os_install_service

os_install_service

This directory contains the OS install D-Bus service. The service is used to install the OS to disk.

The service exposes a single method, StartOsInstall. This method takes no parameters; the service chooses an appropriate disk to install to without any user input. Updates are provided with the OsInstallStatusChanged signal. Currently there is no percentage-complete report, the signal just indicates if the install succeeded or failed. The signal also includes the install log so that error details can be presented.

This service (when included in the OS image) only runs when the OS is live booted from an installer image. This is checked in the upstart script by running is_running_from_installer, which compares the sizes of the root-A and root-B partitions. If they are the same size, then the OS is considered installed, whereas if the sizes are different then the OS is running from an installer image with a stub root-B partition. Note that this check would break if the USB layout is ever changed to include a full-size root-B partition.

Automatic install

To support the creation of mass deployable images install can be started without human intervention. If a specific UEFI variable, ChromiumOSAutoInstall-2a6f93c9-29ea-46bf-b618-271b63baacf3, is present, the service will begin installing when it starts, and request that the OS shut down when install succeeds.

Testing

To test the service manually:

dbus-monitor --system sender=org.chromium.OsInstallService

sudo -u chronos dbus-send --print-reply --system \
    --dest=org.chromium.OsInstallService \
    /org/chromium/OsInstallService \
    org.chromium.OsInstallService.StartOsInstall

To test autoinstall in a VM:

Use virt-firmware to create an OVMF_VARS.fd with the right UEFI variable:

cp /usr/share/OVMF/OVMF_VARS.fd .
echo '{
    "variables": [
        {
            "name": "ChromiumOSAutoInstall",
            "guid": "8be4df61-93ca-11d2-aa0d-00e098032b8c",
            "attr": 7,
            "data": "00"
        }
    ]
}' > var.json
virt-fw-vars -i OVMF_VARS.fd -o OVMF_VARS.fd --set-json var.json

Then run qemu with that OVMF_FARS.fd:

runvm --uefi --ovmf-vars OVMF_VARS.fd <...>
> qemu-system-x86_64 \
>   ...
>   -drive if=pflash,format=raw,readonly=on,file=/.../OVMF_CODE.fd \
>   -drive if=pflash,format=raw,readonly=on,file=OVMF_VARS.fd

Installation should start immediately.

Security

This service is run as root due to all the privileged operations needed for OS installation. The Upstart service runs os_install_service in minijail to restrict some syscalls, and there's an SELinux policy to further restrict what the service can do.