Chromium OS Installer

Files in this folder will be compiled by chromeos-base/chromeos-installer package into few programs, including:

• chromeos-install: A shell script for manual installation from USB.
• chromeos-recovery: The main command for recovery shim to install image.
• chromeos-postinst: The script executed by auto update or recovery shim after image installation is completed, being the symbolic link of /postinst.
• cros_installer: A program meant to be the backend of above commands (but it currently only supports post-install).

Full Installation

When installing manually from USB or from recovery shim, the procedure is:

1. chromeos-install is invoked by either manually or chromeos-recovery.
2. The script will re-create partition table on target storage device.
3. The script will clone all partitions (stateful, rootfs, kernel, OEM, etc...).
4. Start post-install (see below) from /postinst of new rootfs.

Auto Update Installation

Auto update (via update_engine) is slightly different. After the payload is downloaded, update_engine will try to apply the payload (may be full or partial), then invoke the post-install from new rootfs.

Post Install

After the new image is written to disk storage, there may be several steps to activate the new partition. This is done by cros_installer as:

1. Rebuild ‘verity’ information. To reduce download size, the payload from auto update won't include verity hash data because that can be re-constructed. In this step the post-install program will read, verify, and regenerate hash data for the rootfs partition.
2. Update GPT entries to set proper boot priorities on the new kernel partition.
3. For legacy systems, update EFI or syslinux entries.
4. Run a per-board post install script /usr/sbin/board-postinst if it exists. The script is usually installed by chromeos-base/chromeos-bsp-$BOARD or chromeos-base/chromeos-firmware-$BOARD in board overlays.
5. Apply firmware update by running /usr/sbin/chromeos-firmwareupdate --mode=autoupdate if the signing signature (/root/.force_update_firmware) is available. The special signature is needed to prevent unexpected firmware updates from self-built USB images.
6. Apply Cr50 firmware updates if available (error will be ignored).
7. Any other misc housekeeping work. Consult ChromeosChrootPostinst for further details.

To make sure the new kernel and rootfs is bootable, the GPT boot priority may be set with few “try” attempts so the system will revert to the old partition when running out of “tries”.

If the new kernel and rootfs boots, the update_engine will wait for 60 seconds and then:

1. Invoke chromeos-setgoodkernel, to update partition records and mark current kernel as “good” for future reboots.
2. Invoke chromeos-setgoodfirmware to mark active firmware as “good” as well. At the end of chromeos-setgoodfirmware, it will also invoke /usr/sbin/board-setgoodfirmware for board specific configuration if available. The board-setgoodfirmware is usually provided by chromeos-base/chromeos-firmware-$BOARD package.

In summary, if you want to add some board specific task:

• To run on every boot, do it in /usr/sbin/board-setgoodfirmware.
• To run only after each AU or recovery, do it in /usr/sbin/board-postinst.