wip

5_theseus/2_static_transformation.typ

@@ -1,4 +1,4 @@
-#import "../lib.typ": todo, APK, DEX, JAR, OAT, eg, ART, paragraph, jm-note, jfl-note
+#import "../lib.typ": todo, APK, DEX, JAR, OAT, SDK, eg, ART, jm-note, jfl-note
 
 == Code Transformation <sec:th-trans>
 
@@ -156,8 +156,7 @@ We saw in @sec:cl the class loading model of Android.
 When doing dynamic code loading, an application define a new `ClassLoader` that handle the new bytecode, and start accessing its classes using reflection.
 We also saw in @sec:cl that Android now use the multi-dex format, allowing it to handle any number of #DEX files in one classloader.
 Therefore, the simpler way to give access to the dynamically loaded code to static analysis tool is add the dex files to the application.
-This should not impact the classloading model as long as there is no class collision (we will explore this in @sec:th-class-collision) and as long as the original application appliaction did not try to access unaccessible classes. 
-#jm-note[explain? maybe ref to section limitation]
+This should not impact the classloading model as long as there is no class collision (we will explore this in @sec:th-class-collision) and as long as the original application appliaction did not try to access unaccessible classes (we will develop this issue in @sec:th-limits). 
 
 #figure(
   image(
@@ -181,21 +180,20 @@ In @sec:th-trans-cl, we are adding new code.
 By doing so, we increase the probability of having class collisions:
 The developper may have reuse a helper class in both the dynamically loaded bytecode and the application, or an obfuscation process may have rename classes without checking for intersection between the two sources of bytecode.
 When loaded dynamically, the classes are in a different classloader, and the class resolution is resolved at runtime like we saw in @sec:cl-loading.
-We decided to restrain our scope to the use of class loader from the Android SDK.
+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.
 
-#jm-note[
-When we detect a collision, we rename one of the classes colliding in order to be able to differenciate both classes.
+When we detect a collision, we rename one of the colliding classes 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.
+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.
+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
-][this is redundant an messy]
+- 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.
 
 #figure(
   ```python
@@ -229,34 +227,7 @@ The pseudo-code in @lst:renaming-algo show the three steps of this algorithm:
 * #todo[interupting try blocks: catch block might expect temporary registers to still stored the saved value] ?
 */
 
-=== Limitations
+#h(2em)
 
-#paragraph()[Custom Classloaders][
-#jfl-note[The first obvious limitation is that we do not know what custom classloaders do, so we cannot accuratly reproduce statically their behavior.][est ce que c'est une limite des 2 transformations proposées? j'ai l'impression que tu veux faire une 3ieme transformation]
-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. 
-]
-
-#jfl-note[Deporter ces limites en fin de chapitre? je lirai tout ca + tard]
+Now that we saw the transformations we want to make, we know the runtime information we need to do it.
+In the next section, we will propose a solution to collect those informations.