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Extracting a Recovery Kernel from a Recovery Image

If you need to generate a recovery image, use "mod_image_for_recovery.sh --image <image_file>".


<Outdated instructions needed for a few special case CR-48 Users>

Intro

If you find yourself in the need of an officially signed Recovery Kernel, but you only have an officially signed Recovery Image: you're in luck!  You've come to the right place.

What are Recovery Images and Recovery Kernels?
  • A Recovery Image is an image that can be placed onto a USB disk that can boot a particular Chrome OS Notebook and reinstall the default software onto the computer, putting it into a known good state.  Instructions for getting a Recovery Image are usually provided by the manufacturer of a Chrome OS Notebook.
  • A Recovery Kernel is a small part of the the Recovery Image that can be extracted, then used for installing a custom build of the OS.  Sometimes a manufacturer will provide a separate download of the Recovery Kernel as a convenience.
What does it mean for a Recovery Kernel to be officially signed?  It means that it was built by someone (like your device manufacturer) with access to your device's secret recovery key.  That means that your device will allow booting from this Recovery Kernel.  NOTE: every hardware product has a different secret recovery key, so you need to obtain an official Recovery Kernel that is specific to your product.

...you might have read about the Recovery Kernel in such places as the Poking around your Chrome OS Notebook page or the Chromium OS Developer Guide.

Get an officially-signed Recovery Image

These steps are going to be different for every piece of hardware.  However, just to be illustrative, here are the steps for the Cr-48 Chrome Notebook:

mkdir -p ~/trunk/recovery

curl -L 'https://dl.google.com/dl/chromeos/recovery/latest_mario_beta_channel' > ~/trunk/recovery/recovery_image.zip
TMPDIR=`mktemp -d`
unzip ~/trunk/recovery/recovery_image.zip -d ${TMPDIR}
RECOVERY_IMAGE=`ls ${TMPDIR}/*.bin`

Future steps will assume that you've put the recovery image into the RECOVERY_IMAGE variable (as the above instructions do).

Extract the Recovery Kernel

The easiest way to do this is if you've already followed the instructions in the Chromium OS Developer Guide and are running from within the chroot.  In that case, you'll have already the cgpt tool.  That means you can follow the generic instructions.  In that case, the instructions are:

KERNA_START=`cgpt show -b ${RECOVERY_IMAGE} | grep KERN-A | awk '{print $1}'`
KERNA_SIZE=`cgpt show -b ${RECOVERY_IMAGE} | grep KERN-A | awk '{print $2}'`

mkdir -p ~/trunk/recovery/
RECOVERY_KERNEL=~/trunk/recovery/recovery_kernel.bin

dd of=${RECOVERY_KERNEL} if=${RECOVERY_IMAGE} \
   bs=512 count=${KERNA_SIZE} skip=${KERNA_START}

If you aren't running from the chroot, most of the commands above are still valid.  You'll just have to figure out your own KERNA_START and KERNA_SIZE.  At the moment, this document doesn't have the instructions for that (sorry!).  However with current images, you could actually just do:

KERNA_START=4096
KERNA_SIZE=32768

...and be done with it.









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