Permission-Based Separation Logic for Multithreaded Java Programs

This paper presents a program logic for reasoning about multithreaded Java-like programs with dynamic thread creation, thread joining and reentrant object monitors. The logic is based on concurrent separation logic. It is the first detailed adaptation of concurrent separation logic to a multithreaded Java-like language. The program logic associates a unique static access permission with each heap location, ensuring exclusive write accesses and ruling out data races. Concurrent reads are supported through fractional permissions. Permissions can be transferred between threads upon thread starting, thread joining, initial monitor entrancies and final monitor exits. In order to distinguish between initial monitor entrancies and monitor reentrancies, auxiliary variables keep track of multisets of currently held monitors. Data abstraction and behavioral subtyping are facilitated through abstract predicates, which are also used to represent monitor invariants, preconditions for thread starting and postconditions for thread joining. Value-parametrized types allow to conveniently capture common strong global invariants, like static object ownership relations. The program logic is presented for a model language with Java-like classes and interfaces, the soundness of the program logic is proven, and a number of illustrative examples are presented

Resource Usage Protocols for Iterators

We discuss usage protocols for iterator objects that prevent concurrent modifications of the underlying collection while iterators are in progress. We formalize these protocols in Java-like object interfaces, enriched with separation logic contracts. We present examples of iterator clients and proofs that they adhere to the iterator protocol, as well as examples of iterator implementations and proofs that they implement the iterator interface

Chapter Dynamic Dispatch for Method Contracts Through Abstract Predicates

Dynamic method dispatch is a core feature of object-oriented programming by which the executed implementation for a polymorphic method is only chosen at runtime. In this paper, we present a specification and verification methodology which extends the concept of dynamic dispatch to design-by-contract specifications. The formal specification language JML has only rudimentary means for polymorphic abstraction in expressions. We promote these to fully flexible specification-only query methods called model methods that can, like ordinary methods, be overridden to give specifications a new semantics in subclasses in a transparent and modular fashion. Moreover, we allow them to refer to more than one program state which give us the possibility to fully abstract and encapsulate two-state specification contexts, i.e., history constraints and method postconditions. Finally, we provide an elegant and flexible mechanism to specify restrictions on specifications in subtypes. Thus behavioural subtyping can be enforced, yet it still allows for other specification paradigms. We provide the semantics for model methods by giving a translation into a first order logic and according proof obligations. We fully implemented this framework in the KeY program verifier and successfully verified relevant examples. We have also implemented an extension to KeY to support permission-based verification of concurrent Java programs. In this context model methods provide a modular specification method to treat code synchronisation through API methods

A Graph-Based Semantics Workbench for Concurrent Asynchronous Programs

A number of novel programming languages and libraries have been proposed that offer simpler-to-use models of concurrency than threads. It is challenging, however, to devise execution models that successfully realise their abstractions without forfeiting performance or introducing unintended behaviours. This is exemplified by SCOOP---a concurrent object-oriented message-passing language---which has seen multiple semantics proposed and implemented over its evolution. We propose a "semantics workbench" with fully and semi-automatic tools for SCOOP, that can be used to analyse and compare programs with respect to different execution models. We demonstrate its use in checking the consistency of semantics by applying it to a set of representative programs, and highlighting a deadlock-related discrepancy between the principal execution models of the language. Our workbench is based on a modular and parameterisable graph transformation semantics implemented in the GROOVE tool. We discuss how graph transformations are leveraged to atomically model intricate language abstractions, and how the visual yet algebraic nature of the model can be used to ascertain soundness.Comment: Accepted for publication in the proceedings of FASE 2016 (to appear

The VerCors tool for verification of concurrent programs

The VerCors tool implements thread-modular static verification of concurrent programs, annotated with functional properties and heap access permissions. The tool supports both generic multithreaded and vector-based programming models. In particular, it can verify multithreaded programs written in Java, specified with JML extended with separation logic. It can also verify parallelizable programs written in a toy language that supports the characteristic features of OpenCL. The tool verifies programs by first encoding the specified program into a much simpler programming language and then applying the Chalice verifier to the simplified program. In this paper we discuss both the implementation of the tool and the features of its specification language

On Using VeriFast, VerCors, Plural, and KeY to Check Object Usage

Marco Giunti: Partially supported by Dstl, reference: ACC2028868. Publisher Copyright: © João Mota Marco Giunti and António Ravara;Typestates are a notion of behavioral types that describe protocols for stateful objects, specifying the available methods for each state. Ensuring methods are called in the correct order (protocol compliance), and that, if and when the program terminates, all objects are in the final state (protocol completion) is crucial to write better and safer programs. Objects of this kind are commonly shared among different clients or stored in collections, which may also be shared. However, statically checking protocol compliance and completion when objects are shared is challenging. To evaluate the support given by state of the art verification tools in checking the correct use of shared objects with protocol, we present a survey on four tools for Java: VeriFast, VerCors, Plural, and KeY. We describe the implementation of a file reader, linked-list, and iterator, check for each tool its ability to statically guarantee protocol compliance and completion, even when objects are shared in collections, and evaluate the programmer’s effort in making the code acceptable to these tools. With this study, we motivate the need for lightweight methods to verify the presented kinds of programs.publishersversionpublishe

Featherweight VeriFast

VeriFast is a leading research prototype tool for the sound modular verification of safety and correctness properties of single-threaded and multithreaded C and Java programs. It has been used as a vehicle for exploration and validation of novel program verification techniques and for industrial case studies; it has served well at a number of program verification competitions; and it has been used for teaching by multiple teachers independent of the authors. However, until now, while VeriFast's operation has been described informally in a number of publications, and specific verification techniques have been formalized, a clear and precise exposition of how VeriFast works has not yet appeared. In this article we present for the first time a formal definition and soundness proof of a core subset of the VeriFast program verification approach. The exposition aims to be both accessible and rigorous: the text is based on lecture notes for a graduate course on program verification, and it is backed by an executable machine-readable definition and machine-checked soundness proof in Coq

Flexible Invariants Through Semantic Collaboration

Modular reasoning about class invariants is challenging in the presence of dependencies among collaborating objects that need to maintain global consistency. This paper presents semantic collaboration: a novel methodology to specify and reason about class invariants of sequential object-oriented programs, which models dependencies between collaborating objects by semantic means. Combined with a simple ownership mechanism and useful default schemes, semantic collaboration achieves the flexibility necessary to reason about complicated inter-object dependencies but requires limited annotation burden when applied to standard specification patterns. The methodology is implemented in AutoProof, our program verifier for the Eiffel programming language (but it is applicable to any language supporting some form of representation invariants). An evaluation on several challenge problems proposed in the literature demonstrates that it can handle a variety of idiomatic collaboration patterns, and is more widely applicable than the existing invariant methodologies.Comment: 22 page

Witnessing the elimination of magic wands

This paper discusses static verification of programs that have been specified using separation logic with magic wands. Magic wands are used to specify incomplete resources in separation logic, i.e., if missing resources are provided, a magic wand allows one to exchange these for the completed resources. One of the applications of the magic wand operator is to describe loop invariants for algorithms that traverse a data structure, such as the imperative version of the tree delete problem (Challenge 3 from the VerifyThis@FM2012 Program Verification Competition), which is the motivating example for our work.\ud \ud Most separation logic based static verification tools do not provide support for magic wands, possibly because validity of formulas containing the magic wand is, by itself, undecidable. To avoid this problem, in our approach the program annotator has to provide a witness for the magic wand, thus circumventing undecidability due to the use of magic wands. A witness is an object that encodes both instructions for the permission exchange that is specified by the magic wand and the extra resources needed during that exchange. We show how this witness information is used to encode a specification with magic wands as a specification without magic wands. Concretely, this approach is used in the VerCors tool set: annotated Java programs are encoded as Chalice programs. Chalice then further translates the program to BoogiePL, where appropriate proof obligations are generated. Besides our encoding of magic wands, we also discuss the encoding of other aspects of annotated Java programs into Chalice, and in particular, the encoding of abstract predicates with permission parameters. We illustrate our approach on the tree delete algorithm, and on the verification of an iterator of a linked list