Benoit Cornu

Dynamic patch generation for null pointer exceptions using metaprogramming

Null pointer exceptions (NPE) are the number one cause of uncaught crashing exceptions in production. In this paper, we aim at exploring the search space of possible patches for null pointer exceptions with metaprogramming. Our idea is to transform the program under repair with automated code transformation, so as to obtain a metaprogram. This metaprogram contains automatically injected hooks, that can be activated to emulate a null pointer exception patch. This enables us to perform a fine-grain analysis of the runtime context of null pointer exceptions. We set up an experiment with 16 real null pointer exceptions that have happened in the field. We compare the effectiveness of our metaprogramming approach against simple templates for repairing null pointer exceptions.

Casper: Automatic Tracking of Null Dereferences to Inception with Causality Traces

Fixing a software error requires understanding its root cause. In this paper, we introduce causality traces, crafted execution traces augmented with the information needed to reconstruct the causal chain from the root cause of a bug to an execution error. We propose an approach and a tool, called Casper, based on code transformation, which dynamically constructs causality traces for null dereference errors. The core idea of Casper is to replace null values with special objects, called “ghosts”, that track the propagation of the nulls from inception to their error-triggering dereference. Causality traces are extracted from these ghosts. We evaluate our contribution by providing and assessing the causality traces of 14 real null dereference bugs collected over six large, popular open-source projects.