This article focuses on the runtime verification of hyperproperties expressed in Hyper- \(\mathsf{rec}\) HML, an expressive yet simple logic for describing properties of sets of traces. To this end, we consider a simple language of monitors that observe sets of system executions and report verdicts w.r.t. a given Hyper- \(\mathsf{rec}\) HML formula. We first employ a unique omniscient monitor that centrally observes all system traces. Since centralized monitors are not ideal for distributed settings, we also provide a language for decentralized monitors, where each trace has a dedicated monitor; these monitors yield a unique verdict by communicating their observations to one another. For both the centralized and the decentralized settings, we provide a synthesis procedure that, given a formula, yields a monitor that is correct (i.e., sound and violation complete). A key step in proving the correctness of the synthesis for decentralized monitors is a result showing that, for each formula, the synthesized centralized monitor and its corresponding decentralized one are weakly bisimilar for a suitable notion of weak bisimulation.

Runtime verification consists in checking whether a system satisfies a given specification by observing the execution trace it produces. In the regular setting, the modal μ-calculus provides a versatile formalism for expressing specifications of the control flow of the system. This paper focuses on the data flow and studies an extension of that logic that allows it to express data-dependent properties, identifying fragments that can be verified at runtime and with what correctness guarantees. The logic studied here is closely related with register automata with guessing. That correspondence yields a monitor synthesis algorithm, and a strict hierarchy among the various fragments of the logic, in contrast to the regular setting. We then exhibit a fragment of the logic that can express all monitorable formulae in the logic without greatest fixed-points but not in the full logic, and show this is the best we can get.

This paper studies the extent to which branching-time properties can be adequately verified using runtime monitors. We depart from the classical setup where monitoring is limited to a single system execution and investigate the enhanced observational capabilities when monitoring a system over multiple runs. To ensure generality, we focus on branching-time properties expressed in the modal µ-calculus, a well-studied foundational logic. Our results show that the proposed setup can systematically extend established monitorability limits for branching-time properties. We validate our results by instantiating them to verify actor-based systems. We also prove bounds that capture the correspondence between the syntactic structure of a property and the number of required system runs.

This paper focuses on the runtime verification of hyperproperties expressed in Hyper-recHML, an expressive yet simple logic for describing properties of sets of traces. To this end, we consider a simple language of monitors that observe sets of system executions and report verdicts w.r.t. a given Hyper-recHML formula. We first employ a unique omniscient monitor that centrally observes all system traces. Since centralised monitors are not ideal for distributed settings, we also provide a language for decentralized monitors, where each trace has a dedicated monitor; these monitors yield a unique verdict by communicating their observations to one another. For both the centralized and the decentralized settings, we provide a synthesis procedure that, given a formula, yields a monitor that is correct (i.e., sound and violation complete). A key step in proving the correctness of the synthesis for decentralized monitors is a result showing that, for each formula, the synthesized centralized monitor and its corresponding decentralized one are weakly bisimilar for a suitable notion of weak bisimulation.

This paper studies the complexity of classical modal logics and of their extension with fixed-point operators, using translations to transfer results across logics. In particular, we show several complexity results for multi-agent logics via translations to and from the $\mu$-calculus and modal logic, which allow us to transfer known upper and lower bounds. We also use these translations to introduce terminating and non-terminating tableau systems for the logics we study, based on Kozen's tableau for the $\mu$-calculus and the one of Fitting and Massacci for modal logic. Finally, we describe these tableaux with $\mu$-calculus formulas, thus reducing the satisfiability of each of these logics to the satisfiability of the $\mu$-calculus, resulting in a general 2EXP upper bound for satisfiability testing.

Existing notions of monitorability for branching-time properties are fairly restrictive. This, in turn, impacts the ability to incorporate prior knowledge about the system under scrutiny - which corresponds to a branching-time property - into the runtime analysis. We propose a definition of optimal monitors that verify the best monitorable under- or over-approximation of a specification, regardless of its monitorability status. Optimal monitors can be obtained for arbitrary branching-time properties by synthesising a sound and complete monitor for their strongest monitorable consequence. We show that the strongest monitorable consequence of specifications expressed in Hennessy-Milner logic with recursion is itself expressible in this logic, and present a procedure to find it. Our procedure enables prior knowledge to be optimally incorporated into runtime monitors.

Weighted monadic second-order logic is a weighted extension of monadic second-order logic that captures exactly the behaviour of weighted automata. Its semantics is parameterized with respect to a semiring on which the values that weighted formulas output are evaluated. Gastin and Monmege (2018) gave abstract semantics for a version of weighted monadic second-order logic to give a more general and modular proof of the equivalence of the logic with weighted automata. We focus on the abstract semantics of the logic and we give a complete axiomatization both for the full logic and for a fragment without general sum, thus giving a more fine-grained understanding of the logic. We discuss how common decision problems for logical languages can be adapted to the weighted setting, and show that many of these are decidable, though they inherit bad complexity from the underlying first- and second-order logics. However, we show that a weighted adaptation of satisfiability is undecidable for the logic when one uses the abstract interpretation.

Abstract We investigate the complexity of modal satisfiability for a family of multi-modal logics with interdependencies among the modalities. In particular, we examine four characteristic multi-modal logics with dependencies and demonstrate that, even if we restrict the formulae to be diamond-free and to have only one propositional variable, these logics still have a high complexity. This result identifies and isolates two sources of complexity: the presence of axiom $D$ for some of the modalities and certain modal interdependencies. We then further investigate and characterize the complexity of the diamond-free, 1-variable fragments of multi-modal logics in a general setting.

This paper establishes a comprehensive theory of runtime monitorability for Hennessy-Milner logic with recursion, a very expressive variant of the modal µ-calculus. It investigates the monitorability of that logic with a linear-time semantics and then compares the obtained results with ones that were previously presented in the literature for a branching-time setting. Our work establishes an expressiveness hierarchy of monitorable fragments of Hennessy-Milner logic with recursion in a linear-time setting and exactly identifies what kinds of guarantees can be given using runtime monitors for each fragment in the hierarchy. Each fragment is shown to be complete, in the sense that it can express all properties that can be monitored under the corresponding guarantees. The study is carried out using a principled approach to monitoring that connects the semantics of the logic and the operational semantics of monitors. The proposed framework supports the automatic, compositional synthesis of correct monitors from monitorable properties.

We introduce a general framework for Runtime Verification, parameterized with respect to a set of conditions. These conditions are encoded in the trace generated by a monitored process, which a monitor can observe. We present this parameterized framework in its general form and prove that it corresponds to a fragment of HML with recursion, extended with these conditions. We then show how this framework can be applied to a number of instantiations of the set of conditions. These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

Silent actions are an essential mechanism for system modelling and specification. They are used to abstractly report the occurrence of computation steps without divulging their precise details, thereby enabling the description of important aspects such as the branching structure of a system. Yet, their use rarely features in specification logics used in runtime verification. We study monitorability aspects of a branching-time logic that employs silent actions, identifying which formulas are monitorable for a number of instrumentation setups. We also consider defective instrumentation setups that imprecisely report silent events, and establish monitorability results for tolerating these imperfections.