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Secure systems are invariably subject to stresses such as those caused by attack, erroneous or malicious inputs, hardware or software faults, unanticipated user behavior, or unexpected environmental changes) that are outside the bounds of "normal operation," and yet the system must continue to deliver essential services in a timely manner, safely and securely. To accomplish this, a system must exhibit system qualities such as robustness, reliability, error tolerance, fault tolerance, performance, and security. All of these system quality attributes depend upon a consistent and comprehensive error-handling that supports the goals of the overall system.

According

ISO/IEC PDTR 24772 Section 6.47, "REU Termination strategy" says: \[[ISO/IEC PDTR 24772|AA. C References#ISO/IEC PDTR 24772]\]

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Expectations that a system will be dependable are based on the confidence that the system will operate as expected and not fail in normal use. The dependability of a system and its fault tolerance can be measured through the component part's reliability, [availability|BB. Definitions#availability], safety and security. Reliability is the ability of a system or component to perform its required functions under stated conditions for a specified period of time \[[IEEE Std 610.12 1990|AA. C References#IEEE Std 610.12 1990]\]. Availability is how timely and reliable the system is to its intended users. Both of these factors matter highly in systems used for safety and security. In spite of the best intentions, systems will encounter a failure, either from internally poorly written software or external forces such as power outages/variations, floods, or other natural disasters. The reaction to a fault can affect the performance of a system and in particular, the safety and security of the system and its users.

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An error handling policy must specify a comprehensive approach to error reporting and response. Components and routines should always generate status indicators, all called routines should have their error returns checked, and all input should be checked for compliance with the formal requirements for such input rather than blindly trusting input data. Moreover, never assume, based on specific knowledge about the system or its domain, that the success of a called routine is guaranteed. The failure to report or properly respond to errors or other anomalies from a system perspective can threaten the survivability of the system as a whole.

ISO/IEC PDTR 24772 Section 6.47, "REU Termination strategy" describes the following mitigation strategies: \[[ISO/IEC PDTR 24772|AA. C References#ISO/IEC PDTR 24772]\]

Software developers can avoid the vulnerability or mitigate its ill effects in the following ways:

• A strategy for fault handling should be decided. Consistency in fault handling should be the same with respect to critically similar parts.
• A multi-tiered approach of fault prevention, fault detection and fault reaction should be used.
• System-defined components that assist in uniformity of fault handling should be used when available. For one example, designing a "runtime constraint handler" (as described in ISO/IEC TR 24731-1) permits the application to intercept various erroneous situations and perform one consistent response, such as flushing a previous transaction and re-starting at the next one.
  • When there are multiple tasks, a fault-handling policy should be specified whereby a task may
  • halt, and keep its resources available for other tasks (perhaps permitting restarting of the faulting task)
  • halt, and remove its resources (perhaps to allow other tasks to use the resources so freed, or to allow a recreation of the task)
  • halt, and signal the rest of the program to likewise halt.

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