jacques-henri.jourdan [at] cnrs [dot] fr
A thunk is a mutable data structure that offers a simple memoization service: it stores either a suspended computation or the result of this computation. Okasaki (1999) presents many data structures that exploit thunks to achieve good amortized time complexity. He analyzes their complexity by associating a debit with every thunk. A debit can be paid off in several increments; a thunk whose debit has been fully paid off can be forced. Quite strikingly, a debit is associated also with future thunks, which do not yet exist in memory. Some of the debit of a faraway future thunk can be transferred to a nearer future thunk. We present a complete machine-checked reconstruction of Okasaki's reasoning rules in Iris, a rich separation logic with time credits. We demonstrate the applicability of the rules by verifying a few operations on streams as well as several of Okasaki's data structures, namely the physicist's queue, implicit queues, and the banker's queue.
In Rust, programs are often written using iterators, but these pose problems for verification: they are non-deterministic, infinite, and often higher-order, effectful and built using adapters. We present a general framework for specifying and reasoning with Rust iterators in first-order logic. Our approach is capable of addressing the challenges set out above, which we demonstrate by verifying real Rust iterators, including a higher-order, effectful map. Using the Creusot verification platform, we evaluate our framework on clients of iterators, showing it leads to efficient verification of complex functional properties.
Rust is a fairly recent programming language for system programming, bringing static guarantees of memory safety through a strict ownership policy. The strong guarantees brought by this feature opens promising progress for deductive verification, which aims at proving the conformity of Rust code with respect to a specification of its intended behavior. We present the foundations of Creusot, a tool for the formal specification and deductive verification of Rust code. A first originality comes from Creusot's specification language, which features a notion of prophecy to reason about memory mutation, working in harmony with Rust's ownership system. A second originality is how Creusot builds upon Rust trait system to provide several advanced abstraction features.
Rust is a systems programming language that offers both low-level memory operations and high-level safety guarantees, via a strong ownership type system that prohibits mutation of aliased state. In prior work, Matsushita et al. developed RustHorn, a promising technique for functional verification of Rust code: it leverages the strong invariants of Rust types to express the behavior of stateful Rust code with first-order logic (FOL) formulas, whose verification is amenable to off-the-shelf automated techniques. RustHorn's key idea is to use prophecies to describe the behavior of mutable borrows. However, the soundness of RustHorn was only established for a safe subset of Rust, and it has remained unclear how to extend it to support various safe APIs that encapsulate unsafe code (i.e., code where Rust's aliasing discipline is relaxed). In this paper, we present RustHornBelt, the first machine-checked proof of soundness for RustHorn-style verification which supports giving FOL specs to safe APIs implemented with unsafe code. RustHornBelt employs the approach of semantic typing used in Jung et al.'s RustBelt framework, but it extends RustBelt's model to reason not only about safety but also functional correctness. The key challenge in RustHornBelt is to develop a semantic model of RustHorn-style prophecies, which we achieve via a new separation-logic mechanism we call parametric prophecies.
We use Cosmo, a modern concurrent separation logic, to formally specify and verify an implementation of a multiple-producer multiple-consumer concurrent queue in the setting of the Multicore OCaml weak memory model. We view this result as a demonstration and experimental verification of the manner in which Cosmo allows modular and formal reasoning about advanced concurrent data structures. In particular, we show how the joint use of logically atomic triples and of Cosmo's views makes it possible to describe precisely in the specification the interaction between the queue library and the weak memory model.
• Rust is the first industry-supported programming language to overcome the longstanding tradeoff between the safety guarantees of higher-level languages (like Java) and the control over resource management provided by lower-level "systems programming" languages (like C and C++).
• It tackles this challenge using a strong type system based on the ideas of ownership and borrowing, which statically prohibits the mutation of shared state. This approach enables many common systems programming pitfalls to be detected at compile time.
• There are a number of data types whose implementations fundamentally depend on shared mutable state and thus cannot be typechecked according to Rust’s strict ownership discipline. To support such data types, Rust embraces the judicious use of unsafe code encapsulated within safe APIs.
• The proof technique of semantic type soundness, together with advances in separation logic and machine-checked proof, has enabled us to begin building rigorous formal foundations for Rust as part of the RustBelt project.
Multicore OCaml extends OCaml with support for shared-memory concurrency. It is equipped with a weak memory model, for which an operational semantics has been published. This begs the question: what reasoning rules can one rely upon while writing or verifying Multicore OCaml code? To answer it, we instantiate Iris, a modern descendant of Concurrent Separation Logic, for Multicore OCaml. This yields a low-level program logic whose reasoning rules expose the details of the memory model. On top of it, we build a higher-level logic, Cosmo, which trades off some expressive power in return for a simple set of reasoning rules that allow accessing nonatomic locations in a data-race-free manner, exploiting the sequentially-consistent behavior of atomic locations, and exploiting the release/acquire behavior of atomic locations. Cosmo allows both low-level reasoning, where the details of the Multicore OCaml memory model are apparent, and high-level reasoning, which is independent of this memory model. We illustrate this claim via a number of case studies: we verify several implementations of locks with respect to a classic, memory-model-independent specification. Thus, a coarse-grained application that uses locks as the sole means of synchronization can be verified in the Concurrent-Separation-Logic fragment of Cosmo, without any knowledge of the weak memory model.
The Rust programming language supports safe systems programming by means of a strong ownership-tracking type system. In their prior work on RustBelt, Jung et al. began the task of setting Rust’s safety claims on a more rigorous formal foundation. Specifically, they used Iris, a Coq-based separation logic framework, to build a machine-checked proof of semantic soundness for a λ-calculus model of Rust, as well as for a number of widely-used Rust libraries that internally employ unsafe language features. However, they also made the significant simplifying assumption that the language is sequentially consistent. In this paper, we adapt RustBelt to account for the relaxed-memory operations that concurrent Rust libraries actually use, in the process uncovering a data race in the Arc library. We focus on the most interesting technical problem: how to reason about resource reclamation under relaxed memory, using a logical construction we call synchronized ghost state.
We verify the partial correctness of a "local generic solver", that is, an on-demand, incremental, memoizing least fixed point computation algorithm. The verification is carried out in Iris, a modern breed of concurrent separation logic. The specification is simple: the solver computes the optimal least fixed point of a system of monotone equations. Although the solver relies on mutable internal state for memoization and for "spying", a form of dynamic dependency discovery, it is apparently pure: no side effects are mentioned in its specification. As auxiliary contributions, we provide several illustrations of the use of prophecy variables, a novel feature of Iris; we establish a restricted form of the infinitary conjunction rule; and we provide a specification and proof of Longley’s modulus function, an archetypical example of spying.
We study a state-of-the-art incremental cycle detection algorithm due to Bender, Fineman, Gilbert, and Tarjan. We propose a simple change that allows the algorithm to be regarded as genuinely online. Then, we exploit Separation Logic with Time Credits to simultaneously verify the correctness and the worst-case amortized asymptotic complexity of the modified algorithm.
We present a machine-checked extension of the program logic Iris with time credits and time receipts, two dual means of reasoning about time. Whereas time credits are used to establish an upper bound on a program’s execution time, time receipts can be used to establish a lower bound. More strikingly, time receipts can be used to prove that certain undesirable events—such as integer overflows—cannot occur until a very long time has elapsed. We present several machine-checked applications of time credits and time receipts, including an application where both concepts are exploited.
A number of tools have been developed for carrying out separation-logic proofs mechanically using an interactive proof assistant. One of the most advanced such tools is the Iris Proof Mode (IPM) for Coq, which offers a rich set of tactics for making separation-logic proofs look and feel like ordinary Coq proofs. However, IPM is tied to a particular separation logic (namely, Iris), thus limiting its applicability.
In this paper, we propose MoSeL, a general and extensible Coq framework that brings the benefits of IPM to a much larger class of separation logics. Unlike IPM, MoSeL is applicable to both affine and linear separation logics (and combinations thereof), and provides generic tactics that can be easily extended to account for the bespoke connectives of the logics with which it is instantiated. To demonstrate the effectiveness of MoSeL, we have instantiated it to provide effective tactical support for interactive and semi-automated proofs in six very different separation logics.
Iris is a framework for higher-order concurrent separation logic, which has been implemented in the Coq proof assistant and deployed very effectively in a wide variety of verification projects. Iris was designed with the express goal of simplifying and consolidating the foundations of modern separation logics, but it has evolved over time, and the design and semantic foundations of Iris itself have yet to be fully written down and explained together properly in one place. Here, we attempt to fill this gap, presenting a reasonably complete picture of the latest version of Iris (version 3.1), from first principles and in one coherent narrative.
Rust is a new systems programming language that promises to overcome the seemingly fundamental tradeoff between high-level safety guarantees and low-level control over resource management. Unfortunately, none of Rust's safety claims have been formally proven, and there is good reason to question whether they actually hold. Specifically, Rust employs a strong, ownership-based type system, but then extends the expressive power of this core type system through libraries that internally use unsafe features. In this paper, we give the first formal (and machine-checked) safety proof for a language representing a realistic subset of Rust. Our proof is extensible in the sense that, for each new Rust library that uses unsafe features, we can say what verification condition it must satisfy in order for it to be deemed a safe extension to the language. We have carried out this verification for some of the most important libraries that are used throughout the Rust ecosystem.
The syntax of the C programming language is described in the C11 standard by an ambiguous context-free grammar, accompanied with English prose that describes the concept of “scope” and indicates how certain ambiguous code fragments should be interpreted. Based on these elements, the problem of implementing a compliant C11 parser is not entirely trivial. We review the main sources of difficulty and describe a relatively simple solution to the problem. Our solution employs the well-known technique of combining an LALR(1) parser with a “lexical feedback” mechanism. It draws on folklore knowledge and adds several original aspects, including: a twist on lexical feedback that allows a smooth interaction with lookahead; a simplified and powerful treatment of scopes; and a few amendments in the grammar. Although not formally verified, our parser avoids several pitfalls that other implementations have fallen prey to. We believe that its simplicity, its mostly-declarative nature, and its high similarity with the C11 grammar are strong informal arguments in favor of its correctness. Our parser is accompanied with a small suite of “tricky” C11 programs. We hope that it may serve as a reference or a starting point in the implementation of compilers and analysis tools.
Concurrent separation logics (CSLs) have come of age, and with age they have accumulated a great deal of complexity Previous work on the Iris logic attempted to reduce the complex logical mechanisms of modern CSLs to two orthogonal concepts: partial commutative monoids (PCMs) and invariants. However, the realization of these concepts in Iris still bakes in several complex mechanisms–such as weakest preconditions and mask-changing view shifts–as primitive notions. In this paper, we take the Iris story to its (so to speak) logical conclusion, applying the reductionist methodology of Iris to Iris itself. Specifically, we define a small, resourceful base logic, which distills the essence of Iris: it comprises only the assertion layer of vanilla separation logic, plus a handful of simple modalities. We then show how the much fancier logical mechanisms of Iris–in particular, its entire program specification layer–can be understood as merely derived forms in our base logic. This approach helps to explain the meaning of Iris’s program specifications at a much higher level of abstraction than was previously possible. We also show that the step-indexed "later" modality of Iris is an essential source of complexity, in that removing it leads to a logical inconsistency. All our results are fully formalized in the Coq proof assistant.
Known algorithms for manipulating octagons do not preserve their sparsity, leading typically to quadratic or cubic time and space complexities even if no relation among variables is known when they are all bounded. In this paper, we present new algorithms, which use and return octagons represented as weakly closed difference bound matrices, preserve the sparsity of their input and have better performance in the case their inputs are sparse. We prove that these algorithms are as precise as the known ones.
In order to develop safer software for critical applications, some static analyzers aim at establishing, with mathematical certitude, the absence of some classes of bug in the input program. A possible limit to this approach is the possibility of a soundness bug in the static analyzer itself, which would nullify the guarantees it is supposed to deliver. In this thesis, we propose to establish formal guarantees on the static analyzer itself: we present the design, implementation and proof of soundness using Coq of Verasco, a formally verified static analyzer based on abstract interpretation handling most of the ISO C99 language, including IEEE754 floating-point arithmetic (except recursion and dynamic memory allocation). Verasco aims at establishing the absence of erroneous behavior of the given programs. It enjoys a modular extendable architecture with several abstract domains and well-specified interfaces. We present the abstract iterator of Verasco, its handling of bounded machine arithmetic, its interval abstract domain, its symbolic abstract domain and its abstract domain of octagons. Verasco led to the development of new techniques for implementing data structure with sharing in Coq.
Floating-point arithmetic is known to be tricky: roundings, formats, exceptional values. The IEEE-754 standard was a push towards straightening the field and made formal reasoning about floating-point computations easier and flourishing. Unfortunately, this is not sufficient to guarantee the final result of a program, as several other actors are involved: programming language, compiler, and architecture. The CompCert formally-verified compiler provides a solution to this problem: this compiler comes with a mathematical specification of the semantics of its source language (a large subset of ISO C99) and target platforms (ARM, PowerPC, x86-SSE2), and with a proof that compilation preserves semantics. In this paper, we report on our recent success in formally specifying and proving correct CompCert’s compilation of floating-point arithmetic. Since CompCert is verified using the Coq proof assistant, this effort required a suitable Coq formalization of the IEEE-754 standard; we extended the Flocq library for this purpose. As a result, we obtain the first formally verified compiler that provably preserves the semantics of floating-point programs.
This paper reports on the design and soundness proof, using the Coq proof assistant, of Verasco, a static analyzer based on abstract interpretation for most of the ISO C 1999 language (excluding recursion and dynamic allocation). Verasco establishes the absence of run-time errors in the analyzed programs. It enjoys a modular architecture that supports the extensible combination of multiple abstract domains, both relational and non-relational. Verasco integrates with the CompCert formally-verified C compiler so that not only the soundness of the analysis results is guaranteed with mathematical certitude, but also the fact that these guarantees carry over to the compiled code.
We report on four different approaches to implementing hash-consing in Coq programs. The use cases include execution inside Coq, or execution of the extracted OCaml code. We explore the different trade-offs between faithful use of pristine extracted code, and code that is fine-tuned to make use of OCaml programming constructs not available in Coq. We discuss the possible consequences in terms of performances and guarantees. We use the running example of binary decision diagrams and then demonstrate the generality of our solutions by applying them to other examples of hash-consed data structures.
In this paper, we focus on finding positive invariants and Lyapunov functions to establish reachability and stability properties, respectively, of polynomial ordinary differential equations (ODEs). In general, the search for such functions is a hard problem. As a result, numerous techniques have been developed to search for polynomial differential variants that yield positive invariants and polynomial Lyapunov functions that prove stability, for systems defined by polynomial differential equations. However, the systematic search for non-polynomial functions is considered a much harder problem, and has received much less attention. In this paper, we combine ideas from computer algebra with the Sum-Of-Squares (SOS) relaxation for polynomial positive semi-definiteness to find non polynomial differential variants and Lyapunov functions for polynomial ODEs. Using the well-known concept of Darboux polynomials, we show how Darboux polynomials can, in many instances, naturally lead to specific forms of Lyapunov functions that involve rational function, logarithmic and exponential terms.We demonstrate the value of our approach by deriving non-polynomial Lyapunov functions for numerical examples drawn from the literature.
We report on three different approaches to use hash-consing in programs certified with the Coq system, using binary decision diagrams (BDD) as running example. The use cases include execution inside Coq, or execution of the extracted OCaml code. There are different trade-offs between faithful use of pristine extracted code, and code that is fine-tuned to make use of OCaml programming constructs not available in Coq. We discuss the possible consequences in terms of performances and guarantees.
Floating-point arithmetic is known to be tricky: roundings, formats, exceptional values. The IEEE-754 standard was a push towards straightening the field and made formal reasoning about floating-point computations easier and flourishing. Unfortunately, this is not sufficient to guarantee the final result of a program, as several other actors are involved: programming language, compiler, architecture. The Comp Certformally-verified compiler provides a solution to this problem: this compiler comes with a mathematical specification of the semantics of its source language (a large subset of ISO C90) and target platforms (ARM, PowerPC, x86-SSE2), and with a proof that compilation preserves semantics. In this paper, we report on our recent success in formally specifying and proving correct Comp Cert's compilation of floating-point arithmetic. Since CompCert is verified using the Coq proof assistant, this effort required a suitable Coq formalization of the IEEE-754 standard, we extended the Flocq library for this purpose. As a result, we obtain the first formally verified compiler that provably preserves the semantics of floating-point programs.
3D integration is a promising advanced manufacturing process offering a variety of new hardware security protection opportunities. This paper presents a way of securing 3D ICs using Hamiltonian paths as hardware integrity verification sensors. As 3D integration consists in the stacking of many metal layers, one can consider surrounding a security-sensitive circuit part by a wire cage. After exploring and comparing different cage construction strategies (and reporting preliminary implementation results on silicon), we introduce a "hardware canary". The canary is a spatially distributed chain of functions F i positioned at the vertices of a 3D cage surrounding a protected circuit. A correct answer (F n ∘ … ∘ F 1)(m) to a challenge m attests the canary's integrity.
An LR(1) parser is a finite-state automaton, equipped with a stack, which uses a combination of its current state and one lookahead symbol in order to determine which action to perform next. We present a validator which, when applied to a context-free grammar G and an automaton A, checks that A and G agree. Validating the parser provides the correctness guarantees required by verified compilers and other high-assurance software that involves parsing. The validation process is independent of which technique was used to construct A. The validator is implemented and proved correct using the Coq proof assistant. As an application, we build a formally-verified parser for the C99 language.
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