Android Interprocess Communication(三)
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Android Interprocess Communication(三)
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Android Interprocess Communication(三)
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摘要: 学习Android Binder的入门文章,浅显易懂,您值得阅读。 -----by Thorsten Schreiber

6. Example IPC Message Flow

6.1. Testing Environment

We used two testing apps running on a virtual device. The first one was an modified example1, which was originally designed to demonstrate an in-app service communication. This is handled quite differently and copes with intercomponent communication, not interprocess communication. It is called SimpleMathService and offers methods for simple mathematical operations.

Testing Environment

The second application was self-programmed and simply uses the remote service provided by the first app.

The applications of the testing environment are illustrated as an extended UML class diagram in Figure 6.1. The UML diagramm is extended with a view of the processes, which instantiated nested Java objects. Their classes and objects (shown in yellow) are compiled from the Android AIDL language. The service app has two components, an activity component called Main and a service component called SimpleMathService. The SimpleMathService class holds an extended anonymous inner class implementing the business logic of the service. The inner class is derived from ISimpleMathService.Stub which is generated by Android SDK from the ISimpleMathService.aidl definition file. This file contains the API definition of the service and must be published if other application developers wish to use this service. The stub class extends the Binder class as well as the proxy class. Accordingly, they are the endpoints of the Binder communication progress.

The application manifest declares the service as remote and therefore it is started in an own process by Android. The activity component will hold a proxy object which is a nested class of the interface ISimpleMathService. The activity runs in an own process, too.

The second application contains the Main activity as component only. This main object holds the UI with a button and a text output and a proxy object for the SimpleMathService, that is more related to IPC.

The applications were compiled with Android SDK for Eclipse and were executed in an Android emulator.

6.2. Message Flow and Call Stacks

Due to the limited size of this paper, only an excerpt can be presented. The binding of the service is presented abstractly, and the remote procedure call is presented in detail.

The user app is executed first. It asks the service manager for a Binder of the SimpleMathService. This Binder is implemented as an anonymous class from ISimpleMathService.Stub of the SimpleMathService application. In this example, the stub object implements the business logic of the service. The bindService() method of the Main class of the user app will create a proxy object for the requested service, which communicates with the stub object on the server side.

Proxy and Stub

At this point we describe in detail what happens, when a remote procedure is called after the binding to the service has been established. The listing 6.1 shows a nested anonymous extended class, which is used as callback.

Callback on Connection Events

This ServiceConnection object was an argument of the earlier called bindService() method. The onServiceConnected callback method returns a proxy object, which delivers all method calls to the remote service. The user application can now handle the object as if it were a local object and marshal methods.

Remote Method Call

Listing 6.2 does a method call on the proxy object to add 40 and 2. In the background, following happens: The call is divided by the proxy object in 6.3 into basic data types, which can be written in a parcel. At first, the receiver is written to the parcel, that is a Binder. The arguments are written serialized in the data packet. A user defined int code is assigned to the transaction. This code relates to the intended method name, because the Binder framework at this point permits only to submit an integer value. To avoid misunderstandings, the remote service as the user application must use the same assignment of code and methods.

Proxy Method

At this point, the interprocess communication is initiated with the transact method.

The parcel is sent to the JNI interface that sends it to the Binder C++ middleware that sends it to the Binder kernel driver. The Binder kernel driver will send the client process to sleep and map the parcel data and the code from client process to the server process. The parcel is send from Binder driver to C++ middleware and then to JNI and on Java API Wrapper Layer the method ontransact of the stub is called.

Stub Method1Stub Method2

In Listing 6.4 the entry point for receiving a message is presented. The code is read first and due to knowledge of the method signature the accurate count of arguments are read from the parcel. Now the method corresponding to the code implementing the business logic is called with extracted arguments. The result is written to a reply parcel.

Again it is routed through the layers to the binder driver, that transfers the parcel and wakes up the sleeping client process and delivers the reply parcel to the proxy object. The relpy is unparceled and returned as the result of the proxy method. Thereafter the result is displayed at the activity window of the client app, refer Figure 6.3.

Testing Environment

  1. Discussion The Binder framework supports basic security features. It ensures that no other application can read or manipulate data by transmitting them over a private channel, namely the Binder kernel module. It acts as mediator and must be trusted by the communicating parties. For identification, the Binder framework provides the UID and PID of the calling Binder. With the UID, an application can check the package signature and identify the app. 7 This is important, because multiple services can be assigned with the same name. The operating system will decide, which service is called, depending on the set priority of the service. However, it is possible for a malicious service to overlap the good service and retrieve information, that is sent by the App believing it is communicating with a trusted service. The application must ensure in security critical situations, e.g. the login to a service, the identity of the service. This is possible and this work could not find a flaw in that system, since the UID and PID are derived from Linux methods, that can be regarded as secure and can not be manipulated by unintended calls or arguments.

The use of Binder as a security token should be audited, because the binder reference number is not chosen randomly. It is incremented from zero in the Binder driver. It could be possible to increase the possible numbers and guess with good probability the right Binder token. But this must be confirmed in a future work.

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