The Spirit of Ghost Code

Jean-Christophe Filliâtre (1, 2) , Léon Gondelman (1, 2) , Andrei Paskevich (1, 2)

1 LRI - Laboratoire de Recherche en Informatique
2 TOCCATA - Formally Verified Programs, Certified Tools and Numerical Computations

In the context of deductive program verification, ghost code is a part of the program that is added for the purpose of specification. Ghost code must not interfere with regular code, in the sense that it can be erased without observable difference in the program outcome. In particular, ghost data cannot participate in regular computations and ghost code cannot mutate regular data or diverge. The idea exists in the folklore since the early notion of auxiliary variables and is implemented in many state-of-the-art program verification tools. However, ghost code deserves rigorous definition and treatment, and few formalizations exist. In this article, we describe a simple ML-style programming language with muta-ble state and ghost code. Non-interference is ensured by a type system with effects, which allows, notably, the same data types and functions to be used in both regular and ghost code. We define the procedure of ghost code erasure and we prove its safety using bisimulation. A similar type system, with numerous extensions which we briefly discuss, is implemented in the program verification environment Why3.

Mots clés: ghost code; deductive software verification; Why3

Domaines: Logique en informatique [cs.LO]; Théorie et langage formel [cs.FL]

Jean-Christophe Filliâtre, Léon Gondelman, Andrei Paskevich. The Spirit of Ghost Code. Formal Methods in System Design, 2016, 48 (3), pp.152-174. ⟨10.1007/s10703-016-0243-x⟩. ⟨hal-01396864⟩