This paper describes our experience applying formal methods to a critical component in the Linux kernel, the just-in-time compilers (“JITs”) for the Berkeley Packet Filter (BPF) virtual machine. We verify these JITs using Jitterbug, the first framework to provide a precise specification of JIT correctness that is capable of ruling out real-world bugs, and an automated proof strategy that scales to practical implementations. Using Jitterbug, we have designed, implemented, and verified a new BPF JIT for 32-bit RISC-V, found and fixed 16 previously unknown bugs in five other deployed JITs, and developed new JIT optimizations; all of these changes have been upstreamed to the Linux kernel. The results show that it is possible to build a verified component within a large, unverified system with careful design of specification and proof strategy.

/pdf/specification-and-verification-in-the-field-applying-formal-2yq1e7mbk2.pdf

Specification and verification in the field: Applying formal methods to {BPF} just-in-time compilers in the Linux kernel

Recent advances in program synthesis convinced us that it was the right time to transform the process of porting an operating system into a program synthesis problem. We set out to synthesize the needed machine dependent code for an existing operating system. This undertaking proved far more challenging than we anticipated. We summarize our experience and lessons learned and propose next steps in realizing such an undertaking.

https://dl.acm.org/doi/pdf/10.1145/3365137.3365401

Trials and Tribulations in Synthesizing Operating Systems

The increasing dependability requirements and hardware diversity of the Internet of Things (IoT) pose a challenge to developers. New approaches for software development that guarantee correct implementations will become indispensable. Specially for Real Time Operating Systems (RTOSs), automatic porting for all current and future devices will also be required. As part of our framework for embedded RTOS portability, based on formal methods and code generation, we present our approach to formally model low-level operating-system functionality using Event-B . We show the part of our RTOS model where the switch into the kernel and back to a task happens, and prove that the model is correct according to the specification. Hardware details are only introduced in late refinements, which allows us to reuse most of the RTOS model and proofs for several target platforms. As a proof of concept, we refine the generic model to two different architectures and prove safety and liveness properties of the models.

A Formal Modeling Approach for Portable Low-Level OS Functionality

The end of Moore’s Law has ushered in a diversity of hardware not seen in decades. Operating system (OS) (and system software) portability is accordingly becoming increasingly critical. Simultaneously, there has been tremendous progress in program synthesis. We set out to explore the feasibility of using modern program synthesis to generate the machine-dependent parts of an operating system. Our ultimate goal is to generate new ports automatically from descriptions of new machines. One of the issues involved is writing specifications, both for machine-dependent operating system functionality and for instruction set architectures. We designed two domain-specific languages: Alewife for machine-independent specifications of machine-dependent operating system functionality and Cassiopea for describing instruction set architecture semantics. Automated porting also requires an implementation. We developed a toolchain that, given an Alewife specification and a Cassiopea machine description, specializes the machine-independent specification to the target instruction set architecture and synthesizes an implementation in assembly language with a customized symbolic execution engine. Using this approach, we demonstrate the successful synthesis of a total of 140 OS components from two pre-existing OSes for four real hardware platforms. We also developed several optimization methods for OS-related assembly synthesis to improve scalability. The effectiveness of our languages and ability to synthesize code for all 140 specifications is evidence of the feasibility of program synthesis for machine-dependent OS code. However, many research challenges remain; we also discuss the benefits and limitations of our synthesis-based approach to automated OS porting.

/pdf/towards-porting-operating-systems-with-program-synthesis-2hnm1kli.pdf

Towards Porting Operating Systems with Program Synthesis

A Study on the Portability of IoT Operating Systems

This technical report describes two of the domain-specific languages used in the Aquarium kernel code synthesis project. It presents the language cores in terms of abstract syntax. Cassiopea is a machine description language for describing the semantics of processor instruction sets. Alewife is a specification language that can be used to write machine-independent specifications for assembly-level instruction blocks. An Alewife specification can be used to verify and synthesize code for any machine described in Cassiopea, given a machine-specific translation for abstractions used in the specification. This article does not include an introduction to either the Aquarium system or the use of the languages. In addition to this version of the article being a draft, the Aquarium project and the languages are work in progress. This article cannot currently be considered either final or complete.

Aquarium: Cassiopea and Alewife Languages

Aquarium Technical Report Preprint.

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Papers

Trials and Tribulations in Synthesizing Operating Systems

Aquarium: Cassiopea and Alewife Languages

Towards Porting Operating Systems with Program Synthesis

Aquarium Technical Report Preprint.