Function flattening for lease-based, information-leak-free systems

Recent research has proposed security-critical real-time embedded systems with provably-strong information containment through the use of hardware-enforced execution leases. Execution leases bound the time and address space used by functions to prevent information leakage between functions. Nested functions, however, require a relatively expensive hardware stack of execution leases. We introduce techniques to flatten nested functions and reduce overhead of the hardware stack. We note that while function flattening is impractical for conventional systems, avoiding information leakage results in constraints on program control that also make flattening possible in this setting. Through a combination of code hoisting and function splitting, we find that leases for nested functions can be substantially flattened in several practical examples. We note that some nested loop and function structures can lead to exponential growth in code size due to flattening, but that our techniques give system designers the ability to trade code size with hardware cost.