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08:30-09:00Coffee & Refreshments
09:00-10:30 Session 85A: Formal Methods for Probabilistic Programs
Location: Taub 1
Data-Driven Invariant Learning for Probabilistic Programs
PRESENTER: Subhajit Roy

ABSTRACT. Morgan and McIver's weakest pre-expectation framework is one of the most well-established methods for deductive verification of probabilistic programs. Roughly, the idea is to generalize binary state assertions to real-valued expectations, which can measure expected values of probabilistic program quantities. While loop-free programs can be analyzed by mechanically transforming expectations, verifying loops usually requires finding an invariant expectation, a difficult task.

We propose a new view of invariant expectation synthesis as a \emph{regression} problem: given an input state, predict the \emph{average} value of the post-expectation. Guided by this perspective, we develop the first \emph{data-driven} invariant synthesis method for probabilistic programs. Unlike prior work on probabilistic invariant inference, our approach can learn piecewise continuous invariants without relying on template expectations, and also works when only given black-box access to the program. We also develop a data-driven approach to learn \emph{sub-invariants} from data, which can be used to upper- or lower-bound expected values. We implement our approaches and demonstrate their effectiveness on a variety of benchmarks from the probabilistic programming literature.

Sound and Complete Certificates for Quantitative Termination Analysis of Probabilistic Programs

ABSTRACT. We consider the quantitative problem of obtaining lower-bounds on the probability of termination of a given non-deterministic probabilistic program. Specifically, given a non-termination threshold p ∈ [0, 1], we aim for certificates proving that the program terminates with probability at least 1 − p. The basic idea of our approach is to find a terminating stochastic invariant, i.e. a subset SI of program states such that (i) the probability of the program ever leaving SI is no more than p, and (ii) almost-surely, the program either leaves SI or terminates. While stochastic invariants are already well-known, we provide the first proof that the idea above is not only sound, but also complete for quantitative termination analysis. We then introduce a novel sound and complete characterization of stochastic invariants that enables template-based approaches for easy synthesis of quantitative termination certificates, especially in affine or polynomial forms. Finally, by combining this idea with the existing martingale-based methods that are relatively complete for qualitative termination analysis, we obtain the first automated, sound, and relatively complete algorithm for quantitative termination analysis. Notably, our completeness guarantees for template-based quantitative termination analysis are as strong as the best-known methods for the qualitative variant.

Our prototype implementation demonstrates the effectiveness of our approach on various probabilistic programs. We also demonstrate that our algorithm certifies lower bounds on termination probability for probabilistic programs that are beyond the reach of previous methods.

Does a Program Yield the Right Distribution? Verifying Probabilistic Programs via Generating Functions
PRESENTER: Lutz Klinkenberg

ABSTRACT. We study discrete probabilistic programs with potentially unbounded looping behaviors over an infinite state space. We present, to the best of our knowledge, the first decidability result for the problem of determining whether such a program generates exactly a specified distribution over its outputs (provided the program terminates almost surely). The class of distributions that can be specified in our formalism consists of standard distributions (geometric, uniform, etc.) and finite convolutions thereof. Our method relies on representing these (possibly infinite-support) distributions as probability generating functions which admit effective arithmetic operations. We have automated our techniques in a tool called PRODIGY, which supports automatic invariance checking, compositional reasoning of nested loops, and efficient queries to the output distribution, as demonstrated by experiments.

Abstraction-Refinement for Hierarchical Probabilistic Models
PRESENTER: Sebastian Junges

ABSTRACT. Markov decision processes are a ubiquitous formalism for modelling systems with non-deterministic and probabilistic behavior. Verification of these models is subject to the famous state space explosion problem. We alleviate this problem by exploiting a hierarchical structure with repetitive parts. This structure not only occurs naturally in robotics, but also in probabilistic programs describing, e.g., network protocols. Such programs often repeatedly call a subroutine with similar behavior. In this paper, we focus on a local case, in which the subroutines have a limited effect on the overall system state. The key ideas to accelerate analysis of such programs are (1) to treat the behavior of the subroutine as uncertain and only remove this uncertainty by a detailed analysis if needed, and (2) to abstract similar subroutines into a parametric template, and then analyse this template. These two ideas are embedded into an abstraction-refinement loop that analyses hierarchical MDPs. A prototypical implementation shows the efficacy of the approach.

10:30-11:00Coffee Break
11:00-12:00 Session 89: Keynote
Harnessing the Power of Formal Verification for the $Trillion Chip Design Industry

ABSTRACT. Formal verification and model checking in particular is a key technology which is widely used for enabling the fast-growing electronic industry, serving many aspects of our digital lives in communication and computing. The pandemic has helped accelerate the growth of this industry boosted by the global digital orientation and remote collaboration. Analysists estimate that the electronic industry total annual revenue will be doubled by 2030 to reach one Trillion USD. Chip design is the heart of it and spans many areas including handheld devices, computer servers and cloud computing, mobile phones, Artificial Intelligence, Internet-of-things, automotive and variety of embedded systems. Cost of chip design is severely growing on the other hand and the industry consistently looks for solutions to address the productivity gaps. Formal Verification plays a significant role to boost verification productivity by an order of magnitude by unleashing formal applications. It enables many domains in the chip design and implementation cycles including functional, safety and security verification, logic optimization at various levels of design abstractions starting from architectural levels down to implementation for both software and hardware models.

The inherit theoretical complexity of model checking presents a big barrier to scale for such complex systems. In this talk, we will show how the industry explores and exploits various techniques in model checking to make it a practical and scalable technology, including key technological and methodological inflection points that made significant innovations and managed to boost formal. We will highlight innovations and exploitation that the industry produced, including for example the concept of formal apps, democratization of formal, the concept of 100% signoff in arithmetic designs, and equivalence checking. Model checking of software is a growing interest in the chip design industry driven by the fast growth of domain specific architectures like AI and DSP chips and dedicated accelerators, where models are implemented in C++ and model checking is required.

Despite the impressive industrial advancements and successful applications of model checking technologies for chip design, the domain is still very young considering its high expected impact on the industry. Therefore, in this talk we are interested in inspiring the academic community and accelerating research in key challenges through a joint and focused research with the industry. The intent is to boost core model checking algorithms and abstraction research, as well as model checking applications in key verification areas such as cybersecurity, automotive safety, and machine learning algorithms. Recently we have observed a rising research front of quantum computing leveraging formal verification technologies which could have major impact on the scalability and applications of quantum machines. Through collaboration, the academic and industrial communities can hugely influence the pace of innovation in these critical areas.

12:30-14:00Lunch Break

Lunches will be held in Taub lobby (CAV, CSF) and in The Grand Water Research Institute (DL, NMR, ITP).


14:00-15:30 Session 90A: Formal Methods for Neural Networks
Location: Taub 1
Shared Certificates for Neural Network Verification

ABSTRACT. Existing neural network verifiers compute a proof that each input is handled correctly under a given perturbation by propagating a symbolic abstraction of reachable values at each layer. This process is repeated from scratch independently for each input (e.g., image) and perturbation (e.g., rotation), leading to an expensive overall proof effort when handling an entire dataset. In this work we introduce a new method for reducing this verification cost without losing precision based on a key insight that abstractions obtained at intermediate layers for different inputs and perturbations can overlap or contain each other. Leveraging our insight, we introduce the general concept of shared certificates, enabling proof effort reuse across multiple inputs to reduce overall verification costs. We perform an extensive experimental evaluation to demonstrate the effectiveness of shared certificates in reducing the verification cost on a range of datasets and attack specifications on image classifiers including the popular patch and geometric perturbations.

Example Guided Synthesis of Linear Approximations for Neural Network Verification
PRESENTER: Brandon Paulsen

ABSTRACT. Linear approximations of nonlinear functions have a wide range of applications such as rigorous global optimization and, recently, verification problems involving neural networks. In the latter case, a linear approximation must be hand-crafted for the neural network's activation functions. This hand-crafting is tedious, potentially error-prone, and requires an expert to prove the soundness of the linear approximation. Such a limitation is at odds with the rapidly advancing deep learning field -- current verification tools either lack the necessary linear approximation, or perform poorly on neural networks with state-of-the-art activation functions. In this work, we consider the problem of automatically synthesizing sound linear approximations for a given neural network activation function. Our approach is example-guided: we develop a procedure to generate examples, and then we leverage machine learning techniques to learn a (static)function that outputs linear approximations. However, since the machine learning techniques we employ do not come with formal guarantees, the resulting synthesized function may produce linear approximations with violations. To remedy this,we bound the maximum violation using rigorous global optimization techniques, then adjust the synthesized linear approximation accordingly to ensure soundness. We evaluate our approach on several neural network verification tasks. Our evaluation shows that the automatically synthesized linear approximations greatly improve the accuracy (i.e., in terms of the number of verification problems solved) compared to hand-crafted linear approximations instate-of-the-art neural network verification tools.

Verifying Neural Networks Against Backdoor Attacks

ABSTRACT. Neural networks have achieved state-of-the-art performance in solving many problems, including many applications in safety/security-critical systems. Researchers also discovered multiple security issues associated with neural networks. One of them is backdoor attacks, i.e., a neural network may be embedded with a backdoor such that a target output is almost always generated in the presence of a trigger. Existing defense approaches mostly focus on detecting whether a neural network is 'backdoored' based on heuristics, e.g., activation patterns. To the best of our knowledge, the only line of work which certifies the absence of backdoor is based on randomized smoothing, which is known to significantly reduce neural network performance. In this work, we propose an approach to verify whether a given neural network is free of backdoor with a certain level of success rate. Our approach integrates statistical sampling as well as abstract interpretation. The experiment results show that our approach effectively verifies the absence of backdoor or generates backdoor triggers.

Trainify: A CEGAR-Driven Training and Verification Framework for Verifiable Deep Reinforcement Learning

ABSTRACT. DeepReinforcementLearning(DRL)has demonstrated its power in developing intelligent systems. These systems shall be formally guaranteed to be trustworthy when they are applied to safety-critical domains, which is typically achieved by formal verification performed after training. This train-then-verify process has two limits: (i) systems are difficult to verify because their state space is continuous and infinite and their AI components (i.e., neural networks) are almost inexplicable, and (ii) the ex post facto detection of bugs increases both the time- and money-wise cost of training and deployment. In this paper, we propose a novel verification-in-the-loop training framework called Trainify for developing verifiable DRL systems, driven by counterexample-guided abstraction and refinement. Specifically, Trainify trains a DRL system on a finite set of coarsely abstracted but efficiently verifiable state spaces. When verification fails, we refine the abstraction based on returned counterexamples and train again on the refined states. The process is iterated until all the properties are verified against the trained system. We demonstrate the effectiveness of our framework by training six classic control systems. Our framework can develop a more reliable DRL system with provable guarantees without sacrificing system performances such as accumulated awards and robustness compared with conventional DRL approaches.

Neural Network Robustness as a Verification Property: A Principled Case Study
PRESENTER: Marco Casadio

ABSTRACT. Neural networks are very successful at detecting patterns in noisy data, and have become the technology of choice in many fields. However, their usefulness is hampered by their susceptibility to adversarial attacks. Recently, many methods for measuring and improving a network’s robustness to adversarial perturbations have been proposed, and this growing body of research has given rise to numerous explicit or implicit notions of robustness. Connections between these notions are often subtle, and a systematic comparison between them is missing in the literature. In this paper we begin addressing this gap, by setting up general principles for the empirical analysis and evaluation of a network’s robustness as a mathematical property — during the network’s training phase, its verification, and after its deployment. We then apply these principles and conduct a case study that showcases the practical benefits of our general approach.

15:30-16:00Coffee Break
16:00-17:30 Session 92A: Software Verification and Model Checking
Location: Taub 1
The Lattice-Theoretic Essence of Property Directed Reachability Analysis
PRESENTER: Mayuko Kori

ABSTRACT. We present LT-PDR, a lattice-theoretic generalization of Bradley’s property directed reachability analysis (PDR) algorithm. LT- PDR identifies the essence of PDR to be an ingenious combination of verification and refutation attempts based on the Knaster–Tarski and Kleene theorems. We introduce four concrete instances of LT-PDR, de- rive their implementation from a generic Haskell implementation of LT- PDR, and experimentally evaluate them. We also present a categorical structural theory that derives these instances.

Affine Loop Invariant Generation via Matrix Algebra

ABSTRACT. Loop invariant generation, which automates the generation of assertions that always hold at the entry of a while loop, has many important applications in program analysis. In this work, we target an important category of while loops, namely affine while loops, that are unnested while loops with affine loop guards and variable updates. Such class of loops widely exists in many programs yet still lacks a general but efficient approach for invariant generation. We propose a novel matrix-algebra approach for automatically synthesizing affine inductive invariants in the form of an affine inequality. The main novelty of our approach is that (i) the approach is general in the sense that it theoretically addresses all the cases of affine invariant generation over an affine while loop, and (ii) it can be efficiently automated through matrix algebra (such as eigenvalue, matrix inverse) methods.

The details of our approach are as follows. First, for the case where the loop guard is a tautology (i.e., `true'), we show that the eigenvalues and their eigenvectors of the matrices derived from the variable updates of the loop body encompass all meaningful affine inductive invariants. Second, for the more general case where the loop guard is a conjunction of affine inequalities, our approach completely addresses the invariant-generation problem by first establishing through matrix inverse the relationship between the invariants and a key parameter in the application of Farkas' lemma, then solving the feasible domain of the key parameter from the inductive conditions, and finally illustrating that a finite number of values suffices for the key parameter w.r.t a tightness condition for the invariants to be generated.

Experimental results show that compared with previous approaches, our approach generates much more accurate affine inductive invariants over affine while loops from existing and new benchmarks, demonstrating the generality and efficiency of our approach.

Data-driven Numerical Invariant Synthesis with Automatic Generation of Attributes

ABSTRACT. We propose a data-driven algorithm for numerical invariant synthesis and verification. The algorithm is based on the ICE-DT schema for learning decision trees from samples that include positive and negative states and additionally implications corresponding to transitions in the program. The main issue we address is the discovery of relevant attributes to be used in the learning process of numerical invariants. We define a method for solving this problem that is guided by the data sample. It is based on the construction of a separator that covers positive states without including negative ones, and that is consistent with the implications. The separator is constructed using an abstract domain representation of convex sets. The generalization mechanism of the decision tree learning from the constraints of the separator allows the inference of general invariants, yet accurate enough for proving the targeted property. We implemented our algorithm and showed its efficiency.

Proof-guided Underapproximation Widening for Bounded Model Checking
PRESENTER: Subhajit Roy

ABSTRACT. Bounded Model Checking (BMC) is a popularly used strategy for program verification and it has been explored extensively over the past decade. Despite such a long history, BMC still faces scalability challenges as programs continue to grow larger and more complex. One approach that has proven to be effective in verifying large programs is called Counter Example Guided Abstraction Refinement (CEGAR). In this work, we propose a complimentary approach to CEGAR. Our strategy works by gradually widening the underapproximation of a program, following proofs of unsatisfiability. We have implemented our ideas in a tool called LEGION. We compare the performance of LEGION against that of CORRAL, a state-of-the-art verifier from Microsoft, that utilizes the CEGAR strategy. We conduct our experiments on 727 Windows and Linux device driver benchmarks. We find that LEGION is able to solve 12% more instances than CORRAL and that LEGION exhibits a complementary behavior to that of CORRAL. Motivated by this, we also build a portfolio verifier, LEGION+, that attempts to draw the best of LEGION and CORRAL. Our portfolio, LEGION+, solves 15% more benchmarks than CORRAL with similar computational resource constraints (i.e. each verifier in the portfolio is run with a time budget that is half of the time budget of CORRAL). Moreover, it is found to be 2.9 times faster than CORRAL on benchmarks that are solved by both CORRAL and LEGION+.

SolCMC: Solidity Compiler’s Model Checker
PRESENTER: Leonardo Alt

ABSTRACT. The practical importance of formally verifying smart contracts can be observed, for instance, in that Ethereum Foundation's Solidity compiler ships with a model checker. The checker, called SolCMC, has been part of the compiler since 2019 and tracks closely the development of the Solidity language. We describe SolCMC's architecture and use from the perspective of developers of both smart contracts and tools for software verification, and show how to analyze nontrivial properties of real life contracts in a fully automated manner.