Finding Likely Errors with Bayesian Specifications

We present a Bayesian framework for learning probabilistic specifications from large, unstructured code corpora, and a method to use this framework to statically detect anomalous, hence likely buggy, program behavior. The distinctive insight here is to build a statistical model that correlates all specifications hidden inside a corpus with the syntax and observed behavior of programs that implement these specifications. During the analysis of a particular program, this model is conditioned into a posterior distribution that prioritizes specifications that are relevant to this program. This allows accurate program analysis even if the corpus is highly heterogeneous. The problem of finding anomalies is now framed quantitatively, as a problem of computing a distance between a "reference distribution" over program behaviors that our model expects from the program, and the distribution over behaviors that the program actually produces. We present a concrete embodiment of our framework that combines a topic model and a neural network model to learn specifications, and queries the learned models to compute anomaly scores. We evaluate this implementation on the task of detecting anomalous usage of Android APIs. Our encouraging experimental results show that the method can automatically discover subtle errors in Android applications in the wild, and has high precision and recall compared to competing probabilistic approaches.