arXiv subjects

Stephanie Balzer

Publications and source records attributed to Stephanie Balzer.

15 recordsLinked to original sources

arXiv2025

TypeDis: A Type System for Disentanglement

Disentanglement is a runtime property of parallel programs guaranteeing that parallel tasks remain oblivious to each other's allocations. As demonstrated in the MaPLe compiler and run-time system, disentanglement can be exploited for fast automatic memory management, especially task-local garbage collection with no synchronization between parallel tasks. However, as a low-level property, disentanglement can be difficult to reason about for programmers. The only means of statically verifying disentanglement so far has been DisLog, an Iris-fueled variant of separation logic, mechanized in the Rocq proof assistant. DisLog is a fully-featured program logic, allowing for proof of functional correctness as well as verification of disentanglement. Yet its employment requires significant expertise and per-program proof effort. This paper explores the route of automatic verification via a type system, ensuring that any well-typed program is disentangled and lifting the burden of carrying out manual proofs from the programmer. It contributes TypeDis, a type system inspired by region types, where each type is annotated with a timestamp, identifying the task that allocated it. TypeDis supports iso-recursive types as well as polymorphism over both types and timestamps. Crucially, timestamps are allowed to change during type-checking, at join points as well as via a form of subtyping, dubbed subtiming. The paper illustrates TypeDis and its features on a range of examples. The soundness of TypeDis and the examples are mechanized in the Rocq proof assistant, using an improved version of DisLog, dubbed DisLog2.

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arXiv2025

Mechanizing a Proof-Relevant Logical Relation for Timed Message-Passing Protocols

Semantic typing has become a powerful tool for program verification, applying the technique of logical relations as not only a proof method, but also a device for prescribing program behavior. In recent work, Yao et al. scaled semantic typing to the verification of timed message-passing protocols, which are prevalent in, e.g., IoT and real-time systems applications. The appeal of semantic typing in this context is precisely because of its ability to support typed and untyped program components alike -- including physical objects -- which caters to the heterogeneity of these applications. Another demand inherent to these applications is timing: constraining the time or time window within which a message exchange must happen. Yao et al. equipped their logical relation not only with temporal predicates, but also with computable trajectories, to supply the evidence that an inhabitant can step from one time point to another one. While Yao et al. provide the formalization for such a verification tool, it lacks a mechanization. Mechanizing the system would not only provide a machine proof for it, but also facilitate scalability for future extensions and applications. This paper tackles the challenge of mechanizing the resulting proof-relevant logical relation in a proof assistant. allowing trajectories to be interleaved, partitioned, and concatenated, while the intended equality on trajectories is the equality of their graphs when seen as processes indexed by time. Unfortunately, proof assistants based on intensional type theory only have modest support for such equations, forcing a prolific use of transports. This paper reports on the process of mechanizing Yao et al.'s results, comprising the logical relation, the algebra of computable trajectories with supporting lemmas, and the fundamental theorem of the logical relation, in the Rocq theorem prover.

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arXiv2025

Mechanized Metatheory of Forward Reasoning for End-to-End Linearizability Proofs

In the past decade, many techniques have been developed to prove linearizability, the gold standard of correctness for concurrent data structures. Intuitively, linearizability requires that every operation on a concurrent data structure appears to take place instantaneously, even when interleaved with other operations. Most recently, Jayanti et al. presented the first sound and complete "forward reasoning" technique for proving linearizability that relates the behavior of a concurrent data structure to a reference atomic data structure as time moves forward. This technique can be used to produce machine-checked proofs of linearizability in TLA+. However, while Jayanti et al.'s approach is shown to be sound and complete, a mechanization of this important metatheoretic result is still outstanding. As a result, it is not possible to produce verified end-to-end proofs of linearizability. To reduce the size of this trusted computing base, we formalize this forward reasoning technique and mechanize proofs of its soundness and completeness in Rocq. As a case study, we use the approach to produce a verified end-to-end proof of linearizability for a simple concurrent register.

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arXiv2025

A Language-Agnostic Logical Relation for Message-Passing Protocols

Today's computing landscape has been gradually shifting to applications targeting distributed and *heterogeneous* systems, such as cloud computing and Internet of Things (IoT) applications. These applications are predominantly *concurrent*, employ *message-passing*, and interface with *foreign objects*, ranging from externally implemented code to actual physical devices such as sensors. Verifying that the resulting systems adhere to the intended protocol of interaction is challenging -- the usual assumption of a common implementation language, let alone a type system, no longer applies, ruling out any verification method based on them. This paper develops a framework for certifying *protocol compliance* of heterogeneous message-passing systems. It contributes the first mechanization of a *language-agnostic logical relation*, asserting that its inhabitants comply with the protocol specified. This definition relies entirely on a labelled transition-based semantics, accommodating arbitrary inhabitants, typed and untyped alike, including foreign objects. As a case study, the paper considers two scenarios: (1) *per-instance verification* of a specific application or hardware device, and (2) *once-and-for-all verification* of well-typed applications for a given type system. The logical relation and both scenarios are mechanized in the Coq theorem prover.

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arXiv2024

Semantic Logical Relations for Timed Message-Passing Protocols (Extended Version)

Many of today's message-passing systems not only require messages to be exchanged in a certain order but also to happen at a certain \emph{time} or within a certain \emph{time window}. Such correctness conditions are particularly prominent in Internet of Things (IoT) and real-time systems applications, which interface with hardware devices that come with inherent timing constraints. Verifying compliance of such systems with the intended \emph{timed protocol} is challenged by their \emph{heterogeneity} -- ruling out any verification method that relies on the system to be implemented in one common language, let alone in a high-level and typed programming language. To address this challenge, this paper contributes a \emph{logical relation} to verify that its inhabitants (the applications and hardware devices to be proved correct) comply with the given timed protocol. To cater to the systems' heterogeneity, the logical relation is entirely \emph{semantic}, lifting the requirement that its inhabitants are syntactically well-typed. A semantic approach enables two modes of use of the logical relation for program verification: (i) \emph{once-and-for-all} verification of an \emph{arbitrary} well-typed application, given a type system, and (ii) \emph{per-instance} verification of a specific application / hardware device (a.k.a. foreign code). To facilitate mode (i), the paper develops a refinement type system for expressing timed message-passing protocols and proves that any well-typed program inhabits the logical relation (fundamental theorem). A type checker for the refinement type system has been implemented in Rust, using an SMT solver to check satisfiability of timing constraints. Then, the paper demonstrates both modes of use based on a small case study of a smart home system for monitoring air quality, consisting of a controller application and various environment sensors.

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arXiv2024

Regrading Policies for Flexible Information Flow Control in Session-Typed Concurrency

Noninterference guarantees that an attacker cannot infer secrets by interacting with a program. Information flow control (IFC) type systems assert noninterference by tracking the level of information learned (pc) and disallowing communication to entities of lesser or unrelated level than the pc. Control flow constructs such as loops are at odds with this pattern because they necessitate downgrading the pc upon recursion to be practical. In a concurrent setting, however, downgrading is not generally safe. This paper utilizes session types to track the flow of information and contributes an IFC type system for message-passing concurrent processes that allows downgrading the pc upon recursion. To make downgrading safe, the paper introduces regrading policies. Regrading policies are expressed in terms of integrity labels, which are also key to safe composition of entities with different regrading policies. The paper develops the type system and proves progress-sensitive noninterference for well-typed processes, ruling out timing attacks that exploit the relative order of messages. The type system has been implemented in a type checker, which supports security-polymorphic processes using local security theories.

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arXiv2024

Information Flow Control in Cyclic Process Networks

Protection of confidential data is an important security consideration of today's applications. Of particular concern is to guard against unintentional leakage to a (malicious) observer, who may interact with the program and draw inference from made observations. Information flow control (IFC) type systems address this concern by statically ruling out such leakage. This paper contributes an IFC type system for message-passing concurrent programs, the computational model of choice for many of today's applications such as cloud computing and IoT applications. Such applications typically either implicitly or explicitly codify protocols according to which message exchange must happen, and to statically ensure protocol safety, behavioral type systems such as session types can be used. This paper marries IFC with session typing and contributes over prior work in the following regards: (1) support of realistic cyclic process networks as opposed to the restriction to tree-shaped networks, (2) more permissive, yet entirely secure, IFC control, exploiting cyclic process networks, and (3) considering deadlocks as another form of side channel, and asserting deadlock-sensitive noninterference (DSNI) for well-typed programs. To prove DSNI, the paper develops a novel logical relation that accounts for cyclic process networks. The logical relation is rooted in linear logic, but drops the tree-topology restriction imposed by prior work.

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arXiv2023

Logical Relations for Session-Typed Concurrency

Program equivalence is the fulcrum for reasoning about and proving properties of programs. For noninterference, for example, program equivalence up to the secrecy level of an observer is shown. A powerful enabler for such proofs are logical relations. Logical relations only recently were adopted for session types -- but exclusively for terminating languages. This paper scales logical relations to general recursive session types. It develops a logical relation for progress-sensitive noninterference (PSNI) for intuitionistic linear logic session types (ILLST), tackling the challenges non-termination and concurrency pose, and shows that logical equivalence is sound and complete with regard to closure of weak bisimilarity under parallel composition, using a biorthogonality argument. A distinguishing feature of the logical relation is its stratification with an observation index (as opposed to a step or unfolding index), a crucial shift to make the logical relation closed under parallel composition in a concurrent setting. To demonstrate practicality of the logical relation, the paper develops an information flow control (IFC) refinement type system for ILLST, with support of secrecy-polymorphic processes, and shows that well-typed programs are self-related by the logical relation and thus enjoy PSNI. The refinement type system has been implemented in a type checker, featuring local security theories to support secrecy-polymorphic processes.

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arXiv2022

Recursive Session Logical Relations

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arXiv2022

Ferrite: A Judgmental Embedding of Session Types in Rust

\emph{Session types} have proved viable in expressing and verifying the protocols of message-passing systems. While message passing is a dominant concurrency paradigm in practice, real world software is written without session types. A limitation of existing session type libraries in mainstream languages is their restriction to linear session types, precluding application scenarios that demand sharing and thus aliasing of channel references. This paper introduces Ferrite, a shallow embedding of session types in Rust that supports both \emph{linear} and \emph{shared} sessions. The formal foundation of Ferrite constitutes the shared session type calculus $\sills$, which Ferrite encodes via a novel \emph{judgmental embedding} technique. The fulcrum of the embedding is the notion of a typing judgment that allows reasoning about shared and linear resources to type a session. Typing rules are then encoded as functions over judgments, with a valid typing derivation manifesting as a well-typed Rust program. This Rust program generated by Ferrite serves as a \emph{certificate}, ensuring that the application will proceed according to the protocol defined by the session type. The paper details the features and implementation of Ferrite and includes a case study on implementing Servo's canvas component in Ferrite.

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arXiv2021

Session Logical Relations for Noninterference

Information flow control type systems statically restrict the propagation of sensitive data to ensure end-to-end confidentiality. The property to be shown is noninterference, asserting that an attacker cannot infer any secrets from made observations. Session types delimit the kinds of observations that can be made along a communication channel by imposing a protocol of message exchange. These protocols govern the exchange along a single channel and leave unconstrained the propagation along adjacent channels. This paper contributes an information flow control type system for linear session types. The type system stands in close correspondence with intuitionistic linear logic. Intuitionistic linear logic typing ensures that process configurations form a tree such that client processes are parent nodes and provider processes child nodes. To control the propagation of secret messages, the type system is enriched with secrecy levels and arranges these levels to be aligned with the configuration tree. Two levels are associated with every process: the maximal secrecy denoting the process' security clearance and the running secrecy denoting the highest level of secret information obtained so far. The computational semantics naturally stratifies process configurations such that higher-secrecy processes are parents of lower-secrecy ones, an invariant enforced by typing. Noninterference is stated in terms of a logical relation that is indexed by the secrecy-level-enriched session types. The logical relation contributes a novel development of logical relations for session typed languages as it considers open configurations, allowing for more nuanced equivalence statement.

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arXiv2021

Manifestly Phased Communication via Shared Session Types

Session types denote message protocols between concurrent processes, allowing a type-safe expression of inter-process communication. Although previous work demonstrate a well-defined notion of subtyping where processes have different perceptions of the protocol, these formulations were limited to linear session types where each channel of communication has a unique provider and client. In this paper, we extend subtyping to shared session types where channels can now have multiple clients instead of a single client. We demonstrate that this generalization can statically capture protocol requirements that span multiple phases of interactions of a client with a shared service provider, something not possible in prior proposals. Moreover, the phases are manifest in the type of the client.

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arXiv2020

Ferrite: A Judgmental Embedding of Session Types in Rust

This paper introduces Ferrite, a shallow embedding of session types in Rust. In contrast to existing session type libraries and embeddings for mainstream languages, Ferrite not only supports linear session types but also shared session types. Shared session types allow sharing (aliasing) of channels while preserving session fidelity (preservation) using type modalities for acquiring and releasing sessions. Ferrite adopts a propositions as types approach and encodes typing derivations as Rust functions, with the proof of successful type-checking manifesting as a Rust program. We provide an evaluation of Ferrite using Servo as a practical example, and demonstrate how safe communication can be achieved in the canvas component using Ferrite.

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arXiv2020

Proceedings of the 12th International Workshop on Programming Language Approaches to Concurrency- and Communication-cEntric Software

Modern hardware platforms, from the very small to the very large, increasingly provide parallel and distributed computing resources for applications to maximise performance. Many applications therefore need to make effective use of tens, hundreds, and even thousands of compute nodes. Computation in such systems is thus inherently concurrent and communication centric. Effectively programming such applications is challenging; performance, correctness, and scalability are difficult to achieve. The development of effective programming methodologies for this increasingly parallel landscape therefore demands exploration and understanding of a wide variety of foundational and practical ideas. The International Workshop on Programming Language Approaches to Concurrency- and Communication-cEntric Software (PLACES) is dedicated to work in this area. The workshop offers a forum for researchers from different fields to exchange new ideas about these challenges to modern and future programming, where concurrency and distribution are the norm rather than a marginal concern. This volume contains the proceedings of the 12th edition of PLACES, which was co-located with ETAPS 2020 in Dublin, Ireland.

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arXiv2019

Resource-Aware Session Types for Digital Contracts

Programming digital contracts comes with unique challenges, which include (i) expressing and enforcing protocols of interaction, (ii) controlling resource usage, and (iii) preventing the duplication or deletion of a contract's assets. This article presents the design and type-theoretic foundation of Nomos, a programming language for digital contracts that addresses these challenges. To express and enforce protocols, Nomos is based on shared binary session types. To control resource usage, Nomos employs automatic amortized resource analysis. To prevent the duplication or deletion of assets, Nomos uses a linear type system. A monad integrates the effectful session-typed language with a general-purpose functional language. Nomos' prototype implementation features linear-time type checking and efficient type reconstruction that includes automatic inference of resource bounds via off-the-shelf linear optimization. The effectiveness of the language is evaluated with case studies about implementing common smart contracts such as auctions, elections, and currencies. Nomos is completely formalized, including the type system, a cost semantics, and a transactional semantics to instantiate Nomos contracts on a blockchain. The type soundness proof ensures that protocols are followed at run-time and that types establish sound upper bounds on the resource consumption, ruling out re-entrancy and out-of-gas vulnerabilities.

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