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Open Access 2021 | Open Access | Book

# Tools and Algorithms for the Construction and Analysis of Systems

## 27th International Conference, TACAS 2021, Held as Part of the European Joint Conferences on Theory and Practice of Software, ETAPS 2021, Luxembourg City, Luxembourg, March 27 – April 1, 2021, Proceedings, Part II

Editors: Prof. Jan Friso Groote, Prof. Kim Guldstrand Larsen

Publisher:

Book Series: Lecture Notes in Computer Science

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This open access two-volume set constitutes the proceedings of the 27th International Conference on Tools and Algorithms for the Construction and Analysis of Systems, TACAS 2021, which was held during March 27 – April 1, 2021, as part of the European Joint Conferences on Theory and Practice of Software, ETAPS 2021. The conference was planned to take place in Luxembourg and changed to an online format due to the COVID-19 pandemic.

The total of 41 full papers presented in the proceedings was carefully reviewed and selected from 141 submissions. The volume also contains 7 tool papers; 6 Tool Demo papers, 9 SV-Comp Competition Papers.

The papers are organized in topical sections as follows:

Part I: Game Theory; SMT Verification; Probabilities; Timed Systems; Neural Networks; Analysis of Network Communication.

Part II: Verification Techniques (not SMT); Case Studies; Proof Generation/Validation; Tool Papers; Tool Demo Papers; SV-Comp Tool Competition Papers.

#### Verification Techniques (not SMT)

Open Access

##### Directed Reachability for Infinite-State Systems

Numerous tasks in program analysis and synthesis reduce to deciding reachability in possibly infinite graphs such as those induced by Petri nets. However, the Petri net reachability problem has recently been shown to require non-elementary time, which raises questions about the practical applicability of Petri nets as target models. In this paper, we introduce a novel approach for efficiently semi-deciding the reachability problem for Petri nets in practice. Our key insight is that computationally lightweight over-approximations of Petri nets can be used as distance oracles in classical graph exploration algorithms such as $$\mathsf {A}^{*}$$ A ∗ and greedy best-first search. We provide and evaluate a prototype implementation of our approach that outperforms existing state-of-the-art tools, sometimes by orders of magnitude, and which is also competitive with domain-specific tools on benchmarks coming from program synthesis and concurrent program analysis.

Michael Blondin, Christoph Haase, Philip Offtermatt

Open Access

##### Bridging Arrays and ADTs in Recursive Proofs

We present an approach to synthesize relational invariants to prove equivalences between object-oriented programs. The approach bridges the gap between recursive data types and arrays that serve to represent internal states. Our relational invariants are recursively-defined, and thus are valid for data structures of unbounded size. Based on introducing recursion into the proofs by observing and lifting the constraints from joint methods of the two objects, our approach is fully automatic and can be seen as an algorithm for solving Constrained Horn Clauses (CHC) of a specific sort. It has been implemented on top of the SMT-based CHC solver AdtChc and evaluated on a range of benchmarks.

Grigory Fedyukovich, Gidon Ernst

Open Access

##### A Two-Phase Approach for Conditional Floating-Point Verification

Tools that automatically prove the absence or detect the presence of large floating-point roundoff errors or the special values NaN and Infinity greatly help developers to reason about the unintuitive nature of floating-point arithmetic. We show that state-of-the-art tools, however, support or provide non-trivial results only for relatively short programs. We propose a framework for combining different static and dynamic analyses that allows to increase their reach beyond what they can do individually. Furthermore, we show how adaptations of existing dynamic and static techniques effectively trade some soundness guarantees for increased scalability, providing conditional verification of floating-point kernels in realistic programs.

Debasmita Lohar, Clothilde Jeangoudoux, Joshua Sobel, Eva Darulova, Maria Christakis

Open Access

##### Symbolic Coloured SCC Decomposition

Problems arising in many scientific disciplines are often modelled using edge-coloured directed graphs. These can be enormous in the number of both vertices and colours. Given such a graph, the original problem frequently translates to the detection of the graph’s strongly connected components, which is challenging at this scale.We propose a new, symbolic algorithm that computes all the monochromatic strongly connected components of an edge-coloured graph. In the worst case, the algorithm performs $$O(p\cdot n\cdot \log n)$$ O ( p · n · log n ) symbolic steps, where p is the number of colours and n the number of vertices. We evaluate the algorithm using an experimental implementation based on Binary Decision Diagrams (BDDs) and large (up to $$2^{48}$$ 2 48 ) coloured graphs produced by models appearing in systems biology.

Nikola Beneš, Luboš Brim, Samuel Pastva, David Šafránek

#### Case Studies

Open Access

##### Local Search with a SAT Oracle for Combinatorial Optimization

NP-hard combinatorial optimization problems are pivotal in science and business. There exists a variety of approaches for solving such problems, but for problems with complex constraints and objective functions, local search algorithms scale the best. Such algorithms usually assume that finding a non-optimal solution with no other requirements is easy. However, what if it is NP-hard? In such case, a SAT solver can be used for finding the initial solution, but how can one continue solving the optimization problem? We offer a generic methodology, called Local Search with SAT Oracle (LSSO), to solve such problems. LSSO facilitates implementation of advanced local search methods, such as variable neighbourhood search, hill climbing and iterated local search, while using a SAT solver as an oracle. We have successfully applied our approach to solve a critical industrial problem of cell placement and productized our solution at Intel.

Open Access

##### Analyzing Infrastructure as Code to Prevent Intra-update Sniping Vulnerabilities

Infrastructure as Code is a new approach to computing infrastructure management that allows users to leverage tools such as version control, automatic deployments, and program analysis for infrastructure configurations. This approach allows for faster and more homogeneous configuration of a complete infrastructure. Infrastructure as Code languages, such as CloudFormation or TerraForm, use a declarative model so that users only need to describe the desired state of the infrastructure. However, in practice, these languages are not processed atomically. During an upgrade, the infrastructure goes through a series of intermediate states. We identify a security vulnerability that occurs during an upgrade even when the initial and final states of the infrastructure are secure, and we show that those vulnerability are possible in Amazon’s AWS and Google Cloud. We call such attacks intra-update sniping vulnerabilities. In order to mitigate this shortcoming, we present a technique that detects such vulnerabilities and pinpoints the root causes of insecure deployment migrations. We implement this technique in a tool, Häyhä, that uses dataflow graph analysis. We evaluate our tool on a set of open-source CloudFormation templates and find that it is scalable and could be used as part of a deployment workflow.

Julien Lepiller, Ruzica Piskac, Martin Schäf, Mark Santolucito

#### Proof Generation/Validation

Open Access

##### Certifying Proofs in the First-Order Theory of Rewriting

The first-order theory of rewriting is a decidable theory for linear variable-separated rewrite systems. The decision procedure is based on tree automata techniques and recently we completed a formalization in the Isabelle proof assistant. In this paper we present a certificate language that enables the output of software tools implementing the decision procedure to be formally verified. To show the feasibility of this approach, we present FORT-h, a reincarnation of the decision tool FORT with certifiable output, and the formally verified certifier FORTify.

Fabian Mitterwallner, Alexander Lochmann, Aart Middeldorp, Bertram Felgenhauer

Open Access

##### Syntax-Guided Quantifier Instantiation

This paper presents a novel approach for quantifier instantiation in Satisfiability Modulo Theories (SMT) that leverages syntax-guided synthesis (SyGuS) to choose instantiation terms. It targets quantified constraints over background theories such as (non)linear integer, reals and floating-point arithmetic, bit-vectors, and their combinations. Unlike previous approaches for quantifier instantiation in these domains which rely on theory-specific strategies, the new approach can be applied to any (combined) theory, when provided with a grammar for instantiation terms for all sorts in the theory. We implement syntax-guided instantiation in the SMT solver CVC4, leveraging its support for enumerative SyGuS. Our experiments demonstrate the versatility of the approach, showing that it is competitive with or exceeds the performance of state-of-the-art solvers on a range of background theories.

Aina Niemetz, Mathias Preiner, Andrew Reynolds, Clark Barrett, Cesare Tinelli

Open Access

##### Making Theory Reasoning Simpler

Reasoning with quantifiers and theories is at the core of many applications in program analysis and verification. Whilst the problem is undecidable in general and hard in practice, we have been making large pragmatic steps forward. Our previous work proposed an instantiation rule for theory reasoning that produced pragmatically useful instances. Whilst this led to an increase in performance, it had its limitations as the rule produces ground instances which (i) can be overly specific, thus not useful in proof search, and (ii) contribute to the already problematic search space explosion as many new instances are introduced. This paper begins by introducing that specifically addresses these two concerns as it produces general solutions and it is a simplification rule, i.e. it replaces an existing clause by a ‘simpler’ one. Encouraged by initial success with this new rule, we performed an experiment to identify further common cases where the complex structure of theory terms blocked existing methods. This resulted in four further simplification rules for theory reasoning. The resulting extensions are implemented in the Vampire theorem prover and evaluated on SMT-LIB, showing that the new extensions result in a considerable increase in the number of problems solved, including 90 problems unsolved by state-of-the-art SMT solvers.

Giles Reger, Johannes Schoisswohl, Andrei Voronkov

Open Access

##### Deductive Stability Proofs for Ordinary Differential Equations

Stability is required for real world controlled systems as it ensures that those systems can tolerate small, real world perturbations around their desired operating states. This paper shows how stability for continuous systems modeled by ordinary differential equations (ODEs) can be formally verified in differential dynamic logic (dL). The key insight is to specify ODE stability by suitably nesting the dynamic modalities of dL with first-order logic quantifiers. Elucidating the logical structure of stability properties in this way has three key benefits: i) it provides a flexible means of formally specifying various stability properties of interest, ii) it yields rigorous proofs of those stability properties from dL’s axioms with dL’s ODE safety and liveness proof principles, and iii) it enables formal analysis of the relationships between various stability properties which, in turn, inform proofs of those properties. These benefits are put into practice through an implementation of stability proofs for several examples in KeYmaera X, a hybrid systems theorem prover based on dL.

Yong Kiam Tan, André Platzer

#### Tool Papers

Open Access

##### An SMT-Based Approach for Verifying Binarized Neural Networks

Deep learning has emerged as an effective approach for creating modern software systems, with neural networks often surpassing hand-crafted systems. Unfortunately, neural networks are known to suffer from various safety and security issues. Formal verification is a promising avenue for tackling this difficulty, by formally certifying that networks are correct. We propose an SMT-based technique for verifying binarized neural networks — a popular kind of neural network, where some weights have been binarized in order to render the neural network more memory and energy efficient, and quicker to evaluate. One novelty of our technique is that it allows the verification of neural networks that include both binarized and non-binarized components. Neural network verification is computationally very difficult, and so we propose here various optimizations, integrated into our SMT procedure as deduction steps, as well as an approach for parallelizing verification queries. We implement our technique as an extension to the Marabou framework, and use it to evaluate the approach on popular binarized neural network architectures.

Guy Amir, Haoze Wu, Clark Barrett, Guy Katz

Open Access

##### cake_lpr: Verified Propagation Redundancy Checking in CakeML

Modern SAT solvers can emit independently checkable proof certificates to validate their results. The state-of-the-art proof system that allows for compact proof certificates is propagation redundancy (PR). However, the only existing method to validate proofs in this system with a formally verified tool requires a transformation to a weaker proof system, which can result in a significant blowup in the size of the proof and increased proof validation time. This paper describes the first approach to formally verify PR proofs on a succinct representation; we present (i) a new Linear PR (LPR) proof format, (ii) a tool to efficiently convert PR proofs into LPR format, and (iii) cake_lpr, a verified LPR proof checker developed in CakeML. The LPR format is backwards compatible with the existing LRAT format, but extends the latter with support for the addition of PR clauses. Moreover, cake_lpr is verified using CakeML ’s binary code extraction toolchain, which yields correctness guarantees for its machine code (binary) implementation. This further distinguishes our clausal proof checker from existing ones because unverified extraction and compilation tools are removed from its trusted computing base. We experimentally show that LPR provides efficiency gains over existing proof formats and that the strong correctness guarantees are obtained without significant sacrifice in the performance of the verified executable.

Yong Kiam Tan, Marijn J. H. Heule, Magnus O. Myreen

Open Access

##### Deductive Verification of Floating-Point Java Programs in KeY

Deductive verification has been successful in verifying interesting properties of real-world programs. One notable gap is the limited support for floating-point reasoning. This is unfortunate, as floating-point arithmetic is particularly unintuitive to reason about due to rounding as well as the presence of the special values infinity and ‘Not a Number’ (NaN). In this paper, we present the first floating-point support in a deductive verification tool for the Java programming language. Our support in the KeY verifier handles arithmetic via floating-point decision procedures inside SMT solvers and transcendental functions via axiomatization. We evaluate this integration on new benchmarks, and show that this approach is powerful enough to prove the absence of floating-point special values—often a prerequisite for further reasoning about numerical computations—as well as certain functional properties for realistic benchmarks.

Rosa Abbasi, Jonas Schiffl, Eva Darulova, Mattias Ulbrich, Wolfgang Ahrendt

Open Access

##### Helmholtz: A Verifier for Tezos Smart Contracts Based on Refinement Types

A smart contract is a program executed on a blockchain, based on which many cryptocurrencies are implemented, and is being used for automating transactions. Due to the large amount of money that smart contracts deal with, there is a surging demand for a method that can statically and formally verify them.This tool paper describes our type-based static verification tool Helmholtz for Michelson, which is a statically typed stack-based language for writing smart contracts that are executed on the blockchain platform Tezos. Helmholtz is designed on top of our extension of Michelson’s type system with refinement types. Helmholtz takes a Michelson program annotated with a user-defined specification written in the form of a refinement type as input; it then typechecks the program against the specification based on the refinement type system, discharging the generated verification conditions with the SMT solver Z3. We briefly introduce our refinement type system for the core calculus Mini-Michelson of Michelson, which incorporates the characteristic features such as compound datatypes (e.g., lists and pairs), higher-order functions, and invocation of another contract. Helmholtz successfully verifies several practical Michelson programs, including one that transfers money to an account and that checks a digital signature.

Yuki Nishida, Hiromasa Saito, Ran Chen, Akira Kawata, Jun Furuse, Kohei Suenaga, Atsushi Igarashi

Open Access

##### SyReNN: A Tool for Analyzing Deep Neural Networks

Deep Neural Networks (DNNs) are rapidly gaining popularity in a variety of important domains. Formally, DNNs are complicated vector-valued functions which come in a variety of sizes and applications. Unfortunately, modern DNNs have been shown to be vulnerable to a variety of attacks and buggy behavior. This has motivated recent work in formally analyzing the properties of such DNNs. This paper introduces SyReNN, a tool for understanding and analyzing a DNN by computing its symbolic representation. The key insight is to decompose the DNN into linear functions. Our tool is designed for analyses using low-dimensional subsets of the input space, a unique design point in the space of DNN analysis tools. We describe the tool and the underlying theory, then evaluate its use and performance on three case studies: computing Integrated Gradients, visualizing a DNN’s decision boundaries, and patching a DNN.

Open Access

##### MachSMT: A Machine Learning-based Algorithm Selector for SMT Solvers

In this paper, we present MachSMT, an algorithm selection tool for Satisfiability Modulo Theories (SMT) solvers. MachSMT supports the entirety of the SMT-LIB language. It employs machine learning (ML) methods to construct both empirical hardness models (EHMs) and pairwise ranking comparators (PWCs) over state-of-the-art SMT solvers. Given an SMT formula $$\mathcal {I}$$ I as input, MachSMT leverages these learnt models to output a ranking of solvers based on predicted run time on the formula $$\mathcal {I}$$ I . We evaluate MachSMT on the solvers, benchmarks, and data obtained from SMT-COMP 2019 and 2020. We observe MachSMT frequently improves on competition winners, winning $$54$$ 54 divisions outright and up to a $$198.4$$ 198.4 % improvement in PAR-2 score, notably in logics that have broad applications (e.g., BV, LIA, NRA, etc.) in verification, program analysis, and software engineering. The MachSMT tool is designed to be easily tuned and extended to any suitable solver application by users. MachSMT is not a replacement for SMT solvers by any means. Instead, it is a tool that enables users to leverage the collective strength of the diverse set of algorithms implemented as part of these sophisticated solvers.

Joseph Scott, Aina Niemetz, Mathias Preiner, Saeed Nejati, Vijay Ganesh

Open Access

##### dtControl 2.0: Explainable Strategy Representation via Decision Tree Learning Steered by Experts

Recent advances have shown how decision trees are apt data structures for concisely representing strategies (or controllers) satisfying various objectives. Moreover, they also make the strategy more explainable. The recent tool dtControl had provided pipelines with tools supporting strategy synthesis for hybrid systems, such as SCOTS and Uppaal Stratego. We present dtControl 2.0, a new version with several fundamentally novel features. Most importantly, the user can now provide domain knowledge to be exploited in the decision tree learning process and can also interactively steer the process based on the dynamically provided information. To this end, we also provide a graphical user interface. It allows for inspection and re-computation of parts of the result, suggesting as well as receiving advice on predicates, and visual simulation of the decision-making process. Besides, we interface model checkers of probabilistic systems, namely STORM and PRISM and provide dedicated support for categorical enumeration-type state variables. Consequently, the controllers are more explainable and smaller.

Pranav Ashok, Mathias Jackermeier, Jan Křetínský, Christoph Weinhuber, Maximilian Weininger, Mayank Yadav

#### Tool Demo Papers

Open Access

##### HLola: a Very Functional Tool for Extensible Stream Runtime Verification

We present HLola, an extensible Stream Runtime Verification (SRV) tool, that borrows from the functional language Haskell (1) rich types for data in events and verdicts; and (2) functional features for parametrization, libraries, high-order specification transformations, etc.SRV is a formal dynamic analysis technique that generalizes Runtime Verification (RV) algorithms from temporal logics like LTL to stream monitoring, allowing the computation of verdicts richer than Booleans (quantitative values and beyond). The keystone of SRV is the clean separation between temporal dependencies and data computations. However, in spite of this theoretical separation previous engines include hardwired implementations of just a few datatypes, requiring complex changes in the tool chain to incorporate new data types. Additionally, when previous tools implement features like parametrization these are implemented in an ad-hoc way. In contrast, HLola is implemented as a Haskell embedded DSL, borrowing datatypes and functional aspects from Haskell, resulting in an extensible engine (The tool is available open-source at http://github.com/imdea-software/hlola ). We illustrate HLola through several examples, including a UAV monitoring infrastructure with predictive characteristics that has been validated in online runtime verification in real mission planning.

Felipe Gorostiaga, César Sánchez

Open Access

##### AMulet 2.0 for Verifying Multiplier Circuits

AMulet 2.0 is a fully automatic tool for the verification of integer multipliers using computer algebra. Our tool models multiplier circuits given as and-inverter graphs as a set of polynomials and applies preprocessing techniques based on elimination theory of Gröbner bases. Finally it uses a polynomial reduction algorithm to verify the correctness of the given circuit. AMulet 2.0 is a re-factorization and improved re-implementation of our previous multiplier verification tool AMulet 1.0.

Daniela Kaufmann, Armin Biere

Open Access

##### RTLola on Board: Testing Real Driving Emissions on your Phone

This paper is about shipping runtime verification to the masses. It presents the crucial technology enabling everyday car owners to monitor the behaviour of their cars in-the-wild. Concretely, we present an Android app that deploys rtlola runtime monitors for the purpose of diagnosing automotive exhaust emissions. For this, it harvests the availability of cheap bluetooth adapters to the On-Board-Diagnostics (obd) ports, which are ubiquitous in cars nowadays. We detail its use in the context of Real Driving Emissions (rde) tests and report on sample runs that helped identify violations of the regulatory framework currently valid in the European Union.

Sebastian Biewer, Bernd Finkbeiner, Holger Hermanns, Maximilian A. Köhl, Yannik Schnitzer, Maximilian Schwenger

Open Access

##### Replicating with Prolonged Retrials: An Experimental Report

Statistical model checking uses Monte Carlo simulation to analyse stochastic formal models. It avoids state space explosion, but requires rare event simulation techniques to efficiently estimate very low probabilities. One such technique is $$\textsc {Restart}$$ R E S T A R T . Villén-Altamirano recently showed—by way of a theoretical study and ad-hoc implementation—that a generalisation of $$\textsc {Restart}$$ R E S T A R T to prolonged retrials offers improved performance. In this paper, we demonstrate our independent replication of the original experimental results. We implemented $$\textsc {Restart}$$ R E S T A R T with prolonged retrials in the and modes tools, and apply them to the models used originally. To do so, we had to resolve ambiguities in the original work, and refine our setup multiple times. We ultimately confirm the previous results, but our experience also highlights the need for precise documentation of experiments to enable replicability in computer science.

Carlos E. Budde, Arnd Hartmanns

Open Access

##### A Web Interface for Petri Nets with Transits and Petri Games

Developing algorithms for distributed systems is an error-prone task. Formal models like Petri nets with transits and Petri games can prevent errors when developing such algorithms. Petri nets with transits allow us to follow the data flow between components in a distributed system. They can be model checked against specifications in LTL on both the local data flow and the global behavior. Petri games allow the synthesis of local controllers for distributed systems from safety specifications. Modeling problems in these formalisms requires defining extended Petri nets which can be cumbersome when performed textually.In this paper, we present a web interface (The web interface is deployed at http://adam.informatik.uni-oldenburg.de .) that allows an intuitive, visual definition of Petri nets with transits and Petri games. The corresponding model checking and synthesis problems are solved directly on a server. In the interface, implementations, counterexamples, and all intermediate steps can be analyzed and simulated. Stepwise simulations and interactive state space generation support the user in detecting modeling errors.

Manuel Gieseking, Jesko Hecking-Harbusch, Ann Yanich

Open Access

##### Momba: JANI Meets Python

JANI-model [6] is a model interchange format for networks of interacting automata. It is well-entrenched in the quantitative model checking community and allows modeling a variety of systems involving concurrency, probabilistic and real-time aspects, as well as continuous dynamics. Python is a general purpose programming language preferred by many for its ease of use and vast ecosystem. In this paper, we present Momba, a flexible Python framework for dealing with formal models centered around the JANI-model format and formalism. Momba strives to deliver an integrated and intuitive experience for experimenting with formal models making them accessible to a broader audience. To this end, it provides a pythonic interface for model construction, validation, and analysis. Here, we demonstrate these capabilities.

Maximilian A. Köhl, Michaela Klauck, Holger Hermanns

#### SV-Comp Tool Competition Papers

Open Access

##### Software Verification: 10th Comparative Evaluation (SV-COMP 2021)

SV-COMP 2021 is the 10th edition of the Competition on Software Verification (SV-COMP), which is an annual comparative evaluation of fully automatic software verifiers for C and Java programs. The competition provides a snapshot of the current state of the art in the area, and has a strong focus on reproducibility of its results. The competition was based on 15 201 verification tasks for C programs and 473 verification tasks for Java programs. Each verification task consisted of a program and a property (reachability, memory safety, overflows, termination). SV-COMP 2021 had 30 participating verification systems from 27 teams from 11 countries.

Dirk Beyer

Open Access

##### cpalockator: Thread-Modular Analysis with Projections
(Competition Contribution)

Our submission to SV-COMP’21 is based on the software verification framework and implements the extension to the thread-modular approach. It considers every thread separately, but in a special environment which models thread interactions. The environment is expressed by projections of normal transitions in each thread. A projection contains a description of possible effects over shared data and synchronization primitives, as well as conditions of its application. Adjusting the precision of the projections, one can find a balance between the speed and the precision of the whole analysis.Implementation on the top of the framework allows combining our approach with existing algorithms and analyses. Evaluation on the sv-benchmarks confirms the scalability and soundness of the approach.

Pavel Andrianov, Vadim Mutilin, Alexey Khoroshilov

Open Access

##### Dartagnan: Leveraging Compiler Optimizations and the Price of Precision (Competition Contribution)

We describe the new features of the bounded model checker Dartagnan for SV-COMP ’21. We participate, for the first time, in the ReachSafety category on the verification of sequential programs. In some of these verification tasks, bugs only show up after many loop iterations, which is a challenge for bounded model checking. We address the challenge by simplifying the structure of the input program while preserving its semantics. For simplification, we leverage common compiler optimizations, which we get for free by using LLVM. Yet, there is a price to pay. Compiler optimizations may introduce bitwise operations, which require bit-precise reasoning. We evaluated an SMT encoding based on the theory of integers + bit conversions against one based on the theory of bit-vectors and found that the latter yields better performance. Compared to the unoptimized version of Dartagnan, the combination of compiler optimizations and bit-vectors yields a speed-up of an order of magnitude on average.

Hernán Ponce-de-León, Thomas Haas, Roland Meyer

Open Access

##### Gazer-Theta: LLVM-based Verifier Portfolio with BMC/CEGAR (Competition Contribution)

Gazer-Theta is a software model checking toolchain including various analyses for state reachability. The frontend, namely Gazer, supports C programs through an LLVM-based transformation and optimization pipeline. Gazer includes an integrated bounded model checker (BMC) and can also employ the Theta backend, a generic verification framework based on abstraction-refinement (CEGAR). On SV-COMP 2021, a portfolio of BMC, explicit-value analysis, and predicate abstraction is applied sequentially in this order.

Zsófia Ádám, Gyula Sallai, Ákos Hajdu

Open Access

##### Goblint: Thread-Modular Abstract Interpretation Using Side-Effecting Constraints
(Competition Contribution)

Goblint is a static analysis framework for C programs specializing in data race analysis. It relies on thread-modular abstract interpretation where thread interferences are accounted for by means of flow-insensitive global invariants.

Simmo Saan, Michael Schwarz, Kalmer Apinis, Julian Erhard, Helmut Seidl, Ralf Vogler, Vesal Vojdani

Open Access

##### Towards String Support in JayHorn (Competition Contribution)

JayHorn is a Horn clause-based model checker for Java programs that has been competing at SV-COMP since 2019. An ongoing research and implementation effort is to add support for String data-type to JayHorn. Since current Horn solvers do not support strings natively, we consider a representation of (unbounded) strings using algebraic data-types, more precisely as lists. This paper discusses Horn clause encodings of different string operations, and presents preliminary results.

Ali Shamakhi, Hossein Hojjat, Philipp Rümmer

Open Access

##### JDart: Portfolio Solving, Breadth-First Search and SMT-Lib Strings (Competition Contribution)

JDart performs dynamic symbolic execution of Java programs: it executes programs with concrete inputs while recording symbolic constraints on executed program paths. A portfolio of constraint solvers is then used for generating new concrete values from recorded constraints that drive execution along previously unexplored paths. For SV-COMP 2021, we improved JDart by implementing exploration strategies, bounded analysis, and path-specific constraint solving strategies, as well as by enabling the use of SMT-Lib string theory for encoding of string operations.

Malte Mues, Falk Howar

Open Access

##### Symbiotic 8: Beyond Symbolic Execution
(Competition Contribution)

Symbiotic 8 extends the traditional combination of static analyses, instrumentation, program slicing, and symbolic execution with one substantial novelty, namely a technique mixing symbolic execution with k-induction. This technique can prove the correctness of programs with possibly unbounded loops, which cannot be done by classic symbolic execution. Symbiotic 8 delivers also several other improvements. In particular, we have modified our fork of the symbolic executor Klee to support the comparison of symbolic pointers. Further, we have tuned the shape analysis tool Predator (integrated already in Symbiotic 7) to perform better on llvm bitcode. We have also developed a light-weight analysis of relations between variables that can prove the absence of out-of-bound accesses to arrays.

Marek Chalupa, Tomáš Jašek, Jakub Novák, Anna Řechtáčková, Veronika Šoková, Jan Strejček

Open Access

##### VeriAbs: A Tool for Scalable Verification by Abstraction (Competition Contribution)

VeriAbs is a strategy selection-based reachability verifier for C programs. The selection of a suitable strategy is from a pre-defined set of strategies and by taking into account the syntax and semantics of the code to be verified. This year we present VeriAbs version 1.4.1 in which a novel preprocessor to strategy selection is introduced. The preprocessor checks for the feasibility of performing a lightweight slicing of the input code using function call graph and variable reference information. By this if the program is found to be sliceable, sub-programs or slices are generated, and the known strategy selection algorithm of VeriAbs is applied to each slice. The verification results of each slice are then composed to derive that of the entire program. This compositional verification has improved the scalability of VeriAbs and presented in this paper.

Priyanka Darke, Sakshi Agrawal, R. Venkatesh