Editors: Georg Weissenbacher, Hana Chockler and Igor Konnov

Authors, Title and AbstractPaperTalk
Jul 16 11:00
Jul 14 14:00

ABSTRACT. We present an extension of propositional dynamic logic called HOT-PDL for specifying temporal properties of higher-order functional programs. The semantics of HOT-PDL is defined over Higher-Order Traces (HOTs) that model execution traces of higher-order programs. A HOT is a sequence of events such as function calls and returns, equipped with two kinds of pointers inspired by the notion of justification pointers from game semantics: one for capturing the correspondence between call and return events, and the other for capturing higher-order control flow involving a function that is passed to or returned by a higher-order function. To allow traversal of the new kinds of pointers, HOT-PDL extends PDL with new path expressions. The extension enables HOT-PDL to specify interesting properties of higher-order programs, including stack-based access control properties and those definable using dependent refinement types. We show that HOT-PDL model checking of higher-order functional programs over bounded integers is decidable via a reduction to modal $\mu$-calculus model checking of higher-order recursion schemes.

Jul 14 11:00

ABSTRACT. We present new algorithms for proving program termination and non-termination using syntax-guided synthesis. They exploit the symbolic encoding of program and automatically construct a formal grammar for symbolic constraints that are used to synthesize either a termination argument or a never-terminating program refinement. The constraints are then added back to the program encoding, and an off-the-shelf constraint solver decides on their fitness and on the progress of the algorithms. The evaluation of our implementation, called FreqTerm, shows that although the number of constraints is always finite and the formal grammar is limited to the syntax of the program, in the majority of cases our algorithms are effective and fast. Importantly, FreqTerm is competitive with the state-of-the-art on a wide range of terminating and non-terminating benchmarks, and it significantly outperforms the state-of-the-art on proving non-termination of a class of programs arising from large-scale Event-Condition-Action systems.

Jul 14 11:15

ABSTRACT. Hyperproperties are properties of sets of computation traces. In this paper, we study quantitative hyperproperties, which we define as hyperproperties that express a bound on the number of traces that may appear in a certain relation. For example, quantitative non-interference limits the amount of information about certain secret inputs that is leaked through the observable outputs of a system. Quantitative non-interference thus bounds the number of traces that have the same observable input but different observable output. We study quantitative hyperproperties in the setting of HyperLTL, a temporal logic for hyperproperties. We show that, while quantitative hyperproperties can be expressed in HyperLTL, the running time of the HyperLTL model checking algorithm is, depending on the type of property, exponential or even doubly exponential in the quantitative bound. We improve this complexity with a new model checking algorithm based on model-counting. The new algorithm needs only logarithmic space in the bound and therefore improves, depending on the property, exponentially or even doubly exponentially over the model checking algorithm of HyperLTL. In the worst case, the running time of the new algorithm is exponential in the size of the system. Our SAT-based prototype implementation demonstrates, however, that the counting approach is viable on systems with nontrivial quantitative information flow requirements such as a passcode checker.

Jul 14 11:30

ABSTRACT. Relational safety specifications describe multiple runs of the same program or relate the behaviors of multiple programs. Approaches to automatic relational verification often compose the programs and analyze the result for safety, but a naively composed program can lead to difficult verification problems. We propose to exploit relational specifications for simplifying the generated verification subtasks. First, we maximize opportunities for synchronizing code fragments. Second, we compute symmetries in the specifications to reveal and avoid redundant subtasks. We have implemented these enhancements in a prototype for verifying k-safety properties on Java programs. Our evaluation confirms that our approach leads to a consistent performance speedup on a range of benchmarks.

Jul 14 11:45

ABSTRACT. We present a bounded model checking tool for verifying Java bytecode, which is built on top of the CProver framework, named Java Bounded Model Checker (JBMC). In summary, JBMC processes Java bytecode together with a model of the standard Java libraries, with the goal of checking a set of desired properties. Experimental results show that JBMC can correctly verify a set of Java benchmarks extracted from the literature and it also performs competitively with JayHorn and JPF, which are state-of-art Java veriers based on constrained Horn clauses and path-based symbolic execution, respectively.

Jul 14 12:00

ABSTRACT. We present a method of abstracting parameterized or infinite-state symbolic model checking problems to finite-state problems, based on propositional skeletons and eager theory explication. The method is extensible in the sense that users can add abstractions (or refine existing abstractions) by providing axiom schemata. We evaluate the method on a collection of parameterized and infinite-state case studies, including FLASH cache coherence protocol and the Virtually Synchronous Paxos distributed consensus protocol. The approach is seen to substantially reduce the complexity of the required auxiliary invariants in comparison to invariant checking using an SMT solver.

Jul 14 12:15

ABSTRACT. We leverage reinforcement learning (RL) to speed up numerical program analysis. The key insight is to establish a correspondence between concepts in RL and program analysis. For instance, a state in RL maps to an abstract program state in the analysis, an action maps to an abstract transformer, and at every state, we have a set of sound transformers (actions) that represent different trade-offs between precision and performance. At each iteration, the agent (analysis in our case) uses a policy that is learned offline by RL to decide on the transformer which minimizes the loss of precision at fixpoint while increasing analysis performance. Our approach leverages the idea of online decomposition (applicable to popular numerical abstract domains) to define a space of approximate transformers with varying degrees of precision and performance. Using a suitably designed set of features that capture key properties of both, abstract program states and available actions, we then apply Q-learning with linear function approximation to compute an optimized context-sensitive policy that chooses transformers during analysis. We implemented our approach for the notoriously expensive Polyhedra domain and evaluated it on a set of Linux device drivers that are expensive to analyze. The results show that our approach can yield massive speedups (up to $515$x) while maintaining precision at fixpoint.

Jul 14 15:00

ABSTRACT. We present an alternative Double Description representation for the domain of NNC (not necessarily closed) polyhedra, together with the corresponding Chernikova-like conversion procedure. The representation uses no slack variable at all and provides a solution to a few technical issues caused by the encoding of an NNC polyhedron as a closed polyhedron in a higher dimension space. A preliminary experimental evaluation shows that the new conversion algorithm is able to achieve significant efficiency improvements.

Jul 14 15:15

ABSTRACT. Given a relational specification between Boolean inputs and outputs, the goal of Boolean functional synthesis is to synthesize each output as a function of the inputs such that the specification is met. In this paper, we first show that unless some hard conjectures in complexity theory are falsified, Boolean functional synthesis must necessarily generate exponential-sized Skolem functions, thereby requiring exponential time, in the worst-case. Given this inherent hardness, what does one do to solve the problem? We present a two-phase algorithm for Boolean functional synthesis, where the first phase is efficient both in terms of time and sizes of synthesized functions, and solves an overwhelming majority of benchmarks. To explain this surprisingly good performance, we provide a sufficient condition under which the first phase must produce exact correct answers. When this condition fails, the second phase builds upon the result of the first phase, possibly requiring exponential time and generating exponential-sized functions in the worst-case. Detailed experimental evaluation shows our algorithm to perform better than state-of-the-art techniques for the vast majority of benchmarks.

Jul 14 16:00

ABSTRACT. Abstract. Program synthesis is the mechanized construction of soft- ware. One of the main difficulties is the efficient exploration of the very large solution space, and tools often require a user-provided syntactic restriction of the search space. We propose a new approach to program synthesis that combines the strengths of a counterexample-guided in- ductive synthesiser with those of a theory solver, exploring the solution space more efficiently without relying on user guidance. We call this approach CEGIS(T ), where T is a first-order theory. In this paper, we focus on one particular application of CEGIS(T ), namely the synthesis of programs that require non-trivial constants, which is a fundamentally difficult task for state-of-the-art synthesisers. We present two exemplars, one based on Fourier-Motzkin (FM) variable elimination and one based on first-order satisfiability. We demonstrate the practical value of CEGIS(T ) by automatically synthesizing programs for a set of intricate benchmarks.

Jul 14 16:15

ABSTRACT. We study the reactive synthesis problem for hyperproperties given as formulas of the temporal logic HyperLTL. Hyperproperties generalize trace properties, i.e., sets of traces, to sets of sets of traces. Typical examples are information-flow policies like noninterference, which stipulate that no sensitive data must leak into the public domain. Such properties cannot be expressed in standard linear or branching-time temporal logics like LTL, CTL, or CTL*. We show that, while the synthesis problem is undecidable for full HyperLTL, it remains decidable for the exists*, exists*forall1, and the linear forall* fragments. Beyond these fragments, the synthesis problem immediately becomes undecidable. For universal HyperLTL, we present a semi-decisionprocedure that constructs implementations and counterexamples up to a given bound. We report encouraging experimental results obtained with a prototype implementation on example specifications with hyperproperties like symmetric responses, secrecy, and information flow.

Jul 14 16:30

ABSTRACT. Reactive synthesis has become a widely-used paradigm for automatically building correct-by-construction implementations for systems that interact with an unknown or adversarial environment. We study how to do reactive synthesis when part of the specification of the system is that its behavior should be random. There are several examples of such systems, including a network protocol fuzz tester whose output should be varied, or a planner for a surveillance robot whose route should be unpredictable. However, existing reactive synthesis techniques do not provide a way to ensure random behavior while maintaining functional correctness. Towards this end, we introduce a notion of randomized reactive synthesis based on the recently-proposed framework of control improvisation (CI). This framework provides a natural way to integrate a randomness requirement with the usual functional specifications of reactive synthesis. We define a reactive version of CI which can be used to solve randomized reactive synthesis problems over finite windows. We theoretically characterize when such problems are realizable, and give a general method for solving them. For specifications given by reachability/safety games or deterministic finite automata, our method yields a polynomial-time synthesis algorithm. For various other types of specifications including temporal logic formulas, we obtain a polynomial-space algorithm and prove matching PSPACE-hardness results. In all of these cases, we show that randomized reactive synthesis is no harder in a complexity-theoretic sense than ordinary reactive synthesis.

Jul 14 16:45

ABSTRACT. Proof by coupling is a classical technique for proving properties about pairs of randomized algorithms by carefully relating (or coupling) two probabilistic executions. In this paper, our goal is to automatically construct such proofs for programs. First, we present f-coupled postconditions, an abstraction describing two coupled programs. Second, we show how specific forms of f-coupled postconditions imply probabilistic properties like uniformity and independence of program variables. Third, we demonstrate how to automate construction of coupling proofs by reducing the problem to solving a purely logical synthesis problem of the form ∃f. ∀X. φ, thus doing away with probabilistic reasoning. Finally, we evaluate our technique in a prototype implementation, and demonstrate its ability to construct a range of coupling proofs for various properties and randomized algorithms.

Jul 14 17:00

ABSTRACT. We address the problem of synthesizing provably correct controllers for linear systems with reach-avoid specifications. Our solution uses a combination of an open-loop controller and a tracking controller, thereby reducing the problem to smaller tractable problems. We show that, once a tracking controller is fixed, the reachable states from an initial neighborhood, subject to any disturbance, can be over-approximated by a sequence of ellipsoids, with sizes that are independent of the open-loop controller. Hence, the open-loop controller can be synthesized independently to meet the reach-avoid specification for an initial neighborhood. Exploiting several techniques for tightening the over-approximations, we reduce the open-loop controller synthesis problem to satisfiability over quantifier-free linear real arithmetic. The overall synthesis algorithm, computes a tracking controller, and then iteratively covers the entire initial set to find open-loop controllers for initial neighborhoods. The algorithm is sound and, for a class of robust systems, is also complete. We present RealSyn, a tool implementing this synthesis algorithm, and we show that it scales to several high-dimensional systems with complex reach-avoid specifications.

Jul 14 17:15

ABSTRACT. Asynchronous interactions are ubiquitous in computing systems and complicate design and programming. Automatic construction (``synthesis'') of asynchronous programs from specifications could ease the difficulty, but known methods are intractable in practice. This work develops substantially simpler methods for asynchronous synthesis. An exponentially more compact automaton construction is developed for the reduction of asynchronous to synchronous synthesis. Experiments with a prototype implementation of the new method demonstrate practical feasibility. Furthermore, it is shown that for several useful classes of temporal properties, automaton-based methods can be avoided altogether and replaced with simpler Boolean constraint solving.

Jul 14 17:30

ABSTRACT. Automatic program synthesis promises to increase the productivity of programmers and end-users of computing devices by automating tedious and error-prone tasks. Despite the practical successes of program synthesis, we still do not have systematic frameworks to synthesize programs that are ``good'' according to certain metrics---e.g., produce programs of reasonable size or with good runtime---and to understand when synthesis can result in such good programs. In this paper, we propose QSyGuS, a unifying framework for describing syntax-guided synthesis problems with quantitative objectives over the syntax of the synthesized programs. QSyGuSbuilds on weighted (tree) grammars, a clean and foundational formalism that provides flexible support for different quantitative objectives, useful closure properties, and practical decision procedures. We then present an algorithm for solving QSyGuS. Our algorithm leverages closure properties of weighted grammars to generate intermediate problems that can be solved using non-quantitative \sygus solvers. Finally, we implement our algorithm in a tool, QuaSi, and evaluate it on 26 quantitative extensions of existing SyGuS benchmarks. QuaSi can synthesize optimal solutions in 15/26 benchmarks with times comparable to those needed to not find an arbitrary solution.

Jul 14 17:45

ABSTRACT. Many example-guided program synthesis techniques use abstractions to prune the search space. While abstraction-based synthesis has proven to be very powerful, a domain expert needs to provide a suitable abstract domain, together with the abstract transformers of each DSL construct. However, coming up with useful abstractions can be non-trivial, as it requires both domain expertise and knowledge about the synthesizer. In this paper, we propose a new technique for learning abstractions that are useful for instantiating a general synthesis framework in a new domain. Given a DSL and a small set of training problems, our method uses tree interpolation to infer reusable predicate templates that speed up synthesis in a given domain. Our method also learns suitable abstract transformers by solving a certain kind of second-order constraint solving problem in a data-driven way. We have implemented the proposed method in a tool called Atlas and evaluate it in the context of the Blaze meta-synthesizer. Our evaluation shows that (a) Atlas can learn useful abstract domains and transformers from few training problems, and (b) the abstractions learned by Atlas allow Blaze to achieve significantly better results compared to manually-crafted abstractions.

Jul 15 10:00

ABSTRACT. Symbolic automata allow transitions to carry predicates over rich alphabet theories, such as linear arithmetic, and therefore extend classic automata to operate over infinite alphabets, such as the set of rational numbers. In this paper, we study the problem of learning symbolic automata and exactly characterize under what conditions symbolic automata can be learned efficiently. First, we present \alg, an $L^*$-style algorithm for learning symbolic automata using membership and equivalence queries, which treats the predicates appearing on transitions as their own learnable entities. The main novelty of \alg is that it can take as input an algorithm $\Lambda$ for learning predicates in the underlying alphabet theory and it uses $\Lambda$ to infer the predicates appearing on the transitions in the target automaton. Using this idea, \alg is able to learn automata operating over alphabets theories in which predicates are efficiently learnable using membership and equivalence queries. Furthermore, we prove that a necessary condition for efficient learnability of an \SFA is that predicates in the underlying algebra are also efficiently learnable using queries. Our results settle the learnability of a large class of \SFA instances, leaving open only a subclass of \SFAs over efficiently learnable alphabet theories. We implement \alg in an open-source library and show that it can efficiently learn automata that cannot be learned using existing algorithms and significantly outperform existing automata learning algorithms over large alphabets.

Jul 15 10:15

ABSTRACT. We address the problem of analysing the reachable set of a nonlinear continuous system by computing or approximating the flow pipe of its dynamics. The most widely employed approach to tackle this problem is to perform a numerical integration over a given time horizon based on Taylor expansion and interval arithmetic. However, this method results to be very conservative when there is a large difference in speed between trajectories as time progresses. In this paper, we propose to use combinations of barrier functions, which we call piecewise barrier tube (PBT), to overapproximate flow pipe. The basic idea of PBT is that for each segment of a flow pipe, a coarse box which is big enough to contain the segment is constructed using sampled simulation and then in the box we compute by linear programming a set of barrier functions (called barrier tube or BT for short) which work together to form a tube surrounding the flow pipe. The benefit of using PBT is that (1) is independent of time and hence can avoid being stretched and deformed by time (2) a small number of BTs can form a tight overapproximation for the flow pipe, which means that the computation required to decide whether the BTs intersect the unsafe set can be reduced significantly. We implemented a prototype tool called PBTS in C++. Experiment on some benchmark systems show that our approach is effective.

Jul 15 11:00

ABSTRACT. Reachability analysis is difficult for hybrid automata with affine differential equations, because the reach set needs to be approximated. Promising abstraction techniques usually employ interval methods or template polyhedra. Interval methods account for dense time and guarantee soundness, and there are interval-based tools that overapproximate affine flowpipes. But interval methods impose bounded and rigid shapes, which make refinement expensive and fixpoint detection difficult. Template polyhedra, on the other hand, can be adapted flexibly and can be unbounded, but sound template refinement for unbounded reachability analysis has been implemented only for systems with piecewise constant dynamics. We capitalize on the advantages of both techniques, combining interval arithmetic and template polyhedra, using the former to abstract time and the latter to abstract space. During a CEGAR loop, whenever a spurious error trajectory is found, we compute additional space constraints and split time intervals, and use these spacetime interpolants to eliminate the counterexample. Spacetime interpolation offers a lazy, flexible framework for increasing precision while guaranteeing soundness, both for error avoidance and fixpoint detection. To the best of out knowledge, this is the first abstraction refinement scheme for the reachability analysis over unbounded and dense time of affine hybrid systems, which is both sound and automatic. We demonstrate the effectiveness of our algorithm with several benchmark examples, which cannot be handled by other tools.

Jul 15 11:15

ABSTRACT. High-performance implementations of distributed and multicore shared objects often guarantee only the weak consistency of their concurrent operations, foregoing the de-facto yet performance-restrictive consistency criterion of linearizability. While such weak consistency is often vital for achieving performance requirements, practical automation for checking weak-consistency is lacking. In principle, algorithmically checking the consistency of executions according to various weak-consistency criteria is hard: in addition to the enumeration of linearizations of an execution’s operations, such criteria generally demand the enumeration of possible visibility relations among the linearized operations; a priori, both enumerations are exponential.

In this work we identify an optimization to weak-consistency checking: rather than enumerating every possible visibility relation, it suffices to consider only the minimal visibility relations which adhere to the various constraints of the given criterion, for a significant class of consistency criteria. We demonstrate the soundness of this optimization, and describe an associated minimal-visibility consistency checking algorithm. Empirically, we show that our algorithm significantly outperforms the baseline weak-consistency checking algorithm, which naïvely enumerates all visibilities, and adds only modest overhead to the baseline linearizability checking algorithm, which does not enumerate visibilities.

Jul 15 11:30

ABSTRACT. This paper presents a numerical algorithm to verify continuous-time Markov chains (CTMCs) against multi-clock deterministic timed automata (DTA). These DTA allow for specifying properties that cannot be expressed in CSL, the logic for CTMCs used by state-of-the-art probabilistic model checkers. The core problem is to compute the probability of timed runs by the CTMC ${\cal C}$ that are accepted by the DTA ${\cal A}$. These likelihoods equal reachability probabilities in an embedded piecewise deterministic Markov process (EPDP) obtained as product ofn${\cal C}$ and ${\cal A}$'s region automaton. This paper provides a numerical algorithm to efficiently solve the PDEs describing these reachability probabilities. The key insight is to solve an ordinary differential equation (ODE) that exploits the specific characteristics of the product EPDP. We provide the numerical precision of our algorithm and present experimental results with a prototypical implementation.

Jul 15 11:45

ABSTRACT. Partial order reduction for timed systems is a challenging topic due to the dependencies among events induced by time acting as a global synchronization mechanism. So far, there has only been a limited success in finding practically applicable solutions yielding significant state space reductions. We suggest a working and efficient method to facilitate stubborn set reduction for timed systems with urgent behaviour. We first describe the framework in the general setting of timed labelled transition systems and then instantiate it to the case of timed-arc Petri nets. The basic idea is that we can employ classical untimed partial order reduction techniques as long as urgent behaviour is enforced. Our solution is implemented in the model checker TAPAAL and the feature is now broadly available to the users of the tool in its latest release from January 2018. By a series of larger case studies, we document the benefits of our method and its applicability to real-world scenarios.

Jul 15 12:00

ABSTRACT. We consider the problem of monitoring a Linear Time Logic (LTL) specification that is defined on infinite paths, over finite traces. For example, we may need to draw a verdict on whether the system satisfies or violates the property ``p holds infinitely often.'' The problem is that there is always a continuation of a finite trace that satisfies the property and a different continuation that violates it.

We propose a two-step approach to address this problem. First, we introduce a counting semantics that computes the number of steps to witness the satisfaction or violation of a formula for each position in the trace. Second, we use this information to make a prediction on inconclusive suffixes. In particular, we consider a \emph{good} suffix to be one that is shorter than the longest witness for a satisfaction, and a \emph{bad} suffix to be shorter than or equal to the longest witness for a violation. Based on this assumption, we provide a verdict assessing whether a continuation of the execution on the same system will presumably satisfy or violate the property.

Jul 15 12:15

ABSTRACT. We present Rabinizer 4, a tool set for translating formulae of linear temporal logic to different types of deterministic omega-automata. The tool set implements and optimizes several recent constructions, including the first implementation translating the frequency extension of LTL. Further, we provide a distribution of PRISM that links Rabinizer and offers model checking procedures for probabilistic systems that are not in the official PRISM distribution. Finally, we evaluate the performance and in cases with any previous implementations we show enhancements both in terms of the size of the automata and the computational time, due to algorithmic as well as implementation improvements.

Jul 15 14:00

ABSTRACT. Strix is a new tool for reactive LTL synthesis combining a direct translation of LTL formulas into deterministic parity automata (DPA) and an efficient, multi-threaded explicit state solver for parity games. In brief, Strix (1) decomposes the given formula into simpler formulas, (2) translates these on-the-fly into DPAs based on the queries of the parity game solver, (3) composes the DPAs into a parity game, and at the same time already solves the intermediate games using strategy iteration, and (4) finally translates the wining strategy, if it exists, into a Mealy automaton or an AIGER cicuit with optional minimization using external tools. We experimentally demonstrate the applicability of our approach by a comparison with Party, BoSy and ltlsynt using the SYNTCOMP2017 benchmarks. In these experiments, our prototype can compete with BoSy and ltlsynt with only Party performing slightly better. In particular, our prototype successfully synthesizes the full and unmodified LTL specification of the AMBA protocol for n = 2 masters.

Jul 15 14:15

ABSTRACT. We describe a new word-level model checking format to capture models of hardware and potentially software in a bit-precise manner. This simple, line-based and easy to parse format can be seen as a sorted extension of the word-level format BTOR. It uses design principles from the bit-level format AIGER and follows the semantics of the SMT-LIB logics of bit-vectors with arrays. This intermediate format can be used in various verification flows and is perfectly suited to establish a word-level model checking competition. A new open source model checker and bit-vector solver support this format. We further provide real-world word-level benchmarks on which these tools are evaluated.

Jul 15 14:30

ABSTRACT. We present Nagini, an automated, modular verifier for statically-typed, concurrent Python 3 programs, built on the Viper verification infrastructure. Combining established concepts with new ideas, Nagini can verify memory safety, functional properties, termination, deadlock freedom, and input/output behavior. Our experiments show that Nagini is able to verify non-trivial properties of real-world Python code.

Jul 15 14:45

ABSTRACT. We introduce Peregrine, the first tool for the analysis and parameterized verification of population protocols. Population protocols are a model of computation very much studied by the distributed computing community, in which mobile anonymous agents interact stochastically to achieve a common task. Peregrine allows users to design protocols, to simulate them both manually and automatically, to gather statistics of properties such as convergence speed, and to verify correctness automatically. This paper describes the features of Peregrine and their implementation.

Jul 15 15:00

ABSTRACT. Approximate circuits with relaxed requirements on functional correctness play an important role in the development of resource-efficient computer systems. Designing approximate circuits is a very complex and time-demanding process trying to find optimal trade-offs between the approximation error and resource savings. In this paper, we present ADAC -- a novel framework for automated design of approximate arithmetic circuits. ADAC integrates in a unique way efficient simulation and formal methods for approximate equivalence checking into a search-based circuit optimisation. To make ADAC easily accessible, it is implemented as a module of the ABC tool: a state-of-the-art system for circuit synthesis and verification. Within several hours, ADAC is able to construct high-quality Pareto sets of complex circuits (including even 32-bit multipliers), providing useful trade-offs between the resource consumption and the error that is formally guaranteed. This demonstrates outstanding performance and scalability compared with other existing approaches

Jul 15 15:15

ABSTRACT. Simple stochastic games can be solved by value iteration (VI), which yields a sequence of under-approximations of the value of the game. This sequence is guaranteed to converge to the value only in the limit. Since no stopping criterion is known, this technique does not provide any guarantees on its results. We provide the first stopping criterion for VI on simple stochastic games. It is achieved by additionally computing a convergent sequence of over-approximations of the value, relying on an analysis of the game graph. Consequently, VI becomes an anytime algorithm returning the approximation of the value and the current error bound. As another consequence, we can provide a simulation-based asynchronous VI algorithm, which yields the same guarantees, but without necessarily exploring the whole game graph.

Jul 15 16:00

ABSTRACT. Computing reachability probabilities is at the heart of probabilistic model checking. All model checkers compute these probabilities in an iterative fashion using value iteration. This technique approximates a fixed point from below by determining reachability probabilities for an increasing number of steps. To avoid results that are significantly off, variants have recently been proposed that converge from both below and above. These procedures require starting values for both sides. We present an alternative that does not require the a priori computation of starting vectors and that converges faster on many benchmarks. The crux of our technique is to give tight and safe bounds — whose computation is cheap — on the reachability probabilities. Lifting this technique to expected rewards is trivial for both Markov chains and MDPs. Experimental results on a large set of benchmarks show its scalability and efficiency.

Jul 15 16:15

ABSTRACT. Apprenticeship learning (AL) is a class of “learning from demonstrations” techniques where the reward function of a Markov Decision Process (MDP) is unknown to the learning agent and the agent has to derive a good policy by observing an expert’s demonstrations. In this paper, we study the problem of how to make AL algorithms inherently safe while still meeting its learning objective. We consider a setting where the unknown reward function is assumed to be a linear combination of a set of state features, and the safety property is specified in Probabilistic Computation Tree Logic (PCTL). By embedding probabilistic model checking inside AL, we propose a novel counterexample-guided approach that can ensure both safety and performance of the learnt policy. We demonstrate the effectiveness of our approach on several challenging AL scenarios where safety is essential.

Jul 15 16:30

ABSTRACT. Probabilistic bisimilarity, due to Larsen and Skou, is an equivalence relation that captures which states of a labelled Markov chain behave the same. Since this behavioural equivalence only identifies states that transition to states that behave exactly the same with exactly the same probability, this notion of equivalence is not robust, as first observed by Giacalone, Jou and Smolka. The probabilistic bisimilarity distances, as first defined by Desharnais, Gupta, Jagadeesan and Panangaden, provide a quantitative generalization of probabilistic bisimilarity. The distance of states captures the similarity of their behaviour. The smaller the distance, the more alike the states behave. In particular, states are probabilistic bisimilar if and only if their distance is zero. This quantitative notion is robust in that small changes in the transition probabilities result in small changes in the distances.

During the last decade, several algorithms have been proposed to approximate and compute the probabilistic bisimilarity distances. The main result of this paper is an algorithm that decides distance one in O(n^2 + m^2), where n is the number of states and m is the number of transitions of the labelled Markov chain. The algorithm is the key new ingredient of our algorithm to compute the distances. The state of the art algorithm, that combines algorithms due to Derisavi, Hermanns and Sanders and due to Bacci, Bacci, Larsen and Mardare, can compute distances for labelled Markov chains up to 150 states. For one such labelled Markov chain, that algorithm takes more than 49 hours. In contrast, our new algorithm only takes 13 milliseconds. Furthermore, our algorithm can compute distances for labelled Markov chains with more than 10,000 states in less than 50 minutes.

Jul 15 16:45

ABSTRACT. We present a graph-based tool for analysing Java programs operating on dynamic data structures. It involves the generation of an abstract state space employing a user-defined graph grammar. LTL model checking is then applied to this state space, supporting both structural and functional correctness properties. The analysis is fully automated, procedure-modular, and provides informative visual feedback including counterexamples in the case of property violations.

Jul 16 09:00

ABSTRACT. We present Typpete, a sound type inferencer that automatically infers Python 3 type annotations. Typpete encodes type constraints as a MaxSMT problem and uses a system of optional constraints and specific quantifier instantiation patterns to make the constraint solving process efficient. Our experimental evaluation shows that Typpete scales to real world Python programs and outperforms state-of-the-art tools.

Jul 16 09:15

ABSTRACT. JKind is an open-source industrial model checker. JKind uses multiple parallel engines to prove or falsify safety properties of infinite state models. It is portable, easy to install, performance competitive with other state-of-the-art model checkers, and has features designed to improve the results presented to users: *inductive validity cores* for proofs and *counterexample smoothing* for test-case generation. It serves as the back-end for various industrial applications.

Jul 16 09:30

ABSTRACT. In this paper we describe the DeepSec prover, a tool for security-protocol analysis deciding equivalence properties, modelled as trace equivalence of two processes in a dialect of the applied pi calculus.

Jul 16 09:45

ABSTRACT. We present a new safety hardware model checker SimpleCAR that serves as the “bottom-line” for evaluating and extending Complementary Approximate Reachability (CAR), a new SAT-based model checking framework inspired by classical reachability analysis. We demonstrate the performance of SimpleCAR on challenging benchmarks from the Hardware Model Checking Competition. Our experiments indicate that SimpleCAR is particularly suited for unsafety checking, or bug-finding; it is able to solve 7 unsafe instances within 1 hour that are not solvable by any other state-of-the-art technique, including BMC and IC3/PDR, within 8 hours. We also report 1 bug and 48 counterexample generation errors in the model checkers compared in our analysis.

Jul 16 10:00

ABSTRACT. In this paper, we introduce StringFuzz: a modular SMT- LIB problem instance transformer and generator for string solvers. We supply a repository of instances generated by StringFuzz in SMT-LIB 2.0/2.5 format. We systematically compare Z3str3, CVC4, Z3str2, and Norn on groups of such instances, and identify those that are particularly challenging for some solvers. We briefly explain our observations and show how StringFuzz helped discover causes of performance degradations in Z3str3.

Jul 16 10:15

ABSTRACT. Information about the memory locations accessed by a program is, for instance, required for program parallelisation and program verification. Existing inference techniques for this information provide only partial solutions for the important class of array-manipulating programs. In this paper, we present a static analysis that infers the memory footprint of an array program in terms of permission pre- and postconditions as used, for example, in separation logic. This formulation allows our analysis to handle concurrent programs and produces specifications that can be used by verification tools.

Our analysis expresses the permissions required by a loop via maximum expressions over the individual loop iterations. These maximum expressions are then solved by a novel maximum elimination algorithm, in the spirit of quantifier elimination.

Our approach is sound and is implemented; an evaluation on existing benchmarks for memory safety of array programs demonstrates accurate results, even for programs with complex access patterns and nested loops.

Jul 16 12:00

ABSTRACT. We study from a computability perspective static program analysis, namely detecting sound program assertions, and verification, namely sound checking of program assertions. We first provide a general computability model for domains of program assertions and corresponding program analysers and verifiers. Next, we formalize and prove an instantiation of Rice's Theorem for static program analysis and verification. Then, within this general model, we provide and show a precise statement of the popular belief that program analysis is a harder problem than program verification: we prove that for finite domains of program assertions, program analysis and verification are equivalent problems, while for infinite domains, program analysis is strictly harder than verification.

Jul 16 12:15

ABSTRACT. The problem of {\em quantitative inclusion} formalizes the goal of comparing quantitative dimensions between systems such as worst-case execution time, resource consumption, and the like. Such systems are typically represented by formalisms such as weighted logics or weighted automata. Despite its significance in analyzing the quality of computing systems, the study of quantitative inclusion has mostly been conducted from a theoretical standpoint. In this work, we conduct the first empirical study of quantitative inclusion for discounted-sum weighted automata (\DS-inclusion, in short).

Currently, two contrasting approaches for \DS-inclusion exist: the linear-programming based \cct{DetLP} and the purely automata-theoretic \cct{BCV}. Theoretical complexity of \DetLPt is exponential in time and space while of \BCVt is \cct{PSPACE}-complete. All practical implementations of \cct{BCV}, however, are also exponential in time and space. Hence, it is not clear which of the two algorithms renders a superior implementation.

In this work we present the first implementations of these algorithms, and perform extensive experimentation to compare between the two approaches. Our empirical analysis shows how the two approaches complement each other. This is a nuanced picture that is much richer than the one obtained from the theoretical study alone.

Jul 17 09:00

ABSTRACT. The design of security protocols is extremely subtle and vulnerable to potentially devastating flaws. As result, many tools and techniques for the automated verification of protocol designs have been developed. Unfortunately, these tools don't have the ability to model and reason about protocols with randomization, which are becoming increasingly prevalent in systems providing privacy and anonymity guarantees. The security metrics of these systems is often formulated through a notion of indistinguishability. In this paper, we give the first algorithms for model checking indistinguishability properties of randomized security protocols against the powerful threat model of a Dolev-Yao attacker. Our techniques are implemented in the Stochastic Protocol ANalayzer (SPAN) and evaluated on several examples. As part of our evaluation, we conduct the first automated analysis of an electronic voting protocol based on the 3-ballot design.

Jul 17 09:15

ABSTRACT. The secure information flow problem, which checks whether low-security outputs of a program are influenced by high-security inputs, has many applications in verifying security properties in programs. In this paper we present lazyself-composition, an approach for verifying secure information flow. It is based on self-composition, where two copies of a program are created on which a safety property is checked. However, rather than an eager duplication of the given program, it uses duplication lazily to reduce the cost of verification. This lazy self-composition is guided by an interplay between symbolic taint analysis on an abstract (single copy) model and safety verification on a refined (two copy) model. We propose two verification methods based on lazy self-composition.The first is a CEGAR-style procedure, where the abstract model associated with taint analysis is refined, on demand, by using a model generated by lazy self-composition. The second is a method based on bounded model checking, where taint queries are generated dynamically during program unrolling to guide lazy self-composition and to conclude an adequate bound for correctness. We have implemented these methods on top of the SEAHORN verification platform and our evaluations show the effectiveness of lazy self-composition.

Jul 17 09:30

ABSTRACT. Power side-channel attacks, capable of deducing secret using statistical analysis techniques, have become a serious threat to devices in cyber-physical systems and the Internet of things. Random masking is a widely used countermeasure for removing the statistical dependence between secret data and side-channel leaks. Although there are some techniques for verifying whether software code has been perfectly masked, they are limited in accuracy and scalability.

To bridge this gap, we propose a refinement-based method for formally verifying masking countermeasures. Our method is more accurate than prior syntactic type inference rule based approaches and more scalable than model-counting based approaches using SAT/SMT solvers.

Indeed, our method can be viewed as a gradual refinement of a set of carefully designed semantic type inference rules for reasoning about distribution types. These rules are kept abstract initially to allow fast deduction, and then made more concrete if necessary, i.e., when the abstract version is not able to completely resolve the verification problem.

We have implemented our method in a tool named SCInfer and evaluated it on cryptographic benchmarks including AES and MAC-Keccak. Our results show that SCInfer significantly outperforms state-of-the-art techniques in terms of both accuracy and scalability.

Jul 17 09:45

ABSTRACT. Given a model and a specification, the fundamental model-checking problem asks for algorithmic verification of whether the model satisfies the specification. We consider graphs and Markov decision processes (MDPs), which are fundamental models for reactive systems. One of the very basic specifications that arise in verification of reactive systems is the strong fairness (aka Streett) objective. Given different types of requests and corresponding grants, the objective requires that for each type, if the request event happens infinitely often, then the corresponding grant event must also happen infinitely often. All \omega-regular objectives can be expressed as Streett objectives and hence they are canonical in verification. To handle the state-space explosion, symbolic algorithms are required that operate on a succinct implicit representation of the system rather than explicitly accessing the system. While explicit algorithms for graphs and MDPs with Streett objectives have been widely studied, there has been no improvement of the basic symbolic algorithms. The worst-case numbers of symbolic steps required for the basic symbolic algorithms are as follows: quadratic for graphs and cubic for MDPs. In this work we present the first sub-quadratic symbolic algorithm for graphs with Streett objectives, and our algorithm is sub-quadratic even for MDPs. Based on our algorithmic insights we present an implementation of the new symbolic approach and show that it improves the existing approach on several academic benchmark examples.

Jul 17 10:00

ABSTRACT. Parity games have important practical applications in formal verification and synthesis, especially to solve the model-checking problem of the modal mu-calculus. They are also interesting from the theory perspective, because they are widely believed to admit a polynomial solution, but so far no such algorithm is known.

We propose a new algorithm to solve parity games based on learning tangles, which are strongly connected subgraphs for which one player has a strategy to win all cycles in the subgraph. We argue that tangles play a fundamental role in the prominent parity game solving algorithms. We show that tangle learning is competitive in practice and the fastest solver for large random games.

Jul 17 10:15

ABSTRACT. Solving nonlinear SMT problems over real numbers has wide applications in robotics and AI. While significant progress is made in solving quantifier-free SMT formulas in the domain, quantified formulas have been much less investigated. We propose the first delta-complete algorithm for solving satisfiability of nonlinear SMT over real numbers with universal quantification and a wide range of nonlinear functions. Our methods combine ideas from counterexample-guided synthesis, interval constraint propagation, and local optimization. In particular, we show how special care is required in handling the interleaving of numerical and symbolic reasoning to ensure delta-completeness. In experiments, we show that the proposed algorithms can handle many new problems beyond the reach of existing SMT solvers.

Jul 17 11:00

ABSTRACT. We present a novel approach for solving quantified bit-vector formulas in Satisfiability Modulo Theories (SMT) based on computing symbolic inverses of bit-vector operators. We derive conditions that precisely characterize when bit-vector constraints are invertible for a representative set of bit-vector operators commonly supported by SMT solvers. We utilize syntax-guided synthesis techniques to aid in establishing these conditions and verify them independently by using several SMT solvers. We show that invertibility conditions can be embedded into quantifier instantiations using Hilbert choice expressions and give experimental evidence that a counterexample-guided approach for quantifier instantiation utilizing these techniques leads to performance improvements with respect to state-of-the-art solvers for quantified bit-vector constraints.

Jul 17 11:15

ABSTRACT. Incremental determinization is a recently proposed algorithm for solving quantified Boolean formulas with one quantifier alternation. In this paper, we formalize incremental determinization as a set of inference rules to help understand the design space of similar algorithms. We then present additional inference rules that extend incremental determinization in two ways. The first extension integrates the popular CEGAR principle and the second extension allows us to expand cases and analyze them in separation. The experimental evaluation demonstrates that the extensions significantly improve the performance.

Jul 17 11:30

ABSTRACT. The resolution proof system has been enormously helpful in deepening our understanding of conflict-driven clause-learning (CDCL) SAT solvers. In the interest of providing a similar proof complexity-theoretic analysis of satisfiability modulo theories (SMT) solvers, we introduce a generalization of resolution called Res(T). We show that many of the known results comparing resolution and CDCL solvers lift to the SMT setting, such as the result of Pipatsrisawat and Darwiche showing that CDCL solvers with ``perfect'' non-deterministic branching and an asserting clause-learning scheme can polynomially simulate general resolution. We also describe a stronger version of Res(T), Res*(T), capturing SMT solvers allowing introduction of new literals.

We analyze the theory EUF of equality with uninterpreted functions, and show that the Res*(EUF) system is able to simulate an earlier calculus introduced by Bjorner and De Moura for the purpose of analyzing DPLL(EUF). Further, we show that Res*(EUF) (and thus SMT algorithms with clause learning over EUF, new literal introduction rules and perfect branching) can simulate the Frege proof system, which is well-known to be far more powerful than resolution. Finally, we prove under the Exponential Time Hypothesis (ETH) that any reduction from EUF to SAT (such as the Ackermann reduction) must, in the worst case, produce an instance of size at least n log n from an instance of size n.

Jul 17 11:45

ABSTRACT. We focus in this paper on generating models of quantified first-order formulas over built-in theories, which is paramount in software verification and bug finding. While standard methods are either geared toward proving the absence of solution or targeted to specific theories, we propose a generic approach based on a reduction to the quantifier-free case. Our technique allows thus to reuse all the efficient machinery developed for that context. Experiments show a substantial improvement over state-of-the-art methods.

Jul 17 12:00

ABSTRACT. We present POCS, a concurrency testing approach that directly samples the partial orders of a concurrent program. POCS uses a novel priority-based scheduling algorithm that naturally considers partial order information dynamically, and guarantees that each partial order will be explored with significant probability. This probabilistic guarantee of error detection is exponentially better state-of-the-art sampling approaches. Besides theoretical guarantees, POCS is extremely simple and lightweight to implement. Evaluation shows that POCS is effective in covering the partial-order space of micro-benchmarks and finding concurrency bugs in real-world programs such as Firefox’s JavaScript engine SpiderMonkey

Jul 17 14:00

ABSTRACT. We present a method for proving that a program running under the Total Store Ordering (TSO) memory model is robust, i.e., all its TSO computations are equivalent to computations under the Sequential Consistency (SC) semantics. This method is inspired by Lipton's reduction theory for proving atomicity of concurrent programs. For programs which are not robust, we introduce an abstraction mechanism that allows to construct robust programs over-approximating their TSO semantics. This enables the use of proof methods designed for the SC semantics in proving invariants that hold on the TSO semantics of a non-robust program. These techniques have been evaluated on a large set of benchmarks using the infrastructure provided by CIVL, a generic tool for reasoning about concurrent programs under the SC semantics.

Jul 17 14:15

ABSTRACT. A dynamic partial order reduction (DPOR) algorithm is optimal when it always explores at most one representative per Mazurkiewicz trace. Existing literature suggests the reduction obtained by the non-optimal state-of-the-art Source-DPOR (SDPOR) algorithm is comparable to optimal DPOR. We show the first program with O(n) Mazurkiewicz traces where SDPOR explores O(2^n) redundant schedules and identify the cause of the blow-up as an NP-hard problem. Our main contribution is a new approach, called Quasi-Optimal POR, that can arbitrarily approximate an optimal exploration using a provided constant k. We present an implementation of our method in a new tool called DPU using specialized data structures. Experiments with DPU, including Debian packages, show that optimality is achieved with low values of k, outperforming state-of-the-art tools.

Jul 17 14:30

ABSTRACT. We address the problem of verifying message passing programs, defined as a set of parallel processes communicating through unbounded FIFO buffers. We introduce a bounded analysis that explores a special type of computations, called k-synchronous. These computations can be viewed as (unbounded) sequences of interaction phases, each phase allowing at most k send actions (by different processes), followed by a sequence of receives corresponding to sends in the same phase. We give a procedure for deciding k-synchronizability of a program, i.e., whether every computation is equivalent (has the same happens-before relation) to one of its k-synchronous computations. We also show that reachability over k-synchronous computations and checking k-synchronizability are both PSPACE-complete. Furthermore, we introduce a class of programs called {\em flow-bounded} for which the problem of deciding whether there exists a k>0 for which the program is k-synchronizable, is decidable.

Jul 17 14:45

ABSTRACT. The cornerstone of dynamic partial order reduction (DPOR) is the notion of independence that is used to decide whether each pair of concurrent events p and t are in a race and thus both p.t and t.p must be explored. We present constrained dynamic partial order reduction (CDPOR), an extension of the DPOR framework which is able to avoid redundant explorations based on the notion of conditional independence --the execution of p and t commutes only when certain independence constraints (ICs) are satisfied. ICs can be declared by the programmer, but importantly, we present a novel SMT-based approach to automatically synthesize ICs in a static pre-analysis. A unique feature of our approach is that we have succeeded to exploit ICs within the state-of-the-art DPOR algorithm, achieving exponential reductions over existing implementations.

Jul 17 15:00

ABSTRACT. Vehicle-to-Vehicle (V2V) communications is a ``connected vehicles'' standard that will likely be mandated in the U.S. within the coming decade. V2V, in which automobiles broadcast to one another, promises improved safety by providing collision warnings, but it also poses a security risk. At the heart of V2V is the communication messaging system, specified in SAE J2735 using the Abstract Syntax Notation One (ASN.1) data description language. Motivated by numerous previous ASN.1 related vulnerabilities, we present the formal verification of an ASN.1 encoder/decoder pair. We describe generating an encoder/decoder pair, implemented in C, using our internally developed ASN.1 toolset. We define self-consistency for encoder/decoder pairs that approximates functional correctness without requiring a formal specification of ASN.1. We then verify self-consistency and memory-safety using symbolic simulation via the Software Analysis Workbench.

Jul 17 16:00

ABSTRACT. We describe formal verification of the open source TLS implementation used in numerous Amazon services, ranging from the Amazon Simple Storage Service (S3) to the Amazon Web Services Software Development Kit (AWS SDK). A key aspect of this proof is continuous checking: to ensure that properties proved remain proved during the lifetime of the software, at each change to the code, proofs are automatically re-established with little to no interaction from the development community. Technical decisions made about the structure of the proof and tools employed were driven by this goal.

Jul 17 16:15

ABSTRACT. Liveness violation bugs are notoriously hard to detect, especially due to the difficulty inherent in applying formal methods to real-world programs. We present a generic and practically useful liveness property which defines a program as being live as long as it will eventually either consume more input or terminate. We show that this property naturally maps to many different kinds of real-world programs. To demonstrate the usefulness of our liveness property, we also present an algorithm that can be efficiently implemented to dynamically find fixed points of the target program during Symbolic Execution. This extends Symbolic Execution, a well known dynamic testing technique, to find a new class of program defects, namely liveness violations, while only incurring a small runtime and memory overhead, as evidenced by our evaluation. The implementation of our method found a total of five previously undiscovered software defects in BusyBox and the GNU Coreutils. All five defects have been confirmed and fixed by the respective maintainers after shipping for years, most of them well over a decade.

Jul 17 16:30

ABSTRACT. This paper describes our experience with symbolic model checking in an industrial setting. We have proved that the initial boot code running in data centers at Amazon Web Services is memory safe, an essential step in establishing the security of any data center. Standard static analysis tools cannot be easily used on boot code without modification due to issues not commonly found in higher-level code, including memory-mapped device interfaces, byte-level memory access, and linker scripts. This paper describes automated solutions to these issues and their implementation in the C Bounded Model Checker (CBMC). CBMC is now the first source-level static analysis tool to extract the memory layout described in a linker script for use in its analysis.

Jul 17 16:45

ABSTRACT. In this paper, we propose Android Stack Systems (ASS), a formal model to capture key mechanisms of Android multi-tasking such as activities, the back stack, launch modes, as well as task affinities. The model is based on pushdown systems with multiple stacks, and focuses on the evolution of the back stack of the Android system when interacting with activities carrying specific launch modes and task affinities. For formal analysis, we study the reachability problem of ASS. While the general problem is shown to be undecidable, we identify expressive fragments for which various verification techniques for pushdown systems or their decidable extensions are harnessed to show that the reachability problem is decidable.

Jul 17 17:00

ABSTRACT. We report on a machine assisted verification of an efficient implementation of Montgomery Multiplication which is a widely used method in cryptography for efficient computation of modular exponentiation. We shortly describe the method, give a brief survey of the VeriFun system used for verification, verify a classical as well as a more recent algorithm for computing multiplicative inverses, illustrate proof-technical obstacles encountered upon verification, present the formal proofs and finally report on the effort when creating the proofs. Our work uncovered a serious fault in a state-of-the-art algorithm for computing multiplicative inverses based on Newton-Raphson iteration, thus providing further evidence for the benefit of computer-aided verification.

Jul 17 17:15

ABSTRACT. Delay differential equations are fundamental for modelingvnetworked control systems where the underlying network induces delay for retrieving values from sensors or delivering orders to actuators. They are notoriously difficult to integrate, as these are actually functional equations, the initial state being a function. We propose a scheme to compute inner and outer approximated flowpipes for such equations with uncertain initial states and parameters. Inner-approximated flowpipes are guaranteed to contain only reachable states, while outer-approximated flowpipes enclose all reachable states. We also introduce a notion of robust inner-approximation, which we believe opens promising perspectives for verification, beyond property falsification. The efficiency of our approach relies on the combination of Taylor models in time, with an abstraction or parameterization in space based on affine forms, or zonotopes. It also relies on an extension of the mean-value theorem, which allows us to deduce inner-approximated flowpipes, from flowpipes outerapproximating the solution of the DDE and its Jacobian with respect to constant but uncertain parameters and initial conditions. We present some experimental results obtained with our C++ implementation.

Jul 17 17:30