How to recognise that the user has just uninstalled your app

Presented during Droidcon.de 2015 barcamp session.

Hacking Android to get a notification when your app is being uninstalled. Mixing JNI and native C code to do something that was never meant to be done ;-)

Transcript

1. How to recognise that the user has just uninstalled your Android app fb.me/pjakubczyk +AleksanderPiotrowski @pelotasplus
2. Opera Max
4. The Java way
5. Read the broadcast <receiver android:name=".PackageWatcher"> <intent-filter> <action android:name="android.intent.action. PACKAGE_ADDED"/> <action android:name="android.intent.action. PACKAGE_REMOVED"/> <action android:name="android.intent.action. PACKAGE_REPLACED" /> <data android:scheme="package"/> </intent-filter> </receiver>
6. Read the broadcast void onReceive(Context context, Intent intent) { Bundle bundle = intent.getExtras(); Iterator<String> it = bundle.keySet().iterator; while (it.hasNext()) { String key = it.next(); Log.e("DDD", key +"="+bundle.get(key)); }
7. Usually we see (install) E/DDD (29199): Dumping Intent start [android.intent.extra.UID=10089] [android.intent.extra.user_handle=0] E/DDD (29199): Dumping Intent end
8. Usually we see (reinstall) E/DDD (29199): Dumping Intent start [android.intent.extra.REMOVED_FOR_ALL_USERS=false] [android.intent.extra.UID=10089] [android.intent.extra.DATA_REMOVED=false] [android.intent.extra.REPLACING=true] [android.intent.extra.user_handle=0] E/DDD (29199): Dumping Intent end
9. Usually we see (uninstall) E/DDD (29199): Dumping Intent start [android.intent.extra.REMOVED_FOR_ALL_USERS=true] [android.intent.extra.UID=10089] [android.intent.extra.DATA_REMOVED=true] [android.intent.extra.user_handle=0] E/DDD (29199): Dumping Intent end
10. Let’s uninstall our app and there’s nothing …. Why ? OS unregisters listener during removal
11. What Opera does? It does not listen for package removal it does some magic ;-) … not in Java code
12. Getting the APK
13. Getting the APK • genymotion with gapps installed • get app from play store • be careful with the right ABI
14. Getting the APK 1. adb shell 2. pm list packages
15. Getting the APK 3. pm path com.opera.max 4. adb pull /data/app/com. opera.max.apk
16. Hacking APK
17. Apktool A tool for reverse engineering Android apk files Made with <3 in Poland ;-)
18. Apktool Easy to use $ apktool d com.opera.max. apk
19. Apktool • decoded XML files • smali assembly code • PNGs, layouts, resources • id-s mapping
20. with Opera Max APK live apktool demo
21. Opera Findings
22. Found a clue! There are *.so files We can inspect them to see more Tools: strings, objdump, nm, readelf
23. rudy$ strings opera/lib/armeabi/libuo.so (II) ... inotify_init inotify_add_watch inotify_rm_watch /data/data/%s/ %s%s
24. inotify framework http://linux.die.net/man/7/inotify The inotify API provides a mechanism for monitoring file system events. Inotify can be used to monitor individual files, or to monitor directories.
25. rudy$ strings opera/lib/armeabi/libuo.so (I) ... Android start android.intent.action.VIEW --user ...
26. am command part of Android system /system/bin/am A way to start apps, intents and whatnot
27. more details $ ps USER PID PPID u0_a91 24318 20265 246900 27716 ffffffff b6edf5cc S com.opera.max u0_a91 24337 24318 856 336 c00e4944 b6f72158 S /data/app-lib/com.opera.max-2/libuo.so
28. The scenario 1. Fork the native process 2. Inside the child process use inotify to watch a file 3. Watcher is woken up on file deletion. Start another native process 4. The last process run the ‘am’ (ActivityManager) command to run intent.
29. Setup JNI
30. local.properties # Location of the SDK. This is only used by Gradle. # For customization when using a Version Control System, please read the sdk.dir=/Users/alek/android-sdk ndk.dir=/Users/alek/android-ndk-r10e
31. build.gradle android.defaultConfig { applicationId "pl.pelotasplus.actionafteruninstall" ndk { moduleName "hello-jni" ldLibs "log", "android" stl "stlport_static" } }
32. MainActivity.java declaring public class MainActivity extends AppCompatActivity { public native String stringFromJNI(); public native void observer(); static { System.loadLibrary("hello-jni"); // System.loadLibrary("/data/data/com.foo.test/lib/liba.so"); } }
33. MainActivity.java calling protected void onCreate(Bundle savedInstanceState) { setContentView(R.layout.activity_main); textView = (TextView) findViewById(R.id.textView); textView.setText(stringFromJNI()); observer(); }
34. project structure
35. Native code JNI
36. Sample by Google jstring Java_pl_pelotasplus_actionafteruninstall_MainActivity_stringFro mJNI (JNIEnv* env, jobject thiz) { return (*env)->NewStringUTF( env, "Hello from JNI ! Compiled with ABI foo." ); }
37. Android.mk LOCAL_PATH := $(call my-dir) include $(CLEAR_VARS) LOCAL_MODULE := hello-jni LOCAL_SRC_FILES := hello-jni.c LOCAL_LDFLAGS += -llog -lpthread include $(BUILD_SHARED_LIBRARY)
38. Application.mk APP_ABI := armeabi-v7a # all APP_STL := stlport_static
39. inotify on Linux int main( int argc, char **argv) { int length, i = 0; int fd; int wd; char buffer[BUF_LEN]; fd = inotify_init(); printf("fd=%d\n", fd); }
40. inotify on Linux int main( int argc, char **argv) { [...] wd = inotify_add_watch(fd, "/var/tmp", IN_MODIFY | IN_CREATE | IN_DELETE); length = read( fd, buffer, BUF_LEN ); printf("length=%d\n", length); if (length < 0) { perror("read"); }
41. inotify on Linux while (i < length) { struct inotify_event *event = (struct inotify_event*)&buffer[ i]; printf("Event len %d\n", event->len); if (event->len) { if (event->mask & IN_DELETE) { if (event->mask & IN_ISDIR) { printf( "The directory %s was deleted.\n", event->name ); } else { printf( "The file %s was deleted.\n", event->name );
42. inotify on Android (pseudo code) void observer(void) { inotify_init(); inotify_add_watch(fd, DIRECTORY, IN_DELETE); if (event->mask & IN_DELETE) { startIntent(); } }
43. first attempt void Java_pl_pelotasplus_actionafteruninstall_MainActivity_observer(JNIEnv* env, jobject thiz) { observer(); } App blocked as native code blocked app
44. second attempt, with thread void Java_pl_pelotasplus_actionafteruninstall_MainActivity_observer (JNIEnv* env, jobject thiz) { pthread_attr_init(&attr); pthread_create(&thread, &attr, &observer_thread, NULL); } App not blocked but native code stopped when stopping app for uninstalling
45. third attempt, with fork void Java_pl_pelotasplus_actionafteruninstall_MainActivity_observer(JNIEnv* env, jobject thiz) { pid_t pid; pid = fork(); if (pid == 0) { __android_log_print(ANDROID_LOG_INFO, TAG, "Fork child\n"); observer(); } }
46. start intent, another fork void startIntent(void) { pid_t p = fork(); if (p == 0) { __android_log_print(ANDROID_LOG_INFO, TAG, "startIntent %d", getpid()); system("/system/bin/am start --user 0 -a android.intent. action.VIEW -d http://droidcon.de"); } }
47. Live demo of our app
48. https://github. com/pelotasplus/A ctionAfterUninstall Check the dirty source code
49. Moral > What happens when I call fork() in JNI code? Will this totally break the > Activity lifecycle model in Android? Don't do this. Just don't. -- Dianne Hackborn Android framework engineer hack...@android.com http://markmail.org/message/ruqp2t6gvhnhv654