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/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 \
--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 currently are @SystemApi, thus available only to privileged apps.

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

Debuggable microdroid

Debugging features

Microdroid supports following debugging features:

VM log
console output
kernel output
logcat output
ramdump
crashdump
adb
gdb

Enabling debugging features

There's two ways to enable the debugging features:

Option 1) Running microdroid on AVF debug policy configured device

microdroid can be started with debugging features by debug policies from the host. Host bootloader may provide debug policies to host OS's device tree for VMs. Host bootloader MUST NOT provide debug policies for locked devices for security reasons.

For protected VM, such device tree will be available in microdroid. microdroid can check which debuging features is enabled.

Here are list of device tree properties for debugging features.

/avf/guest/common/log: <1> to enable kernel log and logcat. Ignored otherwise.
/avf/guest/common/ramdump: <1> to enable ramdump. Ignored otherwise.
/avf/guest/microdroid/adb: <1> to enable adb. Ignored otherwise.

Option 2) Lauching microdroid with debug level.

microdroid can be started with debugging features. To do so, first, delete $TEST_ROOT/instance.img; this is because changing debug settings requires a new instance. Then add the --debug=full flag to the /apex/com.android.virt/bin/vm run-app command. This will enable all debugging features.

ADB

If adb connection is enabled, launch following command.

vm_shell

Done. Now you are logged into Microdroid. Have fun!

Once you have an adb connection with vm_shell, localhost:8000 will be the serial of microdroid.

Debugging the payload on microdroid

Like a normal adb device, you can debug native processes using lldbclient.py script, either by running a new process, or attaching to an existing process. Use vm_shell tool above, and then run lldbclient.py.

adb -s localhost:8000 shell 'mount -o remount,exec /data'
development/scripts/lldbclient.py -s localhost:8000 --chroot . --user '' \
    (-p PID | -n NAME | -r ...)

Note: We need to pass --chroot . to skip verifying device, because microdroid doesn‘t match with the host’s lunch target. We need to also pass --user '' as there is no su binary in microdroid.