Task 4 Papers

[1] Kasper Svendsen, Jean Pichon-Pharabod, Marko Doko, Ori Lahav, and Viktor Vafeiadis. A separation logic for a promising semantics. In ESOP 2018, 2018. [ bib ]
[2] Thomas Bauereiß, Armando Pesenti Gritti, Andrei Popescu, and Franco Raimondi. Cosmed: A confidentiality-verified social media platform. Journal of Automated Reasoning, December 2017. [ bib | DOI | http ]
This paper describes progress with our agenda of formal verification of information flow security for realistic systems. We present CoSMed, a social media platform with verified document confidentiality. The system's kernel is implemented and verified in the proof assistant Isabelle/HOL. For verification, we employ the framework of Bounded-Deducibility (BD) Security, previously introduced for the conference system CoCon. CoSMed is a second major case study in this framework. For CoSMed, the static topology of declassification bounds and triggers that characterized previous instances of BD Security has to give way to a dynamic integration of the triggers as part of the bounds. We also show that, from a theoretical viewpoint, the removal of triggers from the notion of BD Security does not restrict its expressiveness.

[3] Cristina Matache, Victor B. F. Gomes, and Dominic P. Mulligan. Programming and proving with classical types. In APLAS 2017: Proceedings of the 15th Asian Symposium on Programming Languages and Systems, November 2017. [ bib | DOI | http ]
The propositions-as-types correspondence is ordinarily presented as linking the metatheory of typed λ-calculi and the proof theory of intuitionistic logic. Griffin observed that this correspondence could be extended to classical logic through the use of control operators. This observation set off a flurry of further research, leading to the development of Parigot’s λμ-calculus. In this work, we use the λμ-calculus as the foundation for a system of proof terms for classical first-order logic. In particular, we define an extended call-by-value λμ-calculus with a type system in correspondence with full classical logic. We extend the language with polymorphic types, add a host of data types in ‘direct style’, and prove several metatheoretical properties. All of our proofs and definitions are mechanised in Isabelle/HOL, and we automatically obtain an interpreter for a system of proof terms cum programming language—called μML—using Isabelle’s code generation mechanism. Atop our proof terms, we build a prototype LCF-style interactive theorem prover—called μTP—for classical first-order logic, capable of synthesising μML programs from completed tactic-driven proofs. We present example closed μML programs with classical tautologies for types, including some inexpressible as closed programs in the original λμ-calculus, and some example tactic-driven μTP proofs of classical tautologies.

[4] Victor B. F. Gomes, Martin Kleppmann, Dominic P. Mulligan, and Alastair R. Beresford. Verifying strong eventual consistency in distributed systems. In OOPSLA 2017: Proceedings of the ACM SIGPLAN International Conference on Object-Oriented Programming, Systems, Languages, and Applications (OOPSLA), pages 109:1-109:28, October 2017. Distinguished Paper award. [ bib | DOI | http ]
Data replication is used in distributed systems to maintain up-to-date copies of shared data across multiple computers in a network. However, despite decades of research, algorithms for achieving consistency in replicated systems are still poorly understood. Indeed, many published algorithms have later been shown to be incorrect, even some that were accompanied by supposed mechanised proofs of correctness. In this work, we focus on the correctness of Conflict-free Replicated Data Types (CRDTs), a class of algorithm that provides strong eventual consistency guarantees for replicated data. We develop a modular and reusable framework in the Isabelle/HOL interactive proof assistant for verifying the correctness of CRDT algorithms. We avoid correctness issues that have dogged previous mechanised proofs in this area by including a network model in our formalisation, and proving that our theorems hold in all possible network behaviours. Our axiomatic network model is a standard abstraction that accurately reflects the behaviour of real-world computer networks. Moreover, we identify an abstract convergence theorem, a property of order relations, which provides a formal definition of strong eventual consistency. We then obtain the first machine-checked correctness theorems for three concrete CRDTs: the Replicated Growable Array, the Observed-Remove Set, and an Increment-Decrement Counter. We find that our framework is highly reusable, developing proofs of correctness for the latter two CRDTs in a few hours and with relatively little CRDT-specific code.

[5] Andrea Cerone, Alexey Gotsman, and Hongseok Yang. Algebraic laws for weak consistency. In CONCUR 2017: Proceedings of 28th International Conference on Concurrency Theory, September 2017. [ bib | DOI | .html ]
Modern distributed systems often rely on so called weakly-consistent databases, which achieve scalability by sacrificing the consistency guarantee of distributed transaction processing. Such databases have been formalised in two different styles, one based on abstract executions and the other based on dependency graphs. The choice between these styles has been made according to intended applications. The former has been used for specifying and verifying the implementation of these databases, while the latter for proving properties of client programs of the databases. In this paper, we present a set of novel algebraic laws (i.e. inequations) that connect these two styles of specifications. The laws relate binary relations used in a specification based on abstract executions, to those used in a specification based on dependency graphs. We then show that this algebraic connection gives rise to so called robustness criteria, conditions which ensure that a client program of a weakly-consistent database does not exhibit anomalous behaviours due to weak consistency. These criteria make it easy to reason about these client programs, and may become a basis for dynamic or static program analyses. For a certain class of consistency models specifications, we prove a full abstraction result that connects the two styles of specifications.

[6] Philippa Gardner. Verified trustworthy software systems. Philosophical Transactions of the Royal Society of London A: Mathematical, Physical and Engineering Sciences, 375(2104), September 2017. [ bib | DOI | .pdf ]
Modern society is faced with a fundamental problem: the reliability of complex, evolving software systems on which society critically depends cannot be guaranteed by the established, non-mathematical computer engineering techniques such as informal prose specification and ad hoc testing. The situation is worsening: modern companies are moving fast, leaving little time for code analysis and testing; the behaviour of concurrent and distributed programs cannot be adequately assessed using traditional testing methods; users of mobile applications often neglect to apply software fixes; and malicious users increasingly exploit even simple programming errors, causing major security disruptions. Building trustworthy, reliable software is becoming harder and harder to achieve, while new business and cybersecurity challenges make it of escalating critical importance

[7] José Fragoso Santos, Philippa Gardner, Petar Maksimovic, and Daiva Naudziuniene. Towards logic-based verification of JavaScript programs. In Leonardo de Moura, editor, CADE 2017: 26th International Conference on Automated Deduction, volume 10395 of Lecture Notes in Computer Science, pages 8-25. Springer, August 2017. [ bib | DOI | .html ]
[8] Thomas Dinsdale-Young, Pedro da Rocha Pinto, Kristoffer Just Andersen, and Lars Birkedal. Caper: Automatic Verification for Fine-grained Concurrency. In Proc. ESOP 2017: 26th European Symposium on Programming, Lecture Notes in Computer Science. Springer, April 2017. [ bib | .pdf ]
Recent program logics based on separation logic emphasise a modular approach to proving functional correctness for fine-grained concurrent programs. However, these logics have no automation support. In this paper, we present Caper, a prototype tool for automated reasoning in such a logic. Caper is based on symbolic execution, integrating reasoning about interference on shared data and about ghost resources that are used to mediate this interference. This enables Caper to verify the functional correctness of fine-grained concurrent algorithms.

[9] Shale Xiong, Pedro da Rocha Pinto, Gian Ntzik, and Philippa Gardner. Abstract Specifications for Concurrent Maps. In Proc. ESOP 2017: 26th European Symposium on Programming, Lecture Notes in Computer Science. Springer, April 2017. [ bib | .pdf ]
Despite recent advances in reasoning about concurrent data structure libraries, the largest implementations in java.util.concurrent have yet to be verified. The key issue lies in the development of modular specifications, which provide clear logical boundaries between clients and implementations. A solution is to use recent advances in fine-grained concurrency reasoning, in particular the introduction of abstract atomicity to concurrent separation logic reasoning. We present two specifications of concurrent maps, both providing the clear boundaries we seek. We show that these specifications are equivalent, in that they can be built from each other. We show how we can verify client programs, such as a concurrent set and a producer-consumer client. We also give a substantial first proof that the main operations of ConcurrentSkipListMap in java.util.concurrent satisfy the map specification. This work demonstrates that we now have the technology to verify the largest implementations in java.util.concurrent.

[10] Lars Birkedal, Thomas Dinsdale-Young, Guilhem Jaber, Kasper Svendsen, and Nikos Tzevelekos. Trace properties from separation logic specifications. CoRR, abs/1702.02972, February 2017. [ bib | http | http ]
We propose a formal approach for relating abstract separation logic library specifications with the trace properties they enforce on interactions between a client and a library. Separation logic with abstract predicates enforces a resource discipline that constrains when and how calls may be made between a client and a library. Intuitively, this can enforce a protocol on the interaction trace. This intuition is broadly used in the separation logic community but has not previously been formalised. We provide just such a formalisation. Our approach is based on using wrappers which instrument library code to induce execution traces for the properties under examination. By considering a separation logic extended with trace resources, we prove that when a library satisfies its separation logic specification then its wrapped version satisfies the same specification and, moreover, maintains the trace properties as an invariant. Consequently, any client and library implementation that are correct with respect to the separation logic specification will satisfy the trace properties.

[11] Morten Krogh-Jespersen, Kasper Svendsen, and Lars Birkedal. A relational model of types-and-effects in higher-order concurrent separation logic. In POPL 2017: The 44st Annual ACM SIGPLAN-SIGACT Symposium on Principles of Programming Languages, Paris, France, pages 218-231, New York, NY, USA, January 2017. ACM. [ bib | DOI | http ]
Recently we have seen a renewed interest in programming languages that tame the complexity of state and concurrency through refined type systems with more fine-grained control over effects. In addition to simplifying reasoning and eliminating whole classes of bugs, statically tracking effects opens the door to advanced compiler optimizations.

In this paper we present a relational model of a type-and-effect system for a higher-order, concurrent programming language. The model precisely captures the semantic invariants expressed by the effect annotations. We demonstrate that these invariants are strong enough to prove advanced program transformations, including automatic parallelization of expressions with suitably disjoint effects. The model also supports refinement proofs between abstract data types implementations with different internal data representations, including proofs that fine-grained concurrent algorithms refine their coarse-grained counterparts. This is the first model for such an expressive language that supports both effect-based optimizations and data abstraction.

The logical relation is defined in Iris, a state-of-the-art higher-order concurrent separation logic. This greatly simplifies proving well-definedness of the logical relation and also provides us with a powerful logic for reasoning in the model.

[12] Victor B. F. Gomes and Georg Struth. Modal Kleene algebra applied to program correctness. In John S. Fitzgerald, Stefania Gnesi, and Constance L. Heitmeyer, editors, FM 2016: Formal Methods - 21st International Symposium, Limassol, Lecture Notes in Computer Science, pages 310-325. Springer, November 2016. [ bib | DOI | http | .pdf ]
Modal Kleene algebras are relatives of dynamic logics that support program construction and verification by equational reasoning. We describe their application in implementing versatile program correctness components in interactive theorem provers such as Isabelle/HOL. Starting from a weakest precondition based component with a simple relational store model, we show how variants for Hoare logic, strongest postconditions and program refinement can be built in a principled way. Modularity of the approach is demonstrated by variants that capture program termination and recursion, memory models for programs with pointers, and program trace semantics.

[13] Azalea Raad, Aquinas Hobor, Jules Villard, and Philippa Gardner. Verifying concurrent graph algorithms. In Proc. APLAS 2016: 14th Asian Symposium on Programming Languages and Systems, pages 314-334, November 2016. [ bib | DOI | http ]
We show how to verify four challenging concurrent finegrained graph-manipulating algorithms, including graph copy, a speculatively parallel Dijkstra, graph marking and spanning tree. We develop a reasoning method for such algorithms that dynamically tracks the contributions and responsibilities of each thread operating on a graph, even in cases of arbitrary recursive thread creation. We demonstrate how to use a logic without abstraction (CoLoSL) to carry out abstract reasoning in the style of iCAP, by building the abstraction into the proof structure rather than incorporating it into the semantic model of the logic.

[14] Azalea Raad, José Fragoso Santos, and Philippa Gardner. DOM: specification and client reasoning. In Proc. APLAS 2016: 14th Asian Symposium on Programming Languages and Systems, pages 401-422, November 2016. [ bib | DOI | http ]
We present an axiomatic specification of a key fragment of DOM using structural separation logic. This specfication allows us to develop modular reasoning about client programs that call the DOM.

[15] Hannes Mehnert and Louis Gesbert. Conex - establishing trust into data repositories. In Proceedings of the ACM OCaml 2016 Workshop, September 2016. [ bib | .pdf ]
Opam is a software update system, responsible for discovering, downloading, building, and installing packages from community repositories. The data and metadata at each step should be authenticated to originate from a (transitively) trusted author. Opam does not include any mechanism to authenticate any step at the moment. We propose conex, which establishes digital signatures from the package author to the user. Using conex, neither the distribution server nor the transport layer need to be trusted. Clients will install authenticated packages, and notice conex only if signatures cannot be verified. Authors need to adapt their workflow slightly, an updated opam publish will be developed. Janitors (repository maintainers) need to adapt their workflow to emit cryptographic signatures to the repository (in addition to pressing merge PR buttons).

[16] Enguerrand Decorne, Jeremy Yallop, and David Meršinjak. Ocaml inside: a drop-in replacement for libtls (extended abstract). In Proceedings of the ACM OCaml 2016 Workshop, September 2016. [ bib | .pdf ]
The C programming language pervades systems software. An operating system in the Unix tradition consists of a kernel, written in C, and a collection of libraries and executables, also written in C, which communicate in large part via APIs defined as C types and functions. Systems built in C typically suffer from a number of problems, ranging from buffer overflows and other violations that follow inevitably from unrestricted access to memory, to awkward APIs that result from an inexpressive type system and a lack of automatic memory management. The openssl library, which implements the cryptographic protocols TLS and SSL, suffers from both these problems. The lack of bounds checking in C led to the notorious Heartbleed bug in 2014; a study two years earlier found that almost no clients of openssl use the library correctly, apparently due to its unhelpful interface (Georgiev et al. 2012). In response to the Heartbleed bug, the OpenBSD team created libressl, a fork of openssl with the aim of correcting the deficiencies. One early fruit of the libressl project is libtls, a much simpler, more disciplined interface to the TLS protocol implementation in libressl. However, libtls is still built in C, and so is still vulnerable to potential buffer overflows, type errors, and similar defects. In this talk we describe one approach to avoiding these problems, namely replacing libtls with a fully compatible library written in OCaml. Our library, libnqsb-tls1, matches the libtls function-for-function, but the implementation contains no C; instead, it it wraps the pure OCaml TLS implementation ocaml-tls (Kaloper-Meršinjak et al. 2015)

[17] David Kaloper-Meršinjak and Jeremy Yallop. Generic partially-static data (extended abstract). In Proceedings of the 1st International Workshop on Type-Driven Development, TyDe 2016, pages 39-40, New York, NY, USA, September 2016. ACM. [ bib | DOI | http | http ]
We describe a generic approach to defining partially-static data and corresponding operations.

Keywords: generic programming, partial evaluation, staging
[18] Gian Ntzik. Reasoning About POSIX File Systems. PhD thesis, Imperial College London, September 2016. [ bib | .pdf ]
POSIX is a standard for operating systems, with a substantial part devoted to specifying file-system operations. File-system operations exhibit complex concurrent behaviour, comprising multiple actions affecting different parts of the state: typically, multiple atomic reads followed by an atomic update. However, the standard's description of concurrent behaviour is unsatisfactory: it is fragmented; contains ambiguities; and is generally under-specified. We provide a formal concurrent specification of POSIX file systems and demonstrate scalable reasoning for clients. Our speciation is based on a concurrent specification language, which uses a modern concurrent separation logic for reasoning about abstract atomic operations, and an associated refinement calculus. Our reasoning about clients highlights an important difference between reasoning about modules built over a heap, where the interference on the shared state is restricted to the operations of the module, and modules built over a file system, where the interference cannot be restricted as the file system is a public namespace. We introduce specifications conditional on context invariants used to restrict the interference, and apply our reasoning to lock files and named pipes. Program reasoning based on separation logic has been successful at verifying that programs do not crash due to illegal use of resources, such invalid memory accesses. The underlying assumption of separation logics, however, is that machines do not fail. In practice, machines can fail unpredictably for various reasons, such as power loss, corrupting resources or resulting in permanent data loss. Critical software, such as file systems and databases, employ recovery methods to mitigate these effects. We introduce an extension of the Views framework to reason about programs in the presence of such events and their associated recovery methods. We use concurrent separation logic as an instance of the framework to illustrate our reasoning, and explore programs using write-ahead logging, such a stylised ARIES recovery algorithm.

[19] Pedro da Rocha Pinto. Reasoning with Time and Data Abstractions. PhD thesis, Imperial College London, September 2016. [ bib | .pdf ]
In this thesis, we address the problem of verifying the functional correctness of concurrent programs, with emphasis on fine-grained concurrent data structures. Reasoning about such programs is challenging since data can be concurrently accessed by multiple threads: the reasoning must account for the interference between threads, which is often subtle. To reason about interference, concurrent operations should either be at distinct times or on distinct data. We present TaDA, a sound program logic for verifying clients and implementations that use abstract specifications that incorporate both abstract atomicity-the abstraction that operations take effect at a single, discrete instant in time-and abstract disjointness-the abstraction that operations act on distinct data resources. Our key contribution is the introduction of atomic triples, which offer an expressive approach for specifying program modules. We also present Total-TaDA, a sound extension of TaDA with which we can verify total correctness of concurrent programs, i.e. that such programs both produce the correct result and terminate. With Total-TaDA, we can specify constraints on a thread=92s concurrent environment that are necessary to guarantee termination. This allows us to verify total correctness for nonblocking algorithms and express lock- and wait-freedom. More generally, the abstract specifications can express that one operation cannot impede the progress of another, a new non-blocking property that we call non-impedance. Finally, we describe how to extend TaDA for proving abstract atomicity for data structures that make use of helping-where one thread is performing an abstract operation on behalf of another-and speculation-where an abstract operation is determined by future behaviour.

[20] Azalea Raad. Abstraction, Refinement and Concurrent Reasoning. PhD thesis, Imperial College London, September 2016. [ bib | .pdf ]
This thesis explores the challenges in abstract library specification, library refinement and reasoning about fine-grained concurrent programs.For abstract library specification, this thesis applies structural separation logic (SSL) to formally specify the behaviour of several libraries in an abstract, local and compositional manner. This thesis further generalises the theory of SSL to allow for library specifications that are language independent. Most notably, we specify a fragment of the Document Object Model (DOM) library. This result is compelling as it significantly improves upon existing DOM formalisms in that the specifications produced are local, compositional and language-independent. Concerning library refinement, this thesis explores two existing approaches to library refinement for separation logic, identifying their advantages and limitations in different settings. This thesis then introduces a hybrid approach to refinement, combining the strengths of both techniques for simple scalable library refinement. These ideas are then adapted to refinement for SSL by presenting a JavaScript implementation of the DOM fragment studied and establishing its correctness with respect to its specification using the hybrid refinement approach.As to concurrent reasoning, this thesis introduces concurrent local subjective logic (CoLoSL) for compositional reasoning about fine-grained concurrent programs. CoLoSL introduces subjective views, where each thread is verified with respect to a customised local view of the state, as well as the general composition and framing of interference relations, allowing for better proof reuse.

[21] Andreas Haas, Thomas A. Henzinger, Andreas Holzer, Christoph M. Kirsch, Michael Lippautz, Hannes Payer, Ali Sezgin, Ana Sokolova, and Helmut Veith. Local Linearizability for Concurrent Container-Type Data Structures. In 27th International Conference on Concurrency Theory (CONCUR 2016), volume 59 of Leibniz International Proceedings in Informatics (LIPIcs), pages 6:1-6:15, Dagstuhl, Germany, August 2016. Schloss Dagstuhl-Leibniz-Zentrum fuer Informatik. [ bib | DOI | .pdf ]
The semantics of concurrent data structures is usually given by a sequential specification and a consistency condition. Linearizability is the most popular consistency condition due to its simplicity and general applicability. Nevertheless, for applications that do not require all guarantees offered by linearizability, recent research has focused on improving performance and scalability of concurrent data structures by relaxing their semantics. In this paper, we present local linearizability, a relaxed consistency condition that is applicable to container-type concurrent data structures like pools, queues, and stacks. While linearizability requires that the effect of each operation is observed by all threads at the same time, local linearizability only requires that for each thread T, the effects of its local insertion operations and the effects of those removal operations that remove values inserted by T are observed by all threads at the same time. We investigate theoretical and practical properties of local linearizability and its relationship to many existing consistency conditions. We present a generic implementation method for locally linearizable data structures that uses existing linearizable data structures as building blocks. Our implementations show performance and scalability improvements over the original building blocks and outperform the fastest existing container-type implementations.

[22] Victor B. F. Gomes and Georg Struth. Program construction and verification components based on Kleene algebra. Archive of Formal Proofs, 2016, June 2016. [ bib | http ]
Variants of Kleene algebra support program construction and verification by algebraic reasoning. This entry provides a verification component for Hoare logic based on Kleene algebra with tests, verification components for weakest preconditions and strongest postconditions based on Kleene algebra with domain and a component for step-wise refinement based on refinement Kleene algebra with tests. In addition to these components for the partial correctness of while programs, a verification component for total correctness based on divergence Kleene algebras and one for (partial correctness) of recursive programs based on domain quantales are provided. Finally we have integrated memory models for programs with pointers and a program trace semantics into the weakest precondition component.

[23] Pedro da Rocha Pinto, Thomas Dinsdale-Young, Philippa Gardner, and Julian Sutherland. Modular termination verification for non-blocking concurrency. In ESOP 2016: Proceedings of the 25th European Symposium on Programming, pages 176-201, April 2016. [ bib | DOI | http | .pdf ]
We present Total-TaDA, a program logic for verifying the total correctness of concurrent programs: that such programs both terminate and produce the correct result. With Total-TaDA, we can specify constraints on a thread's concurrent environment that are necessary to guarantee termination. This allows us to verify total correctness for non-blocking algorithms, e.g. a counter and a stack. Our specifications can express lock- and wait-freedom. More generally, they can express that one operation cannot impede the progress of another, a new non-blocking property we call non-impedance. Moreover, our approach is modular. We can verify the operations of a module independently, and build up modules on top of each other.

[24] Kasper Svendsen, Filip Sieczkowski, and Lars Birkedal. Transfinite step-indexing: Decoupling concrete and logical steps. In ESOP 2016, pages 727-751, April 2016. [ bib | DOI | http | .pdf ]
Step-indexing has proven to be a powerful technique for defining logical relations for languages with advanced type systems and models of expressive program logics. In both cases, the model is stratified using natural numbers to solve a recursive equation that has no naive solutions. As a result of this stratification, current models require that each unfolding of the recursive equation - each logical step - must coincide with a concrete reduction step. This tight coupling is problematic for applications where the number of logical steps cannot be statically bounded.

In this paper we demonstrate that this tight coupling between logical and concrete steps is artificial and show how to loosen it using transfinite step-indexing. We present a logical relation that supports an arbitrary but finite number of logical steps for each concrete step.

[25] Victor B. F. Gomes, Walter Guttmann, Peter Höfner, Georg Struth, and Tjark Weber. Kleene algebras with domain. Archive of Formal Proofs, 2016, April 2016. [ bib | http ]
Kleene algebras with domain are Kleene algebras endowed with an operation that maps each element of the algebra to its domain of definition (or its complement) in abstract fashion. They form a simple algebraic basis for Hoare logics, dynamic logics or predicate transformer semantics. We formalise a modular hierarchy of algebras with domain and antidomain (domain complement) operations in Isabelle/HOL that ranges from domain and antidomain semigroups to modal Kleene algebras and divergence Kleene algebras. We link these algebras with models of binary relations and program traces. We include some examples from modal logics, termination and program analysis.

[26] David Kaloper-Meršinjak and Hannes Mehnert. Not-quite-so-broken TLS 1.3 mechanised conformance checking. In TLSv1.3 - Ready or Not? (TRON) workshop, February 2016. [ bib | .pdf ]
We present a set of tools to aid TLS 1.3 implementors, all derived from a single TLS implementation/model. These include an automated offline TLS protocol conformance checker, a test oracle validating recorded sessions, a tool replicating recorded sessions with other implementations, and an interactive online handshake visualisation.

The conformance checker repeatedly runs a user-provided TLS implementation, attempting to establish TLS sessions with it; the checker explores the TLS parameter space to determine feature coverage of the provided implementation. The test oracle takes a recorded session between two endpoints and decides whether the session was conformant with the specification. The replication utility re-runs one side of a recorded session against another TLS implementation, and observes its behaviour. The online visualisation accepts connections from clients and presents the TLS session as an interactive sequence diagram.

All of these tools are based on our clean-slate nqsb-TLS implementation/model. It already supports TLS 1.0-1.2, and interoperates with a broad range of other TLS implementations. We are currently extending nqsb-TLS with TLS 1.3 support, and tracking the progress of the TLS 1.3 draft, adapting our implementation/model accordingly.

We invite the community to use our tools while implementing the TLS 1.3 RFC, and provide feedback on deviations in the interpretation thereof. This process enables the community to converge to a single, mechanically checkable TLS 1.3 model, as implemented by nqsb-TLS.

[27] Gian Ntzik, Pedro da Rocha Pinto, and Philippa Gardner. Fault-tolerant Resource Reasoning. In APLAS 2015: Proceedings of the 13th Asian Symposium on Programming Languages and Systems, pages 169-188, December 2015. [ bib | DOI | http | .pdf ]
Separation logic has been successful at verifying that programs do not crash due to illegal use of resources. The underlying assumption, however, is that machines do not fail. In practice, machines can fail unpredictably for various reasons, e.g. power loss, corrupting resources. Critical software, e.g. file systems, employ recovery methods to mitigate these effects. We introduce an extension of the Views framework to reason about such methods. We use concurrent separation logic as an instance of the framework to illustrate our reasoning, and explore programs using write-ahead logging, e.g. an ARIES recovery algorithm.

[28] Tom Ridge, David Sheets, Thomas Tuerk, Andrea Giugliano, Anil Madhavapeddy, and Peter Sewell. SibylFS: formal specification and oracle-based testing for POSIX and real-world file systems. In SOSP 2015: Proceedings of the 25th Symposium on Operating Systems Principles, Monterey, CA, USA, pages 38-53, October 2015. [ bib | DOI | http | .pdf ]
Systems depend critically on the behaviour of file systems, but that behaviour differs in many details, both between implementations and between each implementation and the POSIX (and other) prose specifications. Building robust and portable software requires understanding these details and differences, but there is currently no good way to systematically describe, investigate, or test file system behaviour across this complex multi-platform interface.

In this paper we show how to characterise the envelope of allowed behaviour of file systems in a form that enables practical and highly discriminating testing. We give a mathematically rigorous model of file system behaviour, SibylFS, that specifies the range of allowed behaviours of a file system for any sequence of the system calls within our scope, and that can be used as a test oracle to decide whether an observed trace is allowed by the model, both for validating the model and for testing file systems against it. SibylFS is modular enough to not only describe POSIX, but also specific Linux, OSX and FreeBSD behaviours. We complement the model with an extensive test suite of over 21000 tests; this can be run on a target file system and checked in less than 5 minutes, making it usable in practice. Finally, we report experimental results for around 40 configurations of many file systems, identifying many differences and some serious flaws.

[29] Gian Ntzik and Philippa Gardner. Reasoning about the POSIX File System: Local Update and Global Pathnames. In OOPSLA 2015: Proceedings of the 30th Annual ACM SIGPLAN Conference on Object-Oriented Programming, Systems, Languages, and Applications, October 2015. [ bib ]
We introduce a program logic for specifying a core sequential subset of the POSIX file system and for reasoning abstractly about client programs working with the file system. The challenge is to reason about the combination of local directory update and global pathname traversal (including '..' and symbolic links) which may overlap the directories being updated. Existing reasoning techniques are either based on first-order logic and do not scale, or on separation logic and can only handle linear pathnames (no '..' or symbolic links). We introduce fusion logic for reasoning about local update and global pathname traversal, introducing a novel effect frame rule to propagate the effect of a local update on overlapping pathnames. We apply our reasoning to the standard recursive remove utility (rm -r), discovering bugs in well-known implementations.

[30] David Kaloper-Mersinjak, Hannes Mehnert, Anil Madhavapeddy, and Peter Sewell. Not-quite-so-broken TLS: lessons in re-engineering a security protocol specification and implementation. In USENIX Security 2015: 24th USENIX Security Symposium, Washington, D.C., USA, pages 223-238, August 2015. [ bib | http | .pdf ]
Transport Layer Security (TLS) implementations have a history of security flaws. The immediate causes of these are often programming errors, e.g. in memory management, but the root causes are more fundamental: the challenges of interpreting the ambiguous prose specification, the complexities inherent in large APIs and code bases, inherently unsafe programming choices, and the impossibility of directly testing conformance between implementations and the specification.

We present nqsb-TLS, the result of our re-engineered approach to security protocol specification and implementation that addresses these root causes. The same code serves dual roles: it is both a specification of TLS, executable as a test oracle to check conformance of traces from arbitrary implementations, and a usable implementation of TLS; a modular and declarative programming style provides clean separation between its components. Many security flaws are thus excluded by construction.

nqsb-TLS can be used in standalone applications, which we demonstrate with a messaging client, and can also be compiled into a Xen unikernel (a specialised virtual machine image) with a TCB that is 4% of a standalone system running a standard Linux/OpenSSL stack, with all network traffic being handled in a memory-safe language; this supports applications including HTTPS, IMAP, Git, and Websocket clients and servers.

[31] Ali Sezgin and Serdar Tasiran. Moving around: Lipton's reduction for TSO - (regular submission). In VSTTE 2015: Verified Software: Theories, Tools, and Experiments - 7th International Conference, San Francisco, CA, USA. Revised Selected Papers, pages 165-182, July 2015. [ bib | DOI | http ]
We generalize Lipton’s reduction theory, hitherto limited to SC, for TSO programs. We demonstrate the use of our theory by specifying the conditions under which a particular write is SC-like (i.e. placing a fence immediately after the write does not constrain the behavior of the overall program) and a library implementation can be safely used (i.e. compositionality). Our theory is complete: a program has only SC behaviors iff there is a proof that establishes that every write in the program is SC-like. We adapt the notion of program abstraction to TSO analysis via our theory. We define precisely what is meant by abstraction, and propose a methodology by which one can obtain via abstraction SC summaries of a program which may have non-SC behaviors. Finally, we show how checking whether a write instruction is SC-like can be mechanized. We describe a transformation in which the execution of each thread of the original program (under TSO) is simulated by the execution of two tightly coupled threads in the new program (under SC).

[32] Pedro da Rocha Pinto, Thomas Dinsdale-Young, and Philippa Gardner. Steps in Modular Specifications for Concurrent Modules (Invited Tutorial Paper). In MFPS 2015: Proceedings of the 31st Conference on the Mathematical Foundations of Programming Semantics, pages 3-18, June 2015. [ bib | DOI | http | .pdf ]
The specification of a concurrent program module is a difficult problem. The specifications must be strong enough to enable reasoning about the intended clients without reference to the underlying module implementation. We survey a range of verification techniques for specifying concurrent modules, in particular highlighting four key concepts: auxiliary state, interference abstraction, resource ownership and atomicity. We show how these concepts combine to provide powerful approaches to specifying concurrent modules.

[33] David Kaloper Meršinjak, Hannes Mehnert, Anil Madhavapeddy, and Peter Sewell. Not-quite-so-broken TLS: lessons in re-engineering a security protocol specification and implementation, May 2015. Presentation at HCSS 2015: the Fifteenth Annual High Confidence Software and Systems Conference. [ bib ]
Transport Layer Security (TLS) implementations have a history of security flaws. The immediate causes of these range from simple programming errors, such as memory management, to subtle violations of the protocol logic. Deeper causes can be seen in the challenges of interpreting the ambiguous prose specification, the complexities inherent in large APIs and code bases, unsafe performance-oriented programming choices, and the impossibility of directly testing conformance between implementations and the specification.

We present nqsb-TLS, the result of our re-engineering approach to improve the quality of security protocol implementations. The same code serves dual roles: it is both a specification of TLS, executable as a test oracle to check conformance of traces from arbitrary implementations, and a secure and usable executable implementation of TLS. nqsb-TLS employs a modular and declarative programming style that also allows it to be compiled into a Xen unikernel (a specialised virtual machine image) with a TCB that is 2.5% of a standalone system running a standard Linux/OpenSSL stack, with all network traffic being handled in a memory-safe language.

nqsb-TLS focuses on protocol-level interoperability, and makes no effort to be compatible with existing (and often poorly designed) library APIs such as OpenSSL. The higher-level API in nqsb-TLS makes it harder to misuse the library, and is demonstrated via several unikernel applications ranging over HTTPS, IMAP, Git, Websocket clients and servers.

[34] Filip Sieczkowski, Kasper Svendsen, Lars Birkedal, and Jean Pichon-Pharabod. A separation logic for fictional sequential consistency. In ESOP 2015, April 2015. [ bib | DOI ]
To improve performance, modern multiprocessors and programming languages typically implement relaxed memory models that do not require all processors/threads to observe memory operations in the same order. To relieve programmers from having to reason directly about these relaxed behaviors, languages often provide efficient synchronization primitives and concurrent data structures with stronger high-level guarantees about memory reorderings. For instance, locks usually ensure that when a thread acquires a lock, it can observe all memory operations of the releasing thread, prior to the release. When used correctly, these synchronization primitives and data structures allow clients to recover a fiction of a sequentially consistent memory model.

In this paper we propose a new proof system, iCAP-TSO, that captures this fiction formally, for a language with a TSO memory model. The logic supports reasoning about libraries that directly exploit the relaxed memory model to achieve maximum efficiency. When these libraries provide sufficient guarantees, the logic hides the underlying complexity and admits standard separation logic rules for reasoning about their more high-level clients.

[35] Azalea Raad, Jules Villard, and Philippa Gardner. CoLoSL: Concurrent local subjective logic. In ESOP 2015, April 2015. [ bib | DOI | .pdf ]
A key difficulty in verifying shared-memory concurrent programs is reasoning compositionally about each thread in isolation. Existing verification techniques for fine-grained concurrency typically require reasoning about either the entire shared state or disjoint parts of the shared state, impeding compositionality. This paper introduces the program logic CoLoSL, where each thread is verified with respect to its subjective view of the global shared state. This subjective view describes only that part of the state accessed by the thread. Subjective views may arbitrarily overlap with each other, and expand and contract depending on the resource required by the thread. This flexibility gives rise to small specifications and, hence, more compositional reasoning for concurrent programs. We demonstrate our reasoning on a range of examples, including a concurrent computation of a spanning tree of a graph.

[36] Soham Chakraborty, Thomas A. Henzinger, Ali Sezgin, and Viktor Vafeiadis. Aspect-oriented linearizability proofs. Logical Methods in Computer Science, 11(1):1-33, April 2015. [ bib | DOI | http ]
Linearizability of concurrent data structures is usually proved by monolithic simulation arguments relying on the identification of the so-called linearization points. Regrettably, such proofs, whether manual or automatic, are often complicated and scale poorly to advanced non-blocking concurrency patterns, such as helping and optimistic updates. In response, we propose a more modular way of checking linearizability of concurrent queue algorithms that does not involve identifying linearization points. We reduce the task of proving linearizability with respect to the queue specification to establishing four basic properties, each of which can be proved independently by simpler arguments. As a demonstration of our approach, we verify the Herlihy and Wing queue, an algorithm that is challenging to verify by a simulation proof.

[37] David Kaloper Meršinjak and Hannes Mehnert. Trustworthy secure modular operating system engineering. Invited talk at 31st Chaos Communication Congress (31C3), January 2015. [ bib ]
We present Mirage OS, a modular library operating system developed from scratch in the functional programming language OCaml. Each service, called unikernel, is an OCaml application using libraries such as a TCP/IP stack, DNS. It is either compiled to a Xen virtual machine image or to a Unix binary (for development). State in 2014 is that it runs on x86 and arm, we implemented a clean-slate TLS (1.0, 1.1, 1.2), X.509, ASN.1 stack, crypto primitives, Off-the-record. We also have TCP/IP, HTTP, a persistent branchable store (similar to git) - all implemented in OCaml. A virtual machine serving data via https is roughly 2MB in size - no libc inside :)

[38] Hannes Mehnert and David Kaloper Mersinjak. Transport Layer Security purely in OCaml. In Proceedings of the ACM OCaml 2014 Workshop, September 2014. [ bib | .pdf ]
Transport Layer Security (TLS) is probably the most widely deployed security protocol on the Internet. It is used to setup virtual private networks, secure various services such as web and email, etc. In this paper we describe our clean slate TLS implementation developed in OCaml. Our motivating goals are reliability, robustness, and API conciseness. While our implementation is still a work in progress and lacks some of the protocol features (such as client authentication and session resumption), it already interoperates with other existing TLS implementations. Preliminary performance evaluation shows that our library is roughly five times slower compared to OpenSSL, but we expect to improve our performance.

[39] Pedro da Rocha Pinto, Thomas Dinsdale-Young, and Philippa Gardner. TaDA: A logic for time and data abstraction. In Richard Jones, editor, ECOOP 2014, volume 8586 of Lecture Notes in Computer Science, pages 207-231. Springer Berlin Heidelberg, July 2014. [ bib | DOI | http ]
To avoid data races, concurrent operations should either be at distinct times or on distinct data. Atomicity is the abstraction that an operation takes effect at a single, discrete instant in time, with linearisability being a well-known correctness condition which asserts that concurrent operations appear to behave atomically. Disjointness is the abstraction that operations act on distinct data resource, with concurrent separation logics enabling reasoning about threads that appear to operate independently on disjoint resources.

We present TaDA, a program logic that combines the benefits of abstract atomicity and abstract disjointness. Our key contribution is the introduction of atomic triples, which offer an expressive approach to specifying program modules. By building up examples, we show that TaDA supports elegant modular reasoning in a way that was not previously possible.

[40] Philippa Gardner, Azalea Raad, Mark J. Wheelhouse, and Adam Wright. Abstract local reasoning for concurrent libraries: Mind the gap. MFPS 2014: Electr. Notes Theor. Comput. Sci., 308:147-166, June 2014. [ bib | DOI | http ]
We study abstract local reasoning for concurrent libraries. There are two main approaches: provide a specification of a library by abstracting from concrete reasoning about an implementation; or provide a direct abstract library specification, justified by refining to an implementation. Both approaches have a significant gap in their reasoning, due to a mismatch between the abstract connectivity of the abstract data structures and the concrete connectivity of the concrete heap representations. We demonstrate this gap using structural separation logic (SSL) for specifying a concurrent tree library and concurrent abstract predicates (CAP) for reasoning about a concrete tree implementation. The gap between the abstract and concrete connectivity emerges as a mismatch between the SSL tree predicates and CAP heap predicates. This gap is closed by an interface function I which links the abstract and concrete connectivity. In the accompanying technical report, we generalise our SSL reasoning and results to arbitrary concurrent data libraries.

[41] Philippa Gardner, Gian Ntzik, and Adam Wright. Local reasoning for the POSIX file system. In ESOP 2014: Programming Languages and Systems - 23rd European Symposium on Programming, Held as Part of the European Joint Conferences on Theory and Practice of Software, ETAPS 2014, Grenoble, France, pages 169-188, April 2014. [ bib | DOI | http ]
We provide a program logic for specifying a core subset of the sequential POSIX file system, and for reasoning abstractly about client programs working with the file system.

[42] Tony Hoare, Stephan van Staden, Bernhard Möller, Georg Struth, Jules Villard, Huibiao Zhu, and Peter W. O'Hearn. Developments in concurrent Kleene algebra. In RAMiCS 2014: Relational and Algebraic Methods in Computer Science - 14th International Conference, Marienstatt, Germany, pages 1-18, April 2014. [ bib | DOI | http ]
This report summarises recent progress in the research of its co-authors towards the construction of links between algebraic presentations of the principles of programming and the exploitation of concurrency in modern programming practice. The research concentrates on the construction of a realistic family of partial order models for Concurrent Kleene Algebra (aka, the Laws of Programming). The main elements of the model are objects and the events in which they engage. Further primitive concepts are traces, errors and failures, and transferrable ownership. In terms of these we can define other concepts which have proved useful in reasoning about concurrent programs, for example causal dependency and independence, sequentiality and concurrency, allocation and disposal, synchrony and asynchrony, sharing and locality, input and output.

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