Checking system rules using system-specific, programmer-written compiler extensions

Systems software such as OS kernels, embedded systems, and libraries must obey many rules for both correctness and performance. Common examples include 'accesses to variable A must be guarded by lock B,' 'system calls must check user pointers for validity before using them,' and 'message handlers should free their buffers as quickly as possible to allow greater parallelism.' Unfortunately, adherence to these rules is largely unchecked. This paper attacks this problem by showing how system implementors can use meta-level compilation (MC) to write simple, system- specific compiler extensions that automatically check their code for rule violations. By melding domain-specific knowledge with the automatic machinery of compilers, MC brings the benefits of language-level checking and optimizing to the higher, 'meta' level of the systems implemented in these languages. This paper demonstrates the effectiveness of the MC approach by applying it to four complex, real systems: Linux, OpenBSD, the Xok exokernel, and the FLASH machine's embedded software. MC extensions found roughly 500 errors in these systems and led to numerous kernel patches. Most extensions were less than a hundred lines of code and written by implementors who had a limited understanding of the systems checked.