Deductive Verification of Legacy Code

Deductive verification is about proving that a piece of code conforms to a given requirement specification. For legacy code, this task is notoriously hard for three reasons: (1) writing specifications posthoc is much more difficult than producing code and its specification simultaneously, (2) verification does not scale as legacy code is often badly modularized, (3) legacy code may be written in a way such that verification requires frequent user interaction. We give examples for which characteristics of (imperative) legacy code impede the specification and verification effort. We also discuss how to handle the challenges of legacy code verification and suggest a strategy for post-hoc verification, together with possible improvements to existing verification approaches. We draw our experience from two case studies for verification of imperative implementations (in Java and C) in which we verified legacy software, i.e., code that was provided by a third party and was not designed to be verified.