Thursday, August 30, 2012

Breakpoints in Smali code

In one of my previous posts I wrote a step-by-step instruction how to debug Smali code with Apktool and NetBeans. However, there were no details about breakpoints, just a short note in steps 12 and 13
12. Set breakpoint. You must select line with some instruction, you can't set breakpoint on lines starting with ".", ":" or "#".
13. Trigger some action in application. If you run at breakpoint, then thread should stop and you will be able to debug step by step, watch variables, etc.
In this post I'm going to share more information about breakpoints in Smali code.
Using Apktool and NetBeans for Smali debugging, we usually face a problem. We connect the debugger to an already running application. Some Smali code has already been executed before we connected the debugger, and will never be executed again. For example, the constructor and onCreate() method of the main activity class. We can't debug such code, so we may miss something important at the beginning of the application.
To solve the problems, we can use several ways. The easiest one is waitForDebugger(). Here is the step-by-step instruction:
  1. Follow the instruction, do steps 1 and 2.
  2. In out/AndroidManifest.xml, find the activity with the following filter
    <intent -filter="-filter">
        <action android:name="android.intent.action.MAIN">
        <category android:name="android.intent.category.LAUNCHER">
  3. Find the the activity class, e.g. Lmy/activity/class/MyActivity;. Then find the onCreate(...) method (the activity class is a descendant of, it overrides the onCreate(...) method). The onCreate(...) method is executed after the constructor of the activity class. In most cases, an application logic starts in onCreate(...).
  4. Insert
    invoke-static {}, Landroid/os/Debug;->waitForDebugger()V
    invoke-super {p0, p1}, Landroid/app/Activity;->onCreate(Landroid/os/Bundle;)V
    in the onCreate(...) method.
  5. Again, follow the instruction, do steps from 3 to 11. You will see just a black screen after you start the application on step 9; don't worry, it's normal. If Android proposes you to close the application because that the application does not respond, reject the proposition. The application is frozen at the very beginning of execution, it is waiting for the debugger.
  6. Set a breakpoint on the first instruction after invoke-static {}, Landroid/os/Debug;->waitForDebugger()V in the onCreate(...) method and continue execution. Your breakpoint will be hit and you will be able to debug the application step by step from the very beginning, watch variables, etc.
From my experience, waitForDebugger() does not work in some cases. Well, it's not a big problem because we can insert an infinite loop instead of waitForDebugger() in the onCreate(...) method. We should just follow the step-by-step instruction above, but use
    sget v0, Lmy/activity/class/MyActivity;->debugFlag:I
    if-nez v0, :debug    
code instead of invoke-static {}, Landroid/os/Debug;->waitForDebugger()V. Do not forget to add the field
    .field static debugFlag:I = 0x01
to the class. As soon as the onCreate(...) method invoked, it goes to an infinite loop while debugFlag is not 0. So, we have a lot of time to connect the debugger to the running application, then use the debugger to set a breakpoint on the first Smali instruction after the loop, and change debugFlag to 0 to escape from the infinite loop. If everything is well-done, our breakpoint is hit and we can start debugging from the very beginning of the application.

That's it :) Any questions are wellcome.

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