen for testing purposes, it is necessary to install appropriate kernel modules and firmware by hand.

To install modules and firmware, simply unpack the initramfs produced by the build system, locate the modules you want to install (e.g. by extracting them from the kernel RPM used by OLPC, from an older initramfs, or from the XO you want to modify)

 FOO=/path/to/modules    

and install the modules into your initramfs.

 cp -r $FOO/lib/modules/* $FOO/lib/firmware/* .

Finally, repack the initramfs according to the instructions above.