We have developed and mechanically verified an ML system called CakeML, which supports a substantial subset of Standard ML. CakeML is implemented as an interactive read-eval-print loop (REPL) in x86-64 machine code. Our correctness theorem ensures that this REPL implementation prints only those results permitted by the semantics of CakeML. Our verification effort touches on a breadth of topics including lexing, parsing, type checking, incremental and dynamic compilation, garbage collection, arbitraryprecision arithmetic, and compiler bootstrapping.Our contributions are twofold. The first is simply in building a system that is end-to-end verified, demonstrating that each piece of such a verification effort can in practice be composed with the others, and ensuring that none of the pieces rely on any over-simplifying assumptions. The second is developing novel approaches to some of the more challenging aspects of the verification. In particular, our formally verified compiler can bootstrap itself: we apply the verified compiler to itself to produce a verified machine-code implementation of the compiler. Additionally, our compiler proof handles diverging input programs with a lightweight approach based on logical timeout exceptions. The entire development was carried out in the HOL4 theorem prover.
We present a formal model of memory that both captures the lowlevel features of C's pointers and memory, and that forms the basis for an expressive implementation of separation logic. At the low level, we do not commit common oversimplifications, but correctly deal with C's model of programming language values and the heap. At the level of separation logic, we are still able to reason abstractly and efficiently. We implement this framework in the theorem prover Isabelle/HOL and demonstrate it on two case studies. We show that the divide between detailed and abstract does not impose undue verification overhead, and that simple programs remain easy to verify. We also show that the framework is applicable to real, security-and safety-critical code by formally verifying the memory allocator of the L4 microkernel.
We present a formal model of memory that both captures the lowlevel features of C's pointers and memory, and that forms the basis for an expressive implementation of separation logic. At the low level, we do not commit common oversimplifications, but correctly deal with C's model of programming language values and the heap. At the level of separation logic, we are still able to reason abstractly and efficiently. We implement this framework in the theorem prover Isabelle/HOL and demonstrate it on two case studies. We show that the divide between detailed and abstract does not impose undue verification overhead, and that simple programs remain easy to verify. We also show that the framework is applicable to real, security-and safety-critical code by formally verifying the memory allocator of the L4 microkernel.
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