Android SystemUpdaterSample (A/B partitioning)

Pre-requisites

• Android device with root permissions and supporting A/B partitioning.
• update_engine daemon is running (https://source.android.com/devices/tech/ota/ab#update-engine)

Development Instructions

This project is derived out of https://android.googlesource.com/platform/bootable/recovery/+/master/updater_sample/ and requires access to android hidden api's, which are otherwise not available. To build this project locally.

• Download android.jar for API 30 and replace <path to android sdk>/platforms/android-30/android.jar

For more information refer https://github.com/anggrayudi/android-hidden-api and anggrayudi/android-hidden-api#84

For the app to be installed on the target and to work. The permissions to REBOOT, ACCESS_CACHE_FILES, RECOVERY are to be provided. These permissions are typically only granted to system apps. On a rooted devices, update the privapp-permissions-platoform.xml file available at /etc/permissions

adb root
adb remount
adb pull /etc/permissions/privapp-permissions-platform.xml .

Update file with the permissions.

    <!-- com.example.android.systemupdatersample -->
    <privapp-permissions package="com.example.android.systemupdatersample">
        <permission name="android.permission.RECOVERY"/>
        <permission name="android.permission.ACCESS_CACHE_FILESYSTEM"/>
        <permission name="android.permission.REBOOT"/>
        <permission name="android.permission.DELETE_CACHE_FILES"/>
    </privapp-permissions>

Push the file back adb push privapp-permissions-platform.xml /etc/permissions/`

Push the built apk to /system/priv-app

adb push app/build/outputs/apk/app-debug.apk /system/priv-app`
adb reboot

Tested on Raspberry PI 4 running LineageOS 18.1 (Android 11). More info Refer https://konstakang.com/devices/rpi4/LineageOS18/

Intro

This app demonstrates how to use Android system updates APIs to install OTA updates. It contains a sample client for update_engine to install A/B (seamless) updates.

A/B (seamless) update is available since Android Nougat (API 24), but this sample targets the latest android.

Workflow

SystemUpdaterSample app shows list of available updates on the UI. User is allowed to select an update and apply it to the device. App shows installation progress, logs can be found in adb logcat. User can stop or reset an update. Resetting the update requests update engine to cancel any ongoing update, and revert if the update has been applied. Stopping does not revert the applied update.

Update Config file

In this sample updates are defined in JSON update config files. The structure of a config file is defined in com.example.android.systemupdatersample.UpdateConfig, example file is located at res/raw/sample.json.

In real-life update system the config files expected to be served from a server to the app, but in this sample, the config files are stored on the device. The directory can be found in logs or on the UI. In most cases it should be located at /data/user/0/com.example.android.systemupdatersample/files/configs/.

SystemUpdaterSample app downloads OTA package from url. In this sample app url is expected to point to file system, e.g. file:///data/my-sample-ota-builds-dir/ota-002.zip.

If ab_install_type is NON_STREAMING then app checks if url starts with file:// and passes url to the update_engine.

If ab_install_type is STREAMING, app downloads only the entries in need, as opposed to the entire package, to initiate a streaming update. The payload.bin entry, which takes up the majority of the space in an OTA package, will be streamed by update_engine directly. The ZIP entries in such a package need to be saved uncompressed (ZIP_STORED), so that their data can be downloaded directly with the offset and length. As payload.bin itself is already in compressed format, the size penalty is marginal.

if ab_config.force_switch_slot set true device will boot to the updated partition on next reboot; otherwise button "Switch Slot" will become active, and user can manually set updated partition as the active slot.

Config files can be generated using tools/gen_update_config.py. Running ./tools/gen_update_config.py --help shows usage of the script.

Sample App State vs UpdateEngine Status

UpdateEngine provides status for different stages of update application process. But it lacks of proper status codes when update fails.

This creates two problems:

1. If sample app is unbound from update_engine (MainActivity is paused, destroyed), app doesn't receive onStatusUpdate and onPayloadApplicationCompleted notifications. If app binds to update_engine after update is completed, only onStatusUpdate is called, but status becomes IDLE in most cases. And there is no way to know if update was successful or not.

2. This sample app demostrates suspend/resume using update_engins's cancel and applyPayload (which picks up from where it left). When cancel is called, status is set to IDLE, which doesn't allow tracking suspended state properly.

To solve these problems sample app implements its own separate update state - UpdaterState. To solve the first problem, sample app persists UpdaterState on a device. When app is resumed, it checks if UpdaterState matches the update_engine's status (as onStatusUpdate is guaranteed to be called). If they doesn't match, sample app calls applyPayload again with the same parameters, and handles update completion properly using onPayloadApplicationCompleted callback. The second problem is solved by adding PAUSED updater state.

Sample App UI

Text fields

• Current Build: - shows current active build.
• Updater state: - SystemUpdaterSample app state.
• Engine status: - last reported update_engine status.
• Engine error: - last reported payload application error.

Buttons

• Reload - reloads update configs from device storage.
• View config - shows selected update config.
• Apply - applies selected update config.
• Stop - cancel running update, calls UpdateEngine#cancel.
• Reset - reset update, calls UpdateEngine#resetStatus, can be called only when update is not running.
• Suspend - suspend running update, uses UpdateEngine#cancel.
• Resume - resumes suspended update, uses UpdateEngine#applyPayload.
• Switch Slot - if ab_config.force_switch_slot config set true, this button will be enabled after payload is applied, to switch A/B slot on next reboot.

Sending HTTP headers from UpdateEngine

Sometimes OTA package server might require some HTTP headers to be present, e.g. Authorization header to contain valid auth token. While performing streaming update, UpdateEngine allows passing on certain HTTP headers; as of writing this sample app, these headers are Authorization and User-Agent.

android.os.UpdateEngine#applyPayload contains information on which HTTP headers are supported.

Used update_engine APIs

UpdateEngine#bind

Binds given callbacks to update_engine. When update_engine successfully initialized, it's guaranteed to invoke callback onStatusUpdate.

UpdateEngine#applyPayload

Start an update attempt to download an apply the provided payload_url if no other update is running. The extra key_value_pair_headers will be included when fetching the payload.

key_value_pair_headers argument also accepts properties other than HTTP Headers. List of allowed properties can be found in system/update_engine/common/constants.cc.

UpdateEngine#cancel

Cancel the ongoing update. The update could be running or suspended, but it can't be canceled after it was done.

UpdateEngine#resetStatus

Reset the already applied update back to an idle state. This method can only be called when no update attempt is going on, and it will reset the status back to idle, deleting the currently applied update if any.

Callback: onStatusUpdate

Called whenever the value of status or progress changes. For progress values changes, this method will be called only if it changes significantly. At this time of writing this doc, delta for progress is 0.005.

onStatusUpdate is always called when app binds to update_engine, except when update_engine fails to initialize.

Callback: onPayloadApplicationComplete

Called whenever an update attempt is completed or failed.

Running on a device

The commands are expected to be run from $ANDROID_BUILD_TOP and for demo purpose only.

Without the privileged system permissions

1. Compile the app mmma -j bootable/recovery/updater_sample.
2. Install the app to the device using adb install <APK_PATH>.
3. Change permissions on /data/ota_package/ to 0777 on the device.
4. Set SELinux mode to permissive. See instructions below.
5. Add update config files; look above at Update Config file.
6. Push OTA packages to the device.
7. Run the sample app.

With the privileged system permissions

To run sample app as a privileged system app, it needs to be installed in /system/priv-app/. This directory is expected to be read-only, unless explicitly remounted.

The recommended way to run the app is to build and install it as a privileged system app, so it's granted the required permissions to access update_engine service as well as OTA package files. Detailed steps are as follows:

1. Prepare to build
2. Add the module (SystemUpdaterSample) to the PRODUCT_PACKAGES list for the lunch target. e.g. add a line containing PRODUCT_PACKAGES += SystemUpdaterSample to device/google/marlin/device-common.mk.
3. Whitelist the sample app
   • Add

    <privapp-permissions package="com.example.android.systemupdatersample">
        <permission name="android.permission.ACCESS_CACHE_FILESYSTEM"/>
    </privapp-permissions>
   

   to frameworks/base/data/etc/privapp-permissions-platform.xml
4. Add privileged: true to SystemUpdaterSample building rule.
5. Build sample app make -j SystemUpdaterSample.
6. Build Android make -j
7. Flash the device
8. Add update config files; look above at ## Update Config file; adb root might be required.
9. Push OTA packages to the device if there is no server to stream packages from; changing of SELinux labels of OTA packages directory might be required chcon -R u:object_r:ota_package_file:s0 /data/my-sample-ota-builds-dir
10. Run the sample app.

Development

• Create a UI with list of configs, current version, control buttons, progress bar and log viewer
• Add PayloadSpec and PayloadSpecs for working with update zip file
• Add UpdateConfig for working with json config files
• Add applying non-streaming update
• Prepare streaming update (partially downloading package)
• Add applying streaming update
• Add stop/reset the update
• Add demo for passing HTTP headers to UpdateEngine#applyPayload
• Package compatibility check
• Deferred switch slot demo
• Add UpdateManager; extract update logic from MainActivity
• Add Sample app update state (separate from update_engine status)
• Add smart update completion detection using onStatusUpdate
• Add pause/resume demo
• Verify system partition checksum for package

Running tests

The commands are expected to be run from $ANDROID_BUILD_TOP.

1. Build make -j SystemUpdaterSample and make -j SystemUpdaterSampleTests.
2. Install app adb install $OUT/system/app/SystemUpdaterSample/SystemUpdaterSample.apk
3. Install tests adb install $OUT/testcases/SystemUpdaterSampleTests/arm64/SystemUpdaterSampleTests.apk
4. Run tests adb shell am instrument -w com.example.android.systemupdatersample.tests/android.support.test.runner.AndroidJUnitRunner
5. Run a test file

   adb shell am instrument \
     -w -e class com.example.android.systemupdatersample.UpdateManagerTest#applyUpdate_appliesPayloadToUpdateEngine \
     com.example.android.systemupdatersample.tests/android.support.test.runner.AndroidJUnitRunner
   

Accessing android.os.UpdateEngine API

android.os.UpdateEngine APIs are marked as @SystemApi, meaning only system apps can access them.

Getting read/write access to /data/ota_package/

Access to cache filesystem is granted only to system apps.

Setting SELinux mode to permissive (0)

local$ adb root
local$ adb shell
android# setenforce 0
android# getenforce

License

SystemUpdaterSample app is released under Apache License 2.0.