In contemporary IoT edge devices with real-time requirements, security is primarily enforced through design-time parameters associated with security tasks, leading to mechanisms that operate in an \emph{opportunistic} manner. As a result, security checks are often performed as secondary operations. This approach can result in systems where no security tasks are executed due to high utilization by other tasks. An alternative approach taken in prior work is to add security mechanisms to every task in the system, resulting in substantially lower performance than that of a system with no security. These approaches have resulted in an \emph{all-or-nothing} scenario for edge device security, motivating numerous studies on the safety-security trade-off in real-time cyber-physical systems (RT-CPS). This study introduces an analytical framework -- REPOSE -- for evaluating the security feasibility of real-time control systems at runtime. REPOSE is developed for \textit{weakly-hard} real-time control systems that facilitate a ``bounded trade-off'' between safety and security. In contrast to imposing additional (pessimistic) design-time overhead as considered in some real-time security literature, REPOSE performs security operations in both \textit{proactive} and \textit{reactive} manners based on the task's current behavior. Our evaluations show that REPOSE can effectively add security operations to RT-CPS with a feasibility overhead of $0.06\%$ at $80\%$ utilization, compared to a $ 29\%$ overhead observed in systems with hard constraints. Through a case study of a classic control system, we also demonstrate that REPOSE provides a robust framework to \textit{analyze and calculate} the safety-security tradeoff.

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