Finding and understanding bugs in software model checkers

Software Model Checking (SMC) is a well-known automatic program verification technique and frequently adopted for checking safety-critical software. Thus, the reliability of SMC tools themselves (i.e., software model checkers) is critical. However, little work exists on validating software model checkers, an important problem that this paper tackles by introducing a practical, automated fuzzing technique. For its simplicity and generality, we focus on control-flow reachability (e.g., whether or how many times a branch is reached) and address two specific challenges for effective fuzzing: oracle and scalability. Given a deterministic program, we (1) leverage its concrete executions to synthesize valid branch reachability properties (thus solving the oracle problem) and (2) fuse such individual properties into a single safety property (thus improving the scalability of fuzzing and reducing manual inspection). We have realized our approach as the MCFuzz tool and applied it to extensively test three state-of-the-art C software model checkers, CPAchecker, CBMC, and SeaHorn. MCFuzz has found 62 unique bugs in all three model checkers - 58 have been confirmed, and 20 have been fixed. We have further analyzed and categorized these bugs (which are diverse), and summarized several lessons for building reliable and robust model checkers. Our testing effort has been well-appreciated by the model checker developers, and also led to improved tool usability and documentation.