Verification for Dummies: SMT and Induction

By OCamlPro.

This book broadly discusses induction as a formal verification technique, which here really means formal program verification. I will use concrete, runnable examples whenever possible. Some of them can run directly in a browser, while others require to run small easy-to-retrieve tools locally. Such is the case for pretty much all examples dealing directly with induction.

The next chapters discuss the following notions:

  • formal logics and formal frameworks;
  • SMT-solving: modern, low-level verification building blocks;
  • declarative transition systems;
  • transition system unrolling;
  • BMC and induction proofs over transition systems;
  • candidate strengthening.

The approach presented here is far from being the only one when it comes to program verification. It happens to be relatively simple to understand, and I believe that familiarity with the notions discussed here makes understanding other approaches significantly easier.

This book thus hopes to serve both as a relatively deep dive into the specific technique of SMT-based induction, as well as an example of the technical challenges inherent to both developing and using automated proof engines.

Some chapters contain a few pieces of Rust code. Usually to provide a runnable version of a system under discussion, or to serve as example of actual code that we want to encode and verify. Some notions of Rust could definitely help in places, but this is not mandatory (probably).

Table of Contents

  • Preface

    High-level presentation of (formal) verification as a formal method.

  • SMT Solvers

    SMT solvers are the basic building blocks for many modern verification tools.

  • Transition Systems

    Transition systems are one way to encode a wide variety of programs in a framework suited for formal verification. Following sections will discuss all notions in the context of transition systems as they are fairly easy to understand. They definitely have downsides, but one can get a surprising mileage out of them if careful.

  • SMT and Transition Systems

    Transition systems are represented by formulas that SMT solver can work on. This chapter lays out the foundation for more complex SMT-based analyses.

  • Unrolling and BMC

    Bounded Model-Checking is, in general, not a verification technique. Still, it is quite useful for finding concrete counterexample, i.e. a concrete behavior of the system that illustrates a problem. It is also a good context to showcase what one can do with a transition system using an SMT solver.

  • BMC: Mikino

    Mikino is a small proof engine that can perform BMC. While it requires getting familiar with its simple input format, it abstracts SMT solvers for us so that we can focus on higher-level concepts.

  • Induction

    Induction is a natural step from BMC: it requires a simple BMC-like base check but also a step check which is simple to encode with SMT solvers. Since induction is a verification technique contrary to BMC, this is where we finally start proving things.

  • Induction: Mikino and Step Cex-s

    In addition to BMC, mikino can also perform induction. It can thus prove inductive properties of a system. Once again, mikino abstracts the SMT solver for us. Mikino is designed with user experience in mind, so by the end of this chapter you will probably be able to experiment by modifying systems introduced so far, or write your own.

  • Candidate Strengthening

    This chapter is quite technical and a bit theoretical. Make sure you are comfortable with all the notions discussed so far before diving in.

    An invariant for a system is not necessarily inductive. This last part of the series focuses on candidate strengthening, which is really about discovering useful, powerful facts about the system's behavior. Such facts can make non-inductive invariants inductive, which is why most modern induction-based verification engines focus heavily on candidate strengthening.

    This chapter, unlike previous ones, aims at proving an actual piece of code by encoding it as a transition system. It also touches on the complexity of verification and the notion of undecidability.