This paper reports our experience applying lightweight formal methods to validate the correctness of ShardStore, a new key-value storage node implementation for the Amazon S3 cloud object storage service. By "lightweight formal methods" we mean a pragmatic approach to verifying the correctness of a production storage node that is under ongoing feature development by a full-time engineering team. We do not aim to achieve full formal verification, but instead emphasize automation, usability, and the ability to continually ensure correctness as both software and its specification evolve over time. Our approach decomposes correctness into independent properties, each checked by the most appropriate tool, and develops executable reference models as specifications to be checked against the implementation. Our work has prevented 16 issues from reaching production, including subtle crash consistency and concurrency problems, and has been extended by non-formal-methods experts to check new features and properties as ShardStore has evolved.

Rust’s ownership type system enforces a strict discipline on how memory locations are accessed and shared. This discipline allows the compiler to statically prevent memory errors, data races, inadvertent side effects through aliasing, and other errors that frequently occur in conventional imperative programs. However, the restrictions imposed by Rust’s type system make it difficult or impossible to implement certain designs, such as data structures that require aliasing (e.g. doubly-linked lists and shared caches). To work around this limitation, Rust allows code blocks to be declared as unsafe and thereby exempted from certain restrictions of the type system, for instance, to manipulate C-style raw pointers. Ensuring the safety of unsafe code is the responsibility of the programmer. However, an important assumption of the Rust language, which we dub the Rust hypothesis , is that programmers use Rust by following three main principles: use unsafe code sparingly, make it easy to review, and hide it behind a safe abstraction such that client code can be written in safe Rust. Understanding how Rust programmers use unsafe code and, in particular, whether the Rust hypothesis holds is essential for Rust developers and testers, language and library designers, as well as tool developers. This paper studies empirically how unsafe code is used in practice by analysing a large corpus of Rust projects to assess the validity of the Rust hypothesis and to classify the purpose of unsafe code. We identify queries that can be answered by automatically inspecting the program’s source code, its intermediate representation MIR, as well as type information provided by the Rust compiler; we complement the results by manual code inspection. Our study supports the Rust hypothesis partially: While most unsafe code is simple and well-encapsulated, unsafe features are used extensively, especially for interoperability with other languages.

Rust's type system ensures memory safety: well-typed Rust programs are guaranteed to not exhibit problems such as dangling pointers, data races, and unexpected side effects through aliased references. Ensuring correctness properties beyond memory safety, for instance, the guaranteed absence of assertion failures or more-general functional correctness, requires static program verification. For traditional system programming languages, formal verification is notoriously difficult and requires complex specifications and logics to reason about pointers, aliasing, and side effects on mutable state. This complexity is a major obstacle to the more-widespread verification of system software. In this paper, we present a novel verification technique that leverages Rust's type system to greatly simplify the specification and verification of system software written in Rust. We analyse information from the Rust compiler and synthesise a corresponding core proof for the program in a flavour of separation logic tailored to automation. To verify correctness properties beyond memory safety, users can annotate Rust programs with specifications at the abstraction level of Rust expressions; our technique weaves them into the core proof to verify modularly whether these specifications hold. Crucially, our proofs are constructed and checked automatically without exposing the underlying formal logic, allowing users to work exclusively at the level of abstraction of the programming language. As such, our work enables a new kind of verification tool, with the potential to impact a wide audience and allow the Rust community to benefit from state-of-the-art verification techniques. We have implemented our techniques for a subset of Rust; our evaluation on several thousand functions from widely-used Rust crates demonstrates its effectiveness.