Android testing via synthetic symbolic execution

Symbolic execution of Android applications is challenging as it involves either building a customized VM for Android or modeling the Android libraries. Since the Android Runtime evolves from one version to another, building a high-fidelity symbolic execution engine involves modeling the effect of the libraries and their evolved versions. Without simulating the behavior of Android libraries, path divergence may occur due to constraint loss when the symbolic values flow into Android framework and these values later affect the subsequent path taken. Previous works such as JPF-Android have relied on the modeling of execution environment such as libraries. In this work, we build a dynamic symbolic execution engine for Android apps, without any manual modeling of execution environment. Environment (or library) dependent control flow decisions in the application will trigger an on-demand program synthesis step to automatically deduce a representation of the library. This representation is refined on-the-fly by running the corresponding library multiple times. The overarching goal of the refinement is to enhance behavioral coverage and to alleviate the path divergence problem during symbolic execution. Moreover, our library synthesis can be made context-specific. Compared to traditional synthesis approaches which aim to synthesize the complete library code, our context-specific synthesis engine can generate more precise expressions for a given context. The evaluation of our dynamic symbolic execution engine, built on top of JDART, shows that the library models obtained from program synthesis are often more accurate than the semi-manual models in JPF-Android. Furthermore, our symbolic execution engine could reach more branch targets, as compared to using the JPF-Android models.