Re: "Pseudo-DIA" between the Linux Kernel Development Community and its Users - 3/5 Safety for Operating Systems involves functional and quality requirements

John MacGregor <john.macgregor@...>

Hi Lukas,

After the workshop, and before my vacation, another veil falls...

For this e-mail, I've enhanced the general V-Model with safety activities / processes. This means that the diagramme covers the general development lifecycle as well as the safety lifecycle. The standards are not so clear about that. Rather than having a separate, parallel V for the safety lifecycle, I've inserted orange "safety" boxes in the nodes representing each development phase.

In the case of ISO 26262, there is the famous illustration of the 3 Vs superimposed over the overview of the standards series (Figure 1 in every standard) and Figures 2 & 3 in Part 4 which replace the hardware and software Vs with boxes. The enclosed illustration seems generally compatible.

It's not generally included in the V-Model, but in the context of safety-critical systems, there should be backwards traceability between the requirements and the work products that implement them.

Two points are immediately noticeable:
1) The standards' requirements mostly only cover the tip of the iceberg of system development activities. (Well, I have to admit I made the orange boxes small so that they wouldn't interfere with the phase titles, however (-: ).
2) There is an overlap between safety functionality and operating system functionality.

Those turquoise boxes represent the development process all application, middleware and, yes, operating system elements in the safety-critical system. The system itself is composed of (using a somewhat arbitrary mixture of terminology):
1) newly-developed safety elements
2) newly-developed non-safety elements
3) pre-existing safety elements that have been used in a similar domain
4) pre-existing safety elements that have been used in another context (at least from the 26262 perspective), i.e. another instance of the same product class
5) pre-existing non-safety elements
6) pre-existing safety components (hardware and software)
7) pre-existing non-safety components (hardware and software)
each of which may have a different certification or qualification route as well as different generic development processes. The difference between elements and components seem nebulous to me and I'd rather call pre-existing things "off-the-shelf", whereby one might have to differentiate whose shelf they come from.

From the previous e-mail (which admittedly considered only non-safety-critical systems), a Linux that is currently being selected for use in an embedded system would belong to category 7 and that is the focus here. It may soon be the case that safety-critical applications will use Linux. There may come a time, where safety functionality has been brought upstream to the Kernel, but these are now not quite the case.

The safety-critical system development process starts by defining the safety-critical system and the environment (context) within which it operates. A hazard and risk analysis is then performed to develop the safety requirements on the system and a corresponding functional safety concept. A technical safety concept is developed in the system architecture phase, which ultimately results in safety requirements on the software architecture, and therefore on the operating system.

At this point the requirements on the operating system should be functional requirements, for safety mechanisms or safety functions, and / or requirements on the qualities of those functions (response time, resource consumption, etc.). Safety functionality, or mechanisms, include such things as monitoring, periodic testing, diagnostic and logging functionalities, tolerance mechanisms for residual design faults in the hardware, environmental stresses, operator mistakes, residual software design faults, data communication errors and overload situations; things that may already exist in the operating system in some form. Refer to 61508-3 a) for a better list.

In other words, the safety-related requirements on the operating system should already be functional or quality requirements that should comparable to other requirements on the operating system.

This principle has already been accepted by a number of accreditation agencies in the context of SafeScrum. There, the hazard analyses result in hazard stories, which are stored in the product backlog. The hazard stories reflect a certain functionality that enable the system to achieve or maintain a safe state in a certain hazard scenario. During a sprint, the developers are instructed in the safety-critical aspects of the hazard story by accompanying safety personnel. The developers develop the hazard functionality while monitoring the safety-related requirements.

In the context of the general development lifecycle of a safety-critical product, i.e. the turquoise boxes, the system developer would again go through the selection and configuration process for an operating system as would be done in developing a conventional system. The operating system has already been developed and therefore has the functionality and qualities that it delivers "off the shelf". The developer must ensure that the operating system meets its functional and quality requirements. Where there are alternatives, the system developer would have the option of choosing the candidate that best meets the requirements, perhaps with an emphasis on its systematic capability (i.e. the potential of an element to lead to a failure of a safety function).

Currently, Linux has not been developed for safety-critical applications, but it may be that it already possesses functionality to perform safety functions with adequate quality. Otherwise, there is the possibility, as in the case with Google and Android, that the system developer can develop, or commission the development of, the necessary functionality. This is the only time there is a development process, and that is not for the operating system but for a feature of the operating system.

In the context of safety, I could foresee see possible Kernel development activities related to new drivers and for the safety functionality.

The point here is that Linux has always (up to now) been developed as functionality. It may be possible to isolate the safety-related parts of that functionality and, as part of the systems engineering part of the development process, attach quality requirements to them and validate that the requirements have been achieved. For me, this would be the development interface for the DIA.

In the next e-mail, I'll look at who might develop that functionality and how the responsibilities might be distributed among the players that perform the development activities.


Mit freundlichen Grüßen / Best regards

John MacGregor

Safety, Security and Privacy (CR/AEX4)
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