notes/docs/lectures/mdp/09_system_services.md

System Services

There are many system services

• Activity Manager
• Notification Manager
• Location Manager
• …

onPause()

All the communication is facilitated by Binder.

System services and APIs

Power Management

• Lock the device in awake power mode
• Only allow this app to unlock this operation

public class MainActivity extends Activity {
    
    private PowerManager.WakeLock wakeLock;
    
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        
        PowerManager pm =
            (PowerManager) getSystemService(Context.POWER_SERVICE);
        wakeLock = pm.newWakeLock(PowerManager.PARTIAL_WAKE_LOCK, "My Tag");
        wakeLock.acquire();
    }
    
    @Override
    protected void onDestroy() {
        super.onDestroy();
        wakeLock.release();
    }
}

• Connect to the service (Power Manager)
• Create a unique token
  • This unlocks the phone, can be passed to other components

This is what actually happens:

Binder Objects and Tokens

Binder Object

• An object that can be accessed through the binder framework
  • When connected to a service, we're given a binder object back which we use as a proxy for accessing the service.
• Implements the IBinder interface
• A unique identity maintained across processes
  • Allocated by the binder driver
    • Cannot be duplicated. Binder objects maintain a unique ID even when parcelled
  • Is a 32-bit ID maintained by the kernel

Process A creates a binder object <- references memory directly

(really asks the Binder driver to allocate it a Binder object for the local object)

Passes it to process B <- referenced by handle (ID)

• Can then be passed to process C the same way

Capability-Based security model

Processes are granted access to a particular resource by giving them a capability in the form of the binder object

• Binder object as token

The possession of a token grants the owning process full access to the Binder object, enabling it to perform Binder transactions on the target object.

• The only way to communicate with a Binder object is to be given a reference to it

Service Manager

A single context manager that maintains references to (system service) Binder objects

• Implemented as a ServiceManager
  • Also hosts many system services within its process
  • As the system user. Not root, but a privileged user
• A Binder instance with a known binder handle (ID=0)
• Knows about other remote services
  • The first to be registered with the Binder
  • Only trusted system services allowed to register
    • System, radio, media
• Binder submits a service name and its Binder handle to the ServiceManager via IPC
  • Client retrieves remote service Binder handle with service name
  • Client communicates with remote service

When wanting to make use of the power manager, I ask the service manager for the binder handle for that particular service.

This means the kernel does not reason about security; however, the service manager does.