Overcoming restraint: Composing verification of foreign functions with Cogent
Authors
Louis Cheung, Liam O'Connor and Christine Rizkallah
School of Computer Science and Engineering
UNSW,
Sydney 2052, Australia
Abstract
Cogent is a restricted functional language designed to
reduce the cost of developing verified systems code. Because of its
sometimes-onerous restrictions, such as the lack of support for
recursion and its strict uniqueness type system, Cogent provides an
escape hatch in the form of a foreign function interface (FFI) to C
code. This poses a problem when verifying Cogent programs, as imported
C components do not enjoy the same level of static guarantees that
Cogent does. Previous verification of file systems implemented in
Cogent merely assumed that their C components were correct and that
they preserved the invariants of Cogent’s type system. In this paper,
we instead prove such obligations. We demonstrate how they smoothly
compose with existing Cogent theorems, and result in a correctness
theorem of the overall Cogent-C system. The Cogent FFI constraints
ensure that key invariants of Cogent’s type system are maintained even
when calling C code. We verify reusable higher-order and polymorphic
functions including a generic loop combinator and array iterators and
demonstrate their application to several examples including binary
search and the BilbyFs file system. We demonstrate the feasibility of
verification of mixed Cogent-C systems, and provide some insight into
verification of software comprised of code in multiple languages with
differing levels of static guarantees.
BibTeX Entry
@inproceedings{Cheung_OR_22,
address = {New York, NY, USA},
author = {Cheung, Louis and O'Connor, Liam and Rizkallah, Christine},
booktitle = {Proceedings of the 11th ACM SIGPLAN International Conference on Certified Programs and Proofs},
doi = {10.1145/3497775.3503686},
isbn = {9781450391825},
keywords = {data-structures, verification, type-systems, compilers, language interoperability},
location = {Philadelphia, PA, USA},
numpages = {14},
pages = {13–26},
paperurl = {https://trustworthy.systems/publications/papers/Cheung_OR_22.pdf},
publisher = {ACM},
series = {CPP 2022},
title = {Overcoming Restraint: Composing Verification of Foreign Functions with {Cogent}},
url = {https://doi.org/10.1145/3497775.3503686},
year = {2022}
}
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