In this talk, I will give an overview of the various formal verification projects around the evolving seL4 microkernel, and discuss our experience in large scale proof engineering and maintenance. In particular, the presentation will draw a picture of what these verifications mean and how they fit together into a whole. Among these are a number of firsts: the first code-level functional correctness proof of a general-purpose OS kernel, the first non-interference proof for such a kernel at the code-level, the first binary-level functional verification of systems code of this complexity, and the first sound worst-case execution-time profile for a protected-mode operating system kernel. Taken together, these projects produced proof artefacts on the order of 400,000 lines of Isabelle/HOL proof scripts. This order of magnitude brings engineering aspects to proofs that we so far mostly associate with software and code. In the second part of the talk, I will report on our experience in proof engineering methods and tools, and pose a number of research questions that we think will be important to solve for the wider scale practical application of such formal methods in industry.
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