CN: Verifying Systems C Code with Separation-Logic Refinement Types

We have a new paper on combining separation logic and refinement types to verify C code, appearing at POPL 2023 in a couple of months. It's called CN: Verifying Systems C Code with Separation-Logic Refinement Types, and it's by Christopher Pulte, Dhruv C. Makwana, Thomas Sewell, Kayvan Memarian, Peter Sewell, and me.

Despite significant progress in the verification of hypervisors, operating systems, and compilers, and in verification tooling, there exists a wide gap between the approaches used in verification projects and conventional development of systems software. We see two main challenges in bringing these closer together: verification handling the complexity of code and semantics of conventional systems software, and verification usability.

We describe an experiment in verification tool design aimed at addressing some aspects of both: we design and implement CN, a separation-logic refinement type system for C systems software, aimed at predictable proof automation, based on a realistic semantics of ISO C. CN reduces refinement typing to decidable propositional logic reasoning, uses first-class resources to support pointer aliasing and pointer arithmetic, features resource inference for iterated separating conjunction, and uses a novel syntactic restriction of ghost variables in specifications to guarantee their successful inference. We implement CN and formalise key aspects of the type system, including a soundness proof of type checking. To demonstrate the usability of CN we use it to verify a substantial component of Google’s pKVM hypervisor for Android.

There's some cool separation logic trickery that might become a blogpost, and of course the CN tool is open source, and is publically available as a backend of the Cerberus executable C semantics. Naturally I (and most of my coauthors) will be in Boston this January for POPL, and the talk about CN will be public then. If you can't wait, earlier this summer I gave a talk about CN which was recorded and on Youtube.