109 lines
3.1 KiB
Markdown
109 lines
3.1 KiB
Markdown
# System Services
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There are many system services
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- Activity Manager
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- Notification Manager
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- Location Manager
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- …
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###### `onPause()`
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All the communication is facilitated by `Binder`.
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### System services and APIs
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##### Power Management
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- Lock the device in *awake* power mode
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- Only allow this app to unlock this operation
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```java
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public class MainActivity extends Activity {
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private PowerManager.WakeLock wakeLock;
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@Override
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protected void onCreate(Bundle savedInstanceState) {
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super.onCreate(savedInstanceState);
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PowerManager pm =
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(PowerManager) getSystemService(Context.POWER_SERVICE);
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wakeLock = pm.newWakeLock(PowerManager.PARTIAL_WAKE_LOCK, "My Tag");
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wakeLock.acquire();
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}
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@Override
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protected void onDestroy() {
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super.onDestroy();
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wakeLock.release();
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}
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}
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```
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- Connect to the service (Power Manager)
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- Create a unique token
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- This unlocks the phone, can be passed to other components
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This is what actually happens:
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#### Binder Objects and Tokens
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###### Binder Object
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- An object that can be accessed through the binder framework
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- When connected to a service, we're given a binder object back which we use as a proxy for accessing the service.
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- Implements the `IBinder` interface
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- A unique identity maintained across processes
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- **Allocated by the binder driver**
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- Cannot be duplicated. Binder objects maintain a unique ID even when parcelled
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- Is a 32-bit ID maintained by the kernel
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Process A creates a binder object <- references memory directly
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> (really asks the Binder driver to allocate it a Binder object for the local object)
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>
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> Passes it to process B <- referenced by handle (ID)
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>
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> - Can then be passed to process C the same way
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###### Capability-Based security model
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Processes are granted access to a particular resource by giving them a *capability* in the form of the binder object
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- Binder object as **token**
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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.
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- The only way to communicate with a Binder object is to be given a reference to it
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### Service Manager
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A single context manager that maintains references to (system service) Binder objects
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- Implemented as a `ServiceManager`
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- Also hosts many system services within its process
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- As the system user. Not root, but a privileged user
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- A Binder instance with a known binder handle (`ID=0`)
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- Knows about other remote services
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- The first to be registered with the Binder
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- Only *trusted* system services allowed to register
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- System, radio, media
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- Binder submits a service name and its Binder handle to the `ServiceManager` via IPC
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- Client retrieves remote service Binder handle with service name
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- Client communicates with remote service
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When wanting to make use of the power manager, I ask the service manager for the binder handle for that particular service.
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This means the kernel does not reason about security; however, the service manager does.
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