Finding Unknown Unknowns using Cyber-Physical System Simulators

Simulation-based approaches are among the most practical means to search for safety violations, bugs, and other unexpected events in cyber-physical systems (CPS). Where existing approaches search for simulations violating a formal specification or maximizing a notion of coverage, in this work we propose a new goal for testing: to discover unknown rare behaviors by examining discrete mode sequences. We assume a CPS simulator outputs mode information, and strive to explore the sequences of modes produced by varying the initial state or time-varying uncertainties. We hypothesize that rare mode sequences are often the most interesting to a designer, and we develop two accelerated sampling algorithms that speed up the process of finding such sequences. We evaluate our approach on several benchmarks, ranging from synthetic examples to Simulink diagrams of a CPS, demonstrating in some cases a speedup of over 100x compared with a random sampling strategy.