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Nancy Leveson suggests that eliminating dynamic memory allocation, recursion, interrupts and multi-processing is done in safety critical systems to make them deterministic. I disagree. Software that uses those techniques can be just as deterministic as software that does not. Further, software without any of those features can be non-deterministic in its behaviour. For example, the non-determinism associated with interrupt handling comes from the unknown timing of the external events and is not affected by the replacement of interrupts with polling. The MOD restrictions do not rule out non-determinism. Further, they do not rule out bad programming. If I am forbidden to use compilers or operating systems with dynamic memory allocation and consequently write complex programs that use "static" variables for many different purposes I am likely to introduce more mistakes than would have been present in the mature dynamic allocation system that I was not allowed to use. If, because I am forbidden to use recursion, I write a complex program that does the stacking and backtracking hidden by recursion, I am likely to introduce more errors than were present in the compiler's well-tested implementation of recursion. Similarly, if I introduce busy waiting and polling in my programs because I cannot use interrupts, I may again make things worse rather than better. Nancy is right in saying that "it is simply untrue that these limitations rule out real systems." Many perfectly horrible real-systems have been written without any of those features. Nor, would I agree that non-determinism is bad. Non-determinism has been demonstrated by Dijkstra (and much earlier by Robert Floyd) to allow programs that are much more easily verified than some deterministic ones. These regulations remind me of the old story about a man found looking carefully under a street lamp. When asked, he said he was looking for a coin he had dropped on the other side of the street. When asked why he was looking on this side, rather than where he dropped the coin, he replied that it was too dark over there. Regulations of this sort are imposed not because they are the right restrictions but because they are easily enforced by people who do not want to read programs carefully. It is much easier to verify that recursion and interrupts have not been used than to verify that the program is well-structured, well-documented, systematically inspected (verified), and adequately tested. Just as it is easier to look for a ring under a street light, it is easier to check for the absence of features than the presence of quality. All my knowledge of the Therac software comes from discussions with Nancy. However, from her description the problem with that software was not the use of multi-tasking but the failure to use well-known (and well-understood) ways of making multi-tasking verifiable. As Nancy says, "The most effective way to increase the reliability of software that we currently know about is simply to make it understandable and predictable." "Sophisticated programming techniques", properly used, can do just that. Unfortunately, we cannot make regulations that define "properly used" so some of us are throwing the baby out with the bath water. David L. Parnas
I have great regard for David Parnas and his opinions, but we do have a difference of opinion with respect to this issue. Perhaps I am using the term "non-deterministic" incorrectly. When interrupts are used, it is not possible to pre-determine in exactly what sequence the statements of the software will be executed because the interrupt-handling code can be executed at any time. When using polling, the programmer has more control over when the input-handling code is executed. It is possible to use priorities to ensure that the code to abort a missile launch is not interrupted, for example, but this just complicates things further and introduces the possibility of different types of errors. At the least, limiting the possible execution sequences allows for more extensive testing given a fixed amount of testing resources. It also allows for more reliable timing analysis as pointed out by someone in a previous posting to Risks. Am I misunderstanding something here? With respect to Therac, it is true that the errors were involved in multi-tasking and its poor implementation. But the use of the "well-known and well-understood ways of implementing multi-tasking" do not guarantee errors will not be introduced. You are assuming that they would implement those features correctly. Although I have great confidence that David Parnas or Edsgar Dijkstra could use ANY techniques with great skill, I have less confidence that this is true for all the programmers writing military software. David is obviously right that using recursive constructs is safer than implementing recursion oneself with stacks and backtracking. But it would be safest (in terms of all types of potential faults including run-time stack overflows) to try to find a simple, non-recursive algorithm to accomplish the same thing. If one does not exist, then the argument for this should be included in the certification procedure. It is easier to grant dispensations to standards when someone provides an argument that it is safer to do so than it is to try to check all software to ensure that superfluous complexity has not been introduced. The person arguing to certify their software must then provide good arguments for introducing complexity rather than the certifier having to determine whether simpler designs exist. Remember, we are talking about a potential nuclear armageddon here and with certifiers who may be no more knowledgeable than the programmers. David is also right that it is better to check for the presence of quality than the absence of features. I would hope that standards would require that quality be built-in and assessed and not just include these simple rules. Unfortunately, I have no way of looking at a program, even a well-designed one, and guaranteeing that there are no errors in it. I do have more confidence that people will make fewer mistakes when their programs are simpler and when they use techniques that rule out various common types of errors. For example, if the design they choose does not involve the potential for deadlock, then I need not worry about it occurring. If the man had carried his coin safely ensconced in his wallet, he would not have had to look for it on either side of the street. Sure, as David says, "perfectly horrible real-systems have been written without any of these features." That is exactly why such standards should contain more than just simplistic rules. But that does not mean that rules are not also useful in the context of an imperfect world. I guess our disagreement comes down to the following: Assuming David's belief that these techniques, properly used, can increase reliability, is it more likely that errors will be introduced (1) because they are improperly used or (2) because they are not used at all? Until we have experimental data on this, it remains a matter of personal opinion. It would be nice if the people who introduce these programming techniques and argue that they increase reliability would do comparative studies to confirm their claims. This type of experimentation is very difficult (as we have found out by trying to do it), but long overdue in our field. Nancy G. Leveson
I have great respect for Nancy Leveson's work and for the great effort she has put into increase everyone's awareness of safety issues, but we continue to have a difference of opinion here. 1) In fact, hardware interrupts are simply hardware implemented polling and the use of interrupts gives the programmer control (because of more rapid response) than can usually be achieved by software polling. Nancy's impression that the programmer loses control comes from the fact that we often use interrupt response code written by others (operating system code) and consequently feel that we have less control. If we allowed someone else to write the polling code, and the code to respond when an event was detected, we would be in exactly the same situation, except that there would be a slower reaction time. Both software polling and interrupt handling can be done badly and both can be done well. Edsger Dijkstra's 1968 papers showed how an interrupt handling system could be set up in a way that the programmers could restrict the order of events as much as they wish. Moreover, the beauty of his approach was that most of the software was unaffected by the choice between interrupts and software polling. The P and V interface hid the difference between events detected by hardware and those detected by software. Twenty years later I meet people who are called "software engineers" but have no knowledge of that work or understanding of similar techniques. Today's hardware and software technology would allow us to do even better than was possible in 1968. 2) Nancy is wrong when she states that I am assuming that programmers would implement those ideas correctly. I have too much experience to make such an assumption. She could equally well be accused of assuming that programmers will avoid those ideas "correctly". There is no idea so good that it cannot be used badly. 3) No one is more in favour of the use of "simple algorithms than I. My vitae carries a quote from A. Einstein, "Everything should be as simple as possible, but not simpler". Further, I don't even believe that there are such things as recursive programs. They are all iterative programs in disguise. I do however find that recursive descriptions of programs programs are often simpler, and easier to verify, than iterative descriptions of the same algorithm. We must never forget the "but not simpler" phrase in Einstein's remark. 4) If Nancy thinks that the danger of deadlock is caused by multi-programming I must also disagree. Every program built using pseudo-parallel processes has an equivalent "sequential program" that can exhibit exactly the same abortive behaviour as its deadlocking counterpart. If Nancy, thinking that deadlock is impossible because there is no multi-programming, does not look for those errors, she will just be surprised and dismayed when the program enters an unending loop or a permanent idle state at unexpected times. I believe that organisations such as MoD would be better advised to introduce regulations requiring the use of certain good programming techniques, requiring the use of highly qualified people, requiring systematic, formal, and detailed documentation, requiring thorough inspection, requiring thorough testing, etc. than to introduce regulations forbidding out the use of perfectly reasonable techniques. Like Nancy, I wish we had experimental data to confirm or disprove my belief in the potential of these techniques but I don't expect to get such data. Other differences, such as the qualifications and interest of the personnell will swamp the effect of the techniques that we are debating. Safety depends on the use of highly qualified, disciplined personnel for the design, documentation and inspection of softwre. By forbidding modern programming techniques, we could drive those people away and achieve just the opposite. David L. Parnas.
Here is a story from FEDERAL COMPUTER WEEK, July 10, 1989, pages 10,11: DOD TARGET SYSTEMS FACE IMPROVEMENT by Gary H. Anthes The Defense Department plans to improve substantially its planning system for the targeting of strategic nuclear weapons. As a first step, it is about to award a contract for a prototype system that would identify targets, allocate weapons and specify the timing and modes of weapons delivery. DOD hopes to reduce the planning-cycle time from 18 months to three days, reduce personnel requirements by a factor of 10 and increase survivability by a factor of eight (see chart, below). The overhaul of the process by which the plans are developed and changed during conflict reflects a shift away from large-scale use of nuclear weapons against fixed targets to more limited conflicts involving mobile targets. It also reflects recent presidential requests for more flexibility in the use of nuclear weapons. The project, called Survivable Adaptive Planning Experiment (SAPE), is being funded jointly by the Defense Advanced Research Projects Agency and the Air Force. Prime bidders on the job are Harris Corp., McDonnell Douglas Corp. and IBM Corp. The United States' plan for nuclear warfare, called the Single Integrated Operational Plan (SIOP), is produced annually under the direction of the Joint Strategic Target Planning Staff (JSTPS), an element of the Joint Chiefs of Staff. The plans are developed at the headquarters of the Strategic Air Command in Omaha, Neb., using intelligence data, force-status information and guidance from the president, the secretary of Defense and other authorities. ``The president decides in broad terms the types of options and results he wants, and the JSTPS takes the guidance and produces the SIOP,'' said Army Lt. Col. Peter W. Sowa, SAPE project manager at DARPA. SAPE is not intended to replace the initial planning process. Rather, it is geared to enabling rapid adaptation of the plan before and during a nuclear conflict while ensuring the survivability of the planning system. Sowa said: ``During a conflict, the situation will be different [from the one anticipated] especially if tactical nuclear weapons are used. Something is sure to be different, so you want to be able to replan quickly.'' It now takes 18 months to create a new plan. Although it could be done faster, Sowa said, a completely new plan could not be developed in three days, which is the SAPE goal. Another SAPE goal is reducing the time required to change targets and to configure the planning network. Retargeting time is of growing importance as the Soviet ballistic missile force becomes more mobile. ``Planning to hold fixed targets at risk is straightforward in concept but exquisitely complex in detail,'' the SAPE statement of work says. ``[But] when targets are capable of relocation over short time periods, accomodating those changes into strategic attack plans becomes enormously difficult.'' SAPE specifications require the ability to change 600 targets per day during a nuclear conflict and to reduce the time required to change a target from eight hours to three minutes. The system also must be able to generate plan options involving 2,000 weapons and up to 4,000 targets in 30 minutes. These objectives can only be overcome by powerful hardware, smart software and a high-speed communications network --- the cornerstones of SAPE. According to Sowa, the system will consist of several mobile processing clusters, each possibly consisting of a large mainframe or supercomputer, data base machines, parallel processors, and workstations. Each local cluster will be able to execute the whole plan, and each will connect to a survivable communications internetwork through a gateway. The core software, most likely written in Ada and Lisp, will consist of mathematical algorithms and artificial-intelligence techniques, Sowa said. ``It will be a software development effort and an expert-knowledge-building effort. A lot of interviewing experts will be needed.'' But the smart software will be of little use if the system is damaged during hostilities. By using redundant, distributed and mobile parts, DARPA aims to increase the system's tolerance to loss from 10 percent to 83 percent and its communications connectivity during a conflict from 10 percent to 50 percent. According to Sowa, the benefits of the more flexible SIOPs include the following: o Enhanced deterrence through the ability to hit mobile targets. o More efficient use of weapons. ``If you don't shoot at empty holes, you don't need as many weapons,'' Sowa said. o Smarter wargaming. The system could be used to conduct what-if analyses - predicting the effects of different options --- as part of planning or negotiating processes. Sowa said SAPE, which will require $20 million to $30 million over five years, is a demonstration project and is not intended to field a system. Some or all may be deployed, or the existing system might be enhanced in some other way, he said. A contract will be awarded in about a month, and it will call for the design and development of an engineering development model. The model must be able to rapidly retarget and generate new options in an environment before a nuclear conflict. In subsequent phases, the model will be expanded to include those functions needed during a nuclear conflict and to create a new nuclear strike plan at the end of hostilities. (This table accompanied the article:) SURVIVABLE ADAPTIVE PLANNING EXPERIMENT PERFORMANCE GOALS FACTOR CURRENT GOAL Number of primary nodes 1 7 Time to replan full SIOP 18 months 3 days Personnel resources 500 persons 50 persons Retarget actions 10 per day 1000 per day Time to retarget 8 hours 5 minutes System tolerance to loss 10 percent 83 percent Communications connectivity Pre-SIOP 99 percent 99 percent Trans-SIOP 10 percent 50 percent Post-SIOP 30 percent 90 percent Reconfiguration time Hours Seconds (end of excerpts from FEDERAL COMPUTER WEEK) - Jonathan Jacky, University of Washington
> [...] Airbus [doesn't agree. They think their stuff is great and saves >weight and can't fail; but]... the A320 does have a partial backup system: "Can't fail"? This is pretty amusing. It's my understanding (possibly incorrect) that Airbus made this same argument about the A-320 control system before the manual backup systems were in place. The US FAA indicated that the A-320 would not be certified without those backups, no matter how reliable Airbus claimed the fly-by-wire system to be. Airbus gave in. During flight tests, the "can't fail" system suffered a total failure, and the A-320 prototype was flown using "peanut" self-powered flight instruments and the back-up manual controls (this event was reported in Aviation Week). Airbus reports that the failure was a freak, they've fixed the problem, and that it won't ever happen again. Now it really can't fail. Right. Do we know that their analysis can't fail... again? >No matter how desperately he or shell hauls back on that sidestick, the >envelope protection system will not let the airplane respond beyond a certain >rate: namely, the rate that limits stress on the airframe to... 2.5G. This is indeed a problem. There are situations where the crew may elect to damage the airframe in return for avoiding a worse alternative. The protection system will not let them do this. It's not clear whether we'll lose more people from not having such a system or because of it. One point worth considering is that second generation fly-by-wire systems for fighters are now being designed to allow maneuvering beyond critical alpha as the disadvantages (high drag, loss of airspeed, structural risk) may allow the pilot to shoot first and thus survive. [re: the 747 loss of control incident] >Airbus, not surprisingly, responds that an A320 never would have [entered the >dive] in the first place. This contention may be incorrect. The flight control laws prevent a pilot from deliberatly maneuvering the aircraft to a control departure, but they don't prevent one from occuring under all conditions. As part of a previous job, I occasionally flew an advanced cockpit flight simulator. I was curious as to the flight limits, and found them rather rapidly. The flight control laws would not allow me to roll the airplane beyond 70 degrees of bank. It seemed obvious to me that an aerodynamic departure could result in a maneuver that would exceed the control law limits. I brought the aircraft to the limiting angle of attack, then reduced thrust on one engine to idle. The resulting asymmetric thrust produced a rolling moment that caused the aircraft to exceed the control limits by a considerable margin. This is virtually the same situation as occured on the 747 mentioned above, and could conceivably occur on an Airbus aircraft with control-law limits. One more anecdote: Several years ago an Air Florida 737 crashed into the Potomac River. One of the contributing causes was faulty engine power indication. The crew believed that maximum power was being applied, but their indications showed considerably more power was being generated than was actually being produced. There was nothing preventing them from advancing the power levels beyond the "max power" setting other than training that predisposed them to not do this. It is now regarded as correct procedure to exceed such limits in an emergency if doing so might save the aircraft. You can always replace the engines after the emergency landing. If you would have crashed without such power application, the engines (among other things) would be lost anyway. If they fail from excessive power application, well, you were going to crash anyway. This is, after all, an effort of last resort. It is a classic error to design systems to handle the majority of cases correctly but omit the handing of limit cases. Advanced systems are intended to increase safety and efficiency. It would be an error to design-out application of control in limit cases that might be necessary to save lives.
There has recently been some considerable discussion of the potential risks associated with 'fly by wire' systems in airplanes, and the way in which the electronics prevent excursions outside a pre-defined `performance envelope'. This sort of thing is not only found in planes - there are now on the market (in the United Kingdom at least, not sure about the States) quite a number of high performance automobiles which do much the same thing. Examples are: electronic monitoring of turbocharger boost/charge temperature to prevent detonation, monitoring of engine speed (prevents overspeed), control of auto transmissions (to prevent shifts to low gears at high road speeds) etc. Most of these limitations are done to prevent possibly hazardous and damaging operation of the vehicle, others are done to comply with the national or regional restrictions on exhaust emissions, or to adapt to the local fuel octane availability. They do, in many cases, mean that the vehicle is also operated well within its design limitations, hence helping to reduce manufacturers warranty claims. There are several organisations in Europe which have, by devious means, gained access to the control programs of these microprocessors (either by disassembling, or by blatant and unashamed bribery of the software writers) and are thus able to modify the software and produce what they then sell as `Superchips' that, for a couple of hundred pounds and ten minutes effort with a chip-puller, can transform the performance of an already rapid car - examples given in UK literature are as follows:: Sierra/Merkur Cosworth - 388 brake horsepower Volvo turbos - 280 brake horsepower Mazda 323 - 180 brake horsepower Nissan 300ZX - 370 brake horsepower SAAB turbos - 280 brake horsepower. Now apart from the potential hazards of improperly programmed microprocessors leading to the destruction of engines/transmissions due to excessive stress that the manufacturer/designer did not forsee, i wonder about the legal position of the car should you be involved in an accident. Cars in Europe have to be 'Type approved', that is to say, there are national testing authorities who oversee the safety of production cars and prevent the sale or import of those vehicles that fail to reach certain standards. Any modifications to a type-approved vehicle should strictly only incorporate parts that have been themselves type-approved. In this context, 'software' is a rather tenuous 'part', again lawyers lagging decades behind technology..... There are, as far as i am aware, no provisions for policing the status of on-board software, nor to prevent its alteration. Since the changes are visually undetectable (who walks round their local friendly used car lot with a signature analyser and an EPROM blower in their pocket?) it would be very hard to spot, either by the licensing authorities, or the insurers who may be asked to meet claims resulting from accidents. Don't any of you think i am against the enhancement of performance of vehicles - i want my 70-90 acceleration times to be as short as is humanly possible (and anyone who has driven the motorway network here will agree on that!), its just that i can forsee potential problems in the future (like what is the risk if you sell such a modified vehicle and fail to disclose the modifications and the purchaser then smashes himself up - can he sue you for failing to tell him about the changes???) Pete Lucas. PHONE: +44 793 411665 JANET: PJML@UK.AC.NERC-WALLINGFORD.IBMA EARN : PJML%UK.AC.NWL.IA@UKACRL
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