How to break on an Android JNI function with IDA Pro Debugger

Question

I have an android application that uses a shared library which I would like to step through with a debugger. I've had success using IDA 6.3 to debug executables with the android_server debug server included with IDA but haven't gotten it to work with shared objects yet.

For a specific example, suppose I have the following Java code (This comes from the hellojni example in the Android NDK):

System.loadLibrary("hello-jni");
tv.setText( stringFromJNI() );

With the JNI C code as:

jstring
Java_com_example_hellojni_HelloJni_stringFromJNI( JNIEnv* env, jobject thiz )
{
        return (*env)->NewStringUTF(env, "Hello from JNI !");
}

If the java code is run only when the application starts up, how can I break in the function Java_com_example_hellojni_HelloJni_stringFromJNI?

Answer 1

There are two options I can see.

1. Start the Dalvik VM manually using app_process. The command line seems to be something like (see am script source):

   app_process /system/bin com.android.commands.am.Am start -a <ACTION>
   

2. Put an endless loop in the beginning of your JNI method, run the app, attach to the new process and skip the loop manually in the debugger.

Answer 2

Newer versions of Android actually include a mechanism like this. It uses jdwp to send a signal to tell the app that you've connected up. See the ndk-gdb script from the NDK =)