Microdroid Payload

Payload disk is a composite disk referencing host APEXes and an APK so that microdroid reads activates APEXes and executes a binary within the APK.

Format

Payload disk has 1 + N(number of APEX/APK payloads) partitions.

The first partition is a Microdroid Signature partition which describes other partitions. And APEXes and an APK are following as separate partitions.

For now, the order of partitions are important.

• partition 1: Microdroid Signature
• partition 2 ~ n: APEX payloads
• partition n + 1: APK payload

It's subject to change in the future, though.

Microdroid Signature

Microdroid Signature contains the signatures of the payloads so that the payloads are verified inside the Guest OS.

Microdroid Signature is composed of header and body.

offset	size	description
0	4	Header. unsigned int32: body length(L) in big endian
4	L	Body. A protobuf message. schema

Payload Partitions

At the end of each payload partition the size of the original payload file (APEX or APK) is stored in 4-byte big endian.

For example, the following code shows how to get the original size of host apex file when the apex is read in microdroid as /dev/block/vdc2,

int fd = open("/dev/block/vdc2", O_RDONLY | O_BINARY | O_CLOEXEC);
uint32_t size;
lseek(fd, -sizeof(size), SEEK_END);
read(fd, &size, sizeof(size));
size = betoh32(size);

How to Create

mk_payload

mk_payload creates a payload image.

$ cat payload_config.json
{
  "system_apexes": [
    "com.android.adbd",
  ],
  "apexes": [
    {
      "name": "com.my.hello",
      "path": "hello.apex"
    }
  ],
  "apk": {
    "name": "com.my.world",
    "path": "/path/to/world.apk"
  }
}
$ adb push payload_config.json hello.apex /data/local/tmp/
$ adb shell 'cd /data/local/tmp; /apex/com.android.virt/bin/mk_payload payload_config.json payload.img
$ adb shell ls /data/local/tmp/*.img
payload.img
payload-footer.img
payload-header.img
payload-signature.img
payload.img.0          # fillers
payload.img.1
...

In the future, VirtualizationService will handle this.