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Microdroid

Microdroid is a (very) lightweight version of Android that is intended to run on on-device virtual machines. It is built from the same source code as the regular Android, but it is much smaller; no system server, no HALs, no GUI, etc. It is intended to host headless & native workloads only.

Prerequisites

Any 64-bit target (either x86_64 or arm64) is supported. 32-bit target is not supported.

The only remaining requirement is that com.android.virt APEX has to be pre-installed. To do this, add the following line in your product makefile.

$(call inherit-product, packages/modules/Virtualization/build/apex/product_packages.mk)

Build the target product after adding the line, and flash it. This step needs to be done only once for the target.

If you are using Pixel 6 and beyond or Cuttlefish (aosp_cf_x86_64_phone) adding above line is not necessary as it's already done.

Building and installing microdroid

Microdroid is part of the com.android.virt APEX. To build it and install to the device:

banchan com.android.virt aosp_arm64
UNBUNDLED_BUILD_SDKS_FROM_SOURCE=true m apps_only dist
adb install out/dist/com.android.virt.apex
adb reboot

If your target is x86_64 (e.g. aosp_cf_x86_64_phone), replace aosp_arm64 with aosp_x86_64.

Building an app

A vm payload is a shared library file that gets executed in microdroid. It is packaged as part of an Android application. The library should have an entry point AVmPayload_main as shown below:

extern "C" int AVmPayload_main() {
  printf("Hello Microdroid!\n");
}

Then build it as a shared library:

cc_library_shared {
  name: "MyMicrodroidPayload",
  srcs: ["**/*.cpp"],
  sdk_version: "current",
}

Embed the shared library file in an APK:

android_app {
  name: "MyApp",
  srcs: ["**/*.java"],
  jni_libs: ["MyMicrodroidPayload"],
  use_embedded_native_libs: true,
  sdk_version: "current",
}

Finally, you build the APK.

TARGET_BUILD_APPS=MyApp m apps_only dist

Running the VM payload on microdroid

First of all, install the APK to the target device.

adb install out/dist/MyApp.apk

There are two ways start a VM and run the payload in it.

  • By manually invoking the vm tool via adb shell.
  • Calling APIs programmatically in the Java app.

Using vm tool

Execute the following commands to launch a VM. The VM will boot to microdroid and then automatically execute your payload (the shared library MyMicrodroidPayload.so).

TEST_ROOT=/data/local/tmp/virt
adb shell /apex/com.android.virt/bin/vm run-app \
--log $TEST_ROOT/log.txt \
--console $TEST_ROOT/console.txt \
PATH_TO_YOUR_APP \
$TEST_ROOT/MyApp.apk.idsig \
$TEST_ROOT/instance.img \
--instance-id-file $TEST_ROOT/instance_id \
--payload-binary-name MyMicrodroidPayload.so

ALL_CAPs below are placeholders. They need to be replaced with correct values:

  • PACKAGE_NAME_OF_YOUR_APP: package name of your app (e.g. com.acme.app).
  • PATH_TO_YOUR_APP: path to the installed APK on the device. Can be obtained via the following command. 
    adb shell pm path PACKAGE_NAME_OF_YOUR_APP
    It shall report a cryptic path similar to /data/app/~~OgZq==/com.acme.app-HudMahQ==/base.apk.

The console output from the VM is stored to $TEST_ROOT/console.txt and logcat is stored to $TEST_ROOT/log.txt file for debugging purpose. If you omit --log or --console option, the console output will be emitted to the current console and the logcat logs are sent to the main logcat in Android.

Stopping the VM can be done by pressing Ctrl+C.

Using the APIs

Use the Android Virtualization Framework Java APIs in your app to create a microdroid VM and run payload in it. The APIs are currently @SystemApi, and only available to preinstalled apps.

If you are looking for an example usage of the APIs, you may refer to the demo app.

Running Microdroid with vendor image

With using vm tool, execute the following commands to launch a VM with vendor partition.

adb shell /apex/com.android.virt/bin/vm run-microdroid \
--vendor $VENDOR_IMAGE

Verification of vendor image

Since vendor image of Microdroid is not part of com.android.virt APEX, the verification process of vendor partition is different from others.

Vendor image uses its hashtree digest for the verifying its data, generated by add_hashtree_footer in avbtool. The value could be seen with following command:

avbtool info_image --image $VENDOR_IMAGE

Fixed path in VM for vendor hashtree digest is written in fstab.microdroid. During first stage init of VM, dm-verity is set up based on vendor hashtree digest by reading fstab.microdroid.

For non-pVM, virtualizationmanager creates DTBO containing vendor hashtree digest, and passes to the VM via crosvm option. The vendor hashtree digest is obtained by virtualizationmanager from the host Android DT under /avf/reference/, which may be populated by the bootloader.

For pVM, VM reference DT included in pvmfw config data is additionally used for validating vendor hashtree digest. Bootloader should append vendor hashtree digest into VM reference DT based on fstab.microdroid. Vendor hashtree digest could be appended as property into descriptors in host Android's vendor image by Makefile when Microdroid vendor image module is defined, so that a bootloader can extract the value and populate into VM reference DT.

Debugging Microdroid

Refer to Debugging protected VMs.