wip

5_theseus/1_static_transformation.typ

@@ -1,4 +1,4 @@
-#import "../lib.typ": todo, APK, DEX, JAR, OAT, eg
+#import "../lib.typ": todo, APK, DEX, JAR, OAT, eg, ART, paragraph
 
 /*
 * Parler de dex lego et du papier qui encode les resultats d'anger en jimple
@@ -140,10 +140,17 @@ Because it is an internal, platform dependant format, we elected to ignore the #
 Practically, #JAR and #APK files are zip files containing #DEX files.
 This means that we only need to find a way to integrate #DEX files to the application.
 
-We elected to simply add the dex files to the application, using the multi-dex feature introduced by the SDK 21 now used by all applications.
+We elected to simply add the dex files to the application, using the multi-dex feature introduced by the SDK 21 now used by all applications as shown in @fig:th-inserting-dex.
 This gives access to the dynamically loaded code to static analysis tool. 
 
-#todo[add drawing of dex insertion]
+#figure(
+  image(
+    "figs/dex_insertion.svg",
+    width: 80%,
+    alt: "A diagram showing a box labelled 'app.apk', a box labelled 'lib.jar', and single file ouside the boxes labelled 'lib.dex'. The lib.jar boxe contains the files classes.dex and classes2.dex. Inside the app.apk box, the files AndroidManifest.xml, resources.arsc, classes.dex, classes2.dex, classes3.dex and the folders lib, res and assets are circled by dashes and labelled 'original files', and, still inside app.apk, the files classes4.dex, classes5.dex and classes5.dex are circled by dashes and labelled 'Added Files'. Arrows go from lib.dex to classes4.dex, from the classes.dex inside lib.jar to classes5.dex inside app.apk and from classe2.dex inside lib.jar to classes6.dex inside app.apk"
+  ),
+  caption: [Inserting #DEX files inside an #APK]
+) <fig:th-inserting-dex>
 
 We decided to leave untouched the original code that load the bytecode.
 At runtime, although the bytecode is already present in the application, the application will still dynamically load the code.
@@ -159,17 +166,76 @@ When loaded dynamically, the classes are in a different classloader, and the cla
 We decided to restrain our scope to the use of class loader from the Android SDK.
 In the abscence of class collision, those class loader behave seamlessly and adding the classes to application maintains the behavior.
 
-When we detect a collision, we rename one of the classes colliding before injecting it to the application.
+#todo[this is redundant an messy:]
+When we detect a collision, we rename one of the classes colliding in order to be able to differenciate both classes.
 To avoid breaking the application, we then need to rename all references to this specific class, an be carefull not to modify references to the other class.
 To do so, we regroup each classes by the classloaders defining them, then, for each colliding class name and each classloader, we check the actual class used by the classloader.
 If the class has been renamed, we rename all reference to this class in the classes defined by this classloader.
 To find the class used by a classloader, we reproduce the behavior of the different classloaders of the Android SDK.
 This is an important step: remember that the delegation process can lead to situation where the class defined by a classloader is not the class that will be loaded when querying the classloader.
+The pseudo-code in @lst:renaming-algo show the three steps of this algorithm: 
+- first we detect collision and rename classes definitions to remove the collisions
+- then we rename the reference to the colliding classes to make sure the right classes are called
+- ultimately, we merge the modified dexfiles of each class loaders into one android application
 
-#todo[renamin algo]
+#figure(
+  ```python
+  defined_classes = set()
+  redifined_classes = set()
 
-=== Pitfalls
+  # Rename the definition of redifined classes
+  for cl in class_loaders:
+    for clz in defined_classes.intersection(cl.defined_classes):
+      cl.rename_definition(clz)
+      redifined_classes.add(clz)
+    defined_classes.update(cl.defined_classes)
 
-#todo[interupting try blocks: catch block might expect temporary registers to still stored the saved value]
-#todo[diferenciating the classloaders]
-#todo[changing classloader with class collision]
+  # Rename reference of redifined classes
+  for cl in class_loaders:
+    for clz in redifined_classes:
+      defining_cl = cl.resolve_class(clz).class_loader
+      cl.rename_reference(clz, defining_cl.new_name(clz))
+
+  # Merge the classloader into a flat APK
+  new_apk = Apk()
+  for cl in class_loaders:
+    for dex in cl.get_dex():
+      new_apk.add_dex(dex)
+  ```,
+  caption: [Pseudo-code of the renaming algorithm]
+) <lst:renaming-algo>
+
+/*
+* Although we limited ourselves to replacing one specific bytecode instruction, we encontered many technical challenges
+* #todo[interupting try blocks: catch block might expect temporary registers to still stored the saved value] ?
+*/
+
+=== Limitations
+
+#paragraph()[Custom Classloaders][
+The first obvious limitation is that we do not know what custom classloaders do, so we cannot accuratly emulate their behavior.
+We elected to fallback to the behavior of the `BaseDexClassLoader`, which is the highest Android specific classloader in the inheritance hierarchy, and whose behavior is shared by all classloaders safe `DelegateLastClassLoader`.
+The current implementation of the #ART enforce some restrictions on the classloaders behavior to optimize the runtime performance by caching classes.
+This gives us some garanties that custom classesloaders will keep a some coherences will the classic classloaders.
+For instance, a class loaded dynamically must have the same name as the name used in `ClassLoader.loadClass()`.
+This make `BaseDexClassLoader` a good estimation for legitimate classloaders, however, an obfuscated application could use the techniques discussed in @sec:cl-cross-obf, in wich case our model would be entirelly wrong.
+]
+
+#paragraph()[Multiple Classloaders for one `Method.invoke()`][
+#todo[explain the problem arrose each time a class is compared to another]
+Although we managed to handle call to different methods from one `Method.invoke()` site, we do not handle calling methods from different classloaders with colliding classes definition.
+The first reason is that it is quite challenging to compare classloaders statically.
+At runtime, each object has an unique identifier that can be used to compare them over the course of the same execution, but this identifier is reset each time the application starts.
+This means we cannot use this identifier in an `if` condition to differentiate the classloaders.
+Ideally, we would combine the hash of the loaded #DEX files, the classloader class and parent to make an unique, static identifier, but the #DEX files loaded by a classloader cannot be accessed at runtime without accessing the process memory at arbitrary locations.
+For some classloaders, the string representation returned by `Object.toString()` list the location of the loaded #DEX file on the file system.
+This is not the case for the commonly used `InMemoryClassLoader`.
+In addition, the #DEX files are often located in the application private folder, whose name is derived from the hash of the #APK itself.
+Because we modify the application, the path of the private folder also change, and so will the string representation of the classloaders.
+Checking the classloader of a classes can also have side-effect on classloaders that delegate to the main application classloader:
+because we inject the classes in the #APK, the classes of the classloader are now already in the main application classloader, which in most case will have priority on the other classloaders, and lead to the class beeing loaded by the application classloader instead of the original classloader.
+If we check for the classloader, we would need to considere such cases en rename each classes of each classloader before reinjecting them to the in the application.
+This would greatly increase the risk of breaking the application during its transformation.
+Instead, we elected to ignore the classloaders when selecting the method to invoque.
+This leads to potential invalid runtime behaviore, as the first method that matching the class name will be called, but the alternative methods from other classloader still appears in the new application, albeit in a block that might be flagged as dead-code by a sufficiently advenced static analyser. 
+]

bibliography.bib

@@ -916,4 +916,3 @@
 	pages = {423--426},
 	file = {IEEE Xplore Abstract Record:/home/histausse/Zotero/storage/QEQLZHMD/7129009.html:text/html;Kriz and Maly - 2015 - Provisioning of application modules to Android dev.pdf:/home/histausse/Zotero/storage/8GRUYQLQ/Kriz and Maly - 2015 - Provisioning of application modules to Android dev.pdf:application/pdf},
 }
-