Verification of programmable logic controller code using model checking and static analysis

摘要

Programmable Logic Controllers (PLCs) are control devices used in industry to control, operate and monitor plants, machines and robots. PLCs comprise input connectors, typically connected to sensors, output connectors, typically connected to actuators, and a program, which controls the behavior, computing new output values based on the input values and an internal state. Since PLCs operate in safety-critical environments, where a malfunction could seriously harm the environment, humans, or the plant, it is recommended to verify their programs using formal methods.This dissertation studies the formal methods model checking and static analysis to prove the correctness of PLC programs. For this, we developed the tool Arcade.PLC, which allow the automatic application of these methods to PLC programs written for different vendors. It extracts a model from the program by abstract simulation, which over-approximates the possible program behavior. The user is then able to verify whether the model obeys a specification, which can be written in the logic CTL or using automata. For applying model checking, we demonstrate how the model can be extracted automatically, such that the approach scales to industrial size programs. For this, we introduce two different abstraction techniques: First, we develop an abstraction refinement guided by the model checker that automatically creates an abstracted model by iteratively analyzing counterexamples. Second, we implemented a predicate abstraction that evaluates a formalized program semantics using an SMT solver. Both techniques are evaluated using different programs from industry and academia. Further, we introduce a simplified formalism to write specifications, which is influenced by an automata-based language established in industry. We implement an algorithm to check programs using this formalism and show that this technique is even able to detect errors in the specification. Finally, we detail how counterexamples generated by the model checker can be analyzed automatically to locate the actual erroneous line in the program.The static analysis we developed is able to detect program errors in a fully automatic way. We detect typical errors such as division by zero and illegal array accesses, but also PLC specific errors, e. g., during a restart. The analysis is based on a value-set analysis, which determines the values of all program variables in each program location. These sets are then verified against the predefined checks or user-provided annotations. We show how to implement this analysis such that it scales to industrial size programs. The approach is evaluated on an industrial case study.