Chrome OS Updater

Update Chrome OS on regular PCs

NOTICE: The main repository became unstable once again. Please, install using the installer found in the latest release instead of the one given here.

Index

• Background
• Installation and Usage
  • Requirements
  • Installation
  • Usage
• How to revert back if something goes wrong
• References

Background

Chrome OS has two root partitions namely ROOT-A and ROOT-B (partition numbered 3 and 5 respectively). On a regular installation, ROOT-A partition is actually the root partition you boot from, and ROOT-B partition is only used when you update your Chromebook from About Chrome OS page. When you update for the first time, the update is installed at ROOT-B partition. So, until you install another update, this partition is the one you boot from. And when another update is installed, it is installed at ROOT-A, and again, you boot from ROOT-A partition (you get the idea). This is also know as A/B update. I followed similar yet somewhat modified approach to develop omaha_cros_update.sh script which can be used to update regular PCs to the latest version of Chrome OS.

There are two types of updates: delta update and full update (similar to Android updates). Delta update is only available for devices which are updated from a (not-very-old) previous version of Chrome OS as it contains only the changes (delta, in science, always seem to denote change). Full update contains the whole update and can be used by Chrome OS of any version. For some obvious reasons, currently only the full updates can be applied through this script.

Previously on a separate project, I've described how to update Chrome OS from another Linux distro, which was slightly easier to develop than use since installing a Linux distro or even using Live OS can be very annoying only for the purpose of updating Chrome OS. That's why I've been working on this project for some time now. The benefit of this project is that you can update Chrome OS directly from Chrome OS itself (without needing another Linux distro). Moreover, this method of updating Chrome OS is very similar to the native process, thus, the security of your OS (though UEFI itself is a culprit) will be much better provided you're only using Chrome OS (ie. without dual or multibooting).

Consequently, it requires another partition namely ROOT-B as well as two optional KERN-A and KERN-B partitions (the last two partitions are there for kernel storage, but can be empty in our case). In a regular installation, the partitions are already there (remember to increase size of both ROOT-A and ROOT-B to at least 4 GB), but if you're multibooting with other OSs following my previous instructions, you'll need to create another partition (with EXT2 partition type and 5 GB size) namely ROOT-B. After you apply update from ROOT-A, ROOT-B will be used as the root partition and vice versa. The benefit of using this approach is that if, for some reason, update doesn't work as expected, you can still be able to use Chrome OS by simply selecting the other partition in the GRUB menu or changing the default partition in the GRUB configuration file.

Installation and Usage

Requirements

There is no requirment for default/regular installation (ie. wiping the whole drive to install Chrome OS). You will need a configuration file if

• (even one or more of) the partition names (also known as partition labels or PARTLABEL) are not standard. The standard partition names (for our purpose) are KERN-A, KERN-B, ROOT-A, ROOT-B, EFI-SYSTEM (KERN-* partitions are optional). If (even one or more of) the partitions of the drive doesn't have the standard labels, you will need this configuration file.
• you're crazy enough to install two or more Chrome OS in the same drive
• you're not going to use swtpm module even if your kernel supports it (you only need TPM=false)

If the configuration file does not exist, the updater will fall back to the standard labels and will issue a warning.

Creating the configuration file

The configuration format is as follows:

ROOTA='<ROOT-A UUID, lowercase>'
ROOTB='<ROOT-B UUID, lowercase>'
EFI='<EFI-SYSTEM UUID, lowercase>'
TPM=true/false

Each value is optional. If a value doesn't exist, default will be used (if available). For swtpm: if you set TPM=false, the swtpm installation will be ignored during the update; otherwise, the updater will automatically determine if swtpm is need.

You can get UUID for a disk ID using the following command:

/sbin/blkid -s UUID -o value /dev/<disk-id>

Notice: partition UUID (goes by the name PARTUUID) and UUID are not the same.

Save this configuration file as cros_update.conf to /usr/local (full URI is /usr/local/cros_update.conf).

Installation

Open crosh shell by pressing Ctrl + Alt + T and the type shell. Now copy and paste the command below (don't use sudo!):

curl -Ls https://github.com/MuntashirAkon/chrome_os_updater/releases/latest/download/installer.sh | bash

or, an easy to remember command:

curl -Ls https://bit.ly/cros_updater | bash

Usage

Run omaha_cros_update.sh -h for more usage.

Check for updates

sudo omaha_cros_update.sh -c

Update Chrome OS

sudo -E omaha_cros_update.sh

Required files will be downloaded automatically. An active Internet connection is required throughout the process.

Switch update channel

It is possible to switch update channel. To switch update channel, you need to export CUSTOM_RELEASE_TRACK environment variable in the crosh terminal before running the updater. Here's an example:

export CUSTOM_RELEASE_TRACK=dev-channel

CUSTOM_RELEASE_TRACK can take one of the following values:

• dev-channel
• canary-channel
• beta-channel
• stable-channel

Custom download directory

It is possible to set custom download directory for users having a small stateful partition. To do this, you need to insert an external drive (e.g. USB pen drive) with a writable file system (e.g. ext*, *fat) which has at least 4 GB free space. By default, any supported file systems are mounted using their associated label at /media/removable directory. So, you need to export CROS_DOWNLOAD_ROOT before running the updater like this:

export CROS_DOWNLOAD_ROOT=/media/removable/<your external drive>

Replace /media/removable/<your external drive> with the mount point of your external drive.

Debugging

Debugging may help sort out the error occurred during the update process. To enable debugging, you need to export CROS_DEBUG=y before running the updater like this:

export CROS_DEBUG=y

The downloaded files will be kept to save your bandwidth!

How to revert back if something goes wrong

There's always a chance that something will go wrong. In this case, don't panic, but you'll need to use Ubuntu Live (or other Linux distro or even Windows or macOS). Boot into it and follow the steps below.

1. Mount the EFI-SYSTEM partition (with vfat type and write permission) using Gparted (available on Ubuntu Live), Disks (comes built-in with Ubuntu), mount or other tools. For Windows and macOS users, this partition should already be mounted.
2. If you're using EFI mode, follow steps 3-5. If you're using legacy mode, follow steps 6-8. Otherwise follow all the steps and don't reboot in step 5.
3. Navigate to efi/boot/grub.cfg inside the disk and open for editing (on Linux, you'll need root permission).
4. In the 9th line, you'll find something like this:

   set default=$defaultA
   

   Here $defaultA might be $defaultB for you. If it is set to $defaultB make it to $defaultA or vice versa. Do the same in the 7th line as well (especially if you've done a clean install).
5. Save it and reboot.
6. Navigate to syslinux/default.cfg inside the disk and open for editing (on Linux, you'll need root permission).
7. You'll find a single line like this:

   DEFAULT chromeos-hd.A
   

   Here chromeos-hd.A might be chromeos-hd.B for you. If it is set to chromeos-hd.B make it to chromeos-hd.A or vice versa.
8. Save and reboot

This should restore Chrome OS to the previously installed version.

References

1. https://www.chromium.org/chromium-os/chromiumos-design-docs/disk-format
2. https://chromium.googlesource.com/chromiumos/platform/crosutils/+/refs/heads/master/build_library/disk_layout_v2.json
3. https://chromium.googlesource.com/chromiumos/platform/crosutils/+/refs/heads/master/build_library/create_legacy_bootloader_templates.sh
4. https://chromium.googlesource.com/chromiumos/third_party/grub2/+/refs/heads/master/grub-core/commands/gptpriority.c
5. https://android.googlesource.com/platform/system/update_engine/+/refs/heads/master
6. https://source.android.com/devices/tech/ota/ab/
7. https://www.chromium.org/chromium-os/developer-information-for-chrome-os-devices/cr-48-chrome-notebook-developer-information/how-to-boot-ubuntu-on-a-cr-48