Active DSU: Dynamic Software Updates for Active Functions

Dynamic Software Updating (DSU) is a technique for updating programs without restarting them. It is useful in systems that provide live programming or in system with high availability needs. As a common limitation, existing DSU systems cannot update active functions. Hence, they are unable to update long-running functions, such as the main loop in a web server, or a state machine in an embedded system implemented as a loop. Updating active functions is challenging as it requires updating local variables and control flow to create a consistent state in the new version of the function. In this paper, we propose Active DSU, which updates the call stack to migrate data and control flow of active functions without the need to wait for the program to reach a particular state beforehand. This is achieved by replacing return addresses with stubs to avoid moving other stack frames on the call stack. Active DSU can migrate control flow automatically without input from the programmer in most cases.