AOSP – Creating a System Service

In the previous post, we built a native daemon in Android native layer. We wrote a simple application to communicate with the daemon using a socket (UDS/UDP/TCP). Most of the services provided by Android using the Android Framewok layer

If for example, the camera application wants to get the current GPS location, it connects to the Location service using the binder. The service load the hardware module using the HAL and returns the position.

To create a system service, we need the following components:

  • AIDL service interface
  • Application to host the service(s)
  • Client library for untrusted applications (from the play store) to use the service
  • Simple client application for testing our service

AIDL and the Client Library

We start with the AIDL file that will generate the stub and proxy. We also build a Manager class for the client application.

Create a folder framework in ~/aosp/device/generic/goldfish

Add a general Android.mk file to scan the child directories

framework/Android.mk

Build the following directory structure:

ISampService.aidl

SampManager.java

Android.mk

To let the untrusted application access the library we need to add a permission file:

com.android.sampservice.xml

The purpose of the permission file is to tell the system where to find the required library so if untrusted application wants to use the library, it should add <uses-library  …. /> tag to AndroidManifest.xml file with the required name

The generated components are: jar file to access the service and xml file for permission

The Service Implementation

To build the service we need to implement the interface and write an application to host it. We can use the system_server but its a bad practice to merge our code with google’s code.

Create a folder app in ~/aosp/device/generic/goldfish

Add a general Android.mk file to scan the child directories

app/Android.mk

Build the following directory structure:

The service implementation

We need to derive from the Stub class and simply implement the interface

SampServiceApp/src/com/android/sampappservice/ISampServiceImpl.java

Now we create an application to host the service. We need to create an object from our implementation and add it to the Service Manager (the binder context)

SampServiceApp.java

To build a system application we need to update the AndroidManifest.xml file.

  • To make it run as system user add android:sharedUserId=”android.uid.system”
  • To make it run automatically on startup add android:persistent=”true”

The following Android.mk build the package (apk)

Note the platform certificate we need to sign the application (LOCAL_CERTIFICATE := platform , LOCAL_PRIVILEGED_MODULE := true)

Build and run the emulator – You will see that the service is not running and in the log you will see the following error:

We need to add SE Linux permissions:

SE Linux

Declare the new type my_service in public/service.te:

In service_contexts file label the service

Add rule: (in public/servicemanager.te)

In android 8.1 google added a new policy language  called the Common Intermediate Language (CIL). To add a system service you need:

in file system/sepolicy/private/compat/26.0/26.0.cil

in file /system/sepolicy/prebuild/api/26.0/nonplat_sepolicy.cil

and add my_service to the line

 

Build and run the emulator again, you will see the service running

 

Create a client Application

First we need to create a jar file for the application developer. The project in Android Studio depends on that library

In AOSP root, run the following command:

You will find the file classes-full-debug.jar in out/target/common/obj/JAVA_LIBRARIES/com.android.sampservice_intermediates/

Create a new Android Studio application, add module and select import jar

import the above jar file and click finish

In the application module settings add module dependency to the jar file module and select compile only

 

To use the library from the device we need to add <uses-library> tag to the manifest file:

 

Now just create an object from the SampManager class and use it:

 

Last thing we need to add SE Linux rule to allow untrusted application. Add the following rule to the untrusted_app.te file

 

Run the application on the emulator and use logcat to see the results

 

 

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1 thought on “AOSP – Creating a System Service

  1. Hello It is very helpful to me,
    Thank you for the post. I am trying to investigate also how can we use Binder to communicate with currently systemServices which already available like ActivityManager, WindowManager and how to influences it also.
    eg : Tell ActivityManager to kill and app or Pause a app..
    I know that AOSP allow me to do that somehow but it will be nice to learn from your experiences .
    Thank you,

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