Disaster Redux!

Published in Embedded Systems Programming January 2005

By Jack Ganssle

Disaster Redux!

 The spacecraft descended towards the planet, accelerating in the high-g field as it drew nearer. Sophisticated electronics measured the vehicle's position and environment with exquisite precision, waiting for just the right moment to deploy the parachute.

Nothing happened. The mission crashed on the surface.

Last month I described how Mars Polar Lander was lost due to a software error. But this scenario played out on yet another planet, the one known as Earth, when on September 8 the Genesis mission impacted at 200 MPH. As I write this the Mishap Investigation Board hasn't released a final report. But they did say the gravity-sensing switches were installed upside down so couldn't detect the Earth's gravitational field.

The origins of Murphy's Law are in some dispute. The best research I can find (http://www.improb.com/airchives/paperair/volume9/v9i5/murphy/murphy1.html) suggests that Captain Ed Murphy complained "If there's any way they can do it wrong, they will" when he discovered that acceleration sensors on a rocket sled were installed backwards. Nearly 60 years later the same sort of mistake doomed Genesis.

Perhaps a corollary to Murphy's Law is George Santanya's oft quoted "those who forget history are condemned to repeat it."

NASA's mantra "test like you fly, fly like you test" serves as an inoculation of sorts against the Murphy virus. We don't as yet know why Genesis' sensors were installed upside down, but a reasonable test regime would have identified the flaw long before launch.

Last month I focused on high profile failures from the space business. Few other industries are exempt from their share of firmware disasters. Some are quite instructive.

Tumor Zappers

The Therac 25 was a radiotherapy instrument designed to treat tumors with carefully regulated doses of radiation. Occasionally operators found that when they pressed the "give the patient a dose" button the machine made a loud clunking sound, and then illuminated the "no dose given" light. Being normal human-type operators, they did what any normal human-type person would do: press the "dose" button again. After a few iterations the patients were screaming in agony.

Between 1985 and 1988 six cases of massive overdosing resulted in three deaths.

The machines were all shut down during an investigation, which found that if the backspace button was pressed within 8 seconds of the "give the patient a dose" control being actuated, the device would give full-bore max X-rays, cooking the patient.

Software killed.

The code used a homebrew RTOS riddled with timing errors. Yet today, nearly two decades later, far too many of us continue to write our own operating systems. This is despite the fact that at least a hundred are available, for prices ranging from free to infinity, from royalty-free licenses to ones probably written by pirates. Even those cool but brain-dead PIC processors that have a max address space of a few hundred words have a $99 RTOS available.

Developers give me lots of technical reasons why it's impossible to use a commercial OS. Too big, too slow, wrong API - the reasons are legion. And mostly pretty dumb. EEs have long used glue logic to make incompatible parts compatible. They'd never consider building a custom UART just because of some technical obstacle. Can you imagine going to your boss and saying "this microprocessor is ideal for our application, except there are two instructions we could do so much better. So I plan to build our own out of 10 million transistors." The boss would have you committed. Yet we software people regularly do the same by building our own code from 10 million bits. Crafting a custom OS is nearly always insane, and in the case of the Therac 25, criminal.

It's tough to pass information between tasks safely in a multithreaded system, which is why a decent RTOS has sophisticated messaging schemes. The homebrew version used in the Therac 25 didn't have such features so global variables were used instead, another contributor to the disaster.

Globals are responsible for all of the evil in the universe, from male pattern baldness to ozone depletion. Of course there are instances where they're unavoidable, but those are rare. Too many of us use them out of laziness. The OOP crowd chants "encapsulation, inheritance, polymorphism;" the faster they can utter that mantra the closer they are to OOP nirvana, it seems. Of the three, encapsulation is the most important. Both Java and C++ support encapsulation! as do assembly and C. Hide the data, bind it to just the routines that need it. Use drivers both for hardware and to access data items.

The Therac's code was, as usual, a convoluted mess. Written mostly by a solo developer, it was utterly unaudited. No code inspections had been performed.

We've known since 1976 that inspections are the best way to rid programs of bugs. Testing and debugging simply don't work; most test regimens only exercise about half the code. It's quite difficult to create truly comprehensive tests, and some features, like exception handlers, are nearly impossible to invoke and exercise.

Decent inspections will find about 70% of a system's bugs for a twentieth of the cost of debugging. The Therac's programmer couldn't be bothered, which was a shame for those three dead patients.

But 1985 was a long time ago. These things just don't

Dateline Panama, 2001. Another radiotherapy device, built by a different company, zapped 28 patients. At least 8 died right after the overexposures; another 15 either already have or are expected to die as a result.

To protect the patient physicians put lead blocks around the tumor. The operator draws the block configuration on the machine's screen using a mouse. Developers apparently expected the users to draw each one individually, though the manual didn't make that a requirement. 50 years of software engineering has taught us that users will always do unexpected things. Since the blocks encircled the tumor a number of doctors drew the entire configuration in one smooth arcing line.

The code printed out a reasonable treatment plan yet in fact delivered its maximum radiation dose.

Software continues to kill.

The FDA found the usual four horsemen of the software apocalypse at fault: inadequate testing, poor requirements, no code inspections, and no use of a defined software process.

Pacemaking

I bet you think pacemakers are immune from firmware defects. Better think again.

In 1997 Guidant announced that one of their new pacemakers occasionally drives the patient's heartbeat to 190 beats per minute. Now, I don't know much about cardiovascular diseases, but suspect 190 BPM to be a really bad thing for a person with a sick heart.

The company reassured the pacemaking public that there wasn't really a problem; the code had been fixed and disks were being sent across the country to doctors. However, the pacemaker is implanted subcutaneously. There's no 'net connection, no USB port or PCMCIA slot.

Turns out that it's possible to hold an inductive loop over the implanted pacemaker. A small coil in the device receives energy to charge the battery. It's possible to modulate the signal and upload new code into Flash. The robopatients were reprogrammed and no one was hurt.

The company was understandably reluctant to discuss the problem so it's impossible to get much insight into the nature of what went wrong. But clearly inadequate was testing.

Guidant is far from alone. A study in the August 15, 2001 Journal of the American Medical Association ("Recalls and Safety Alerts Involving Pacemakers and Implantable Cardioverter-Defibrillator Generators") showed that more than 500,000 implanted pacemakers and cardioverters were recalled between 1990 and 2000. (This month's puzzler: how do you recall one of these things?)

41% of those recalls were due to firmware problems. The recall rate increased between in the second half of that decade over the first. Firmware is getting worse. All five US vendors have an increasing recall rate.

The study said: "engineered (hardware) incidents [are] predictable and therefore preventable, while system (firmware) incidents are inevitable due to complex processes combining in unforeseeable ways."

Baloney.

It's true that the software embedded into these marvels has grown steadily more complex over the years. But that's not an excuse for a greater recall rate. We must build better firmware when the code base grows. As the software content of the world increases a constant bug rate will lead to the collapse of civilization. We do know how to build better code. We chose not to. And that blows my mind.

Plutonium Perils

Remember Los Alamos? Before they were so busily engaged in losing disks bulging with classified material this facility was charged with the final assembly of the US's nuclear weapons. Most or all of that work has stopped, reportedly, but the lab still runs experiments with plutonium.

In 1998 researchers were bringing two subcritical chunks of plutonium together in a "criticality" experiment, which measured the rate of change of neutron flux between the two halves. It would be a Real Bad Thing if the two bits actually got quite close, so they were mounted on small controllable cars, rather like a model railway. An operator uses a joystick to cautiously nudge them towards each other.

The experiment proceeded normally for a time, the cars moving at a snail's pace. Suddenly both picked up speed, careening towards each other at full speed. No doubt with thoughts of a mushroom cloud in his head, the operator hit the "shut down" button mounted on the joystick.

Nothing happened. The cars kept accelerating.

Finally actuating an emergency SCRAM control, his racing heart (happily sans defective embedded pacemaker) slowed when the cars stopped and moved apart.

The joystick had failed. A processor reading this device recognized the problem and sent an error message, a question mark, to the main controller. Unhappily, ? is ASCII 63, the largest number that fits in a 6 bit field. The main CPU interpreted the message as a big number meaning go real fast.

Two issues come to mind: the first is to test everything, even exception handlers. The second is that error handling is intrinsically difficult and must be designed carefully.

Patterns

The handful of disaster stories I've shared over the last two columns have many common elements. On Mars Polar Lander and Deep Space 2, the Mars Expedition Rover and Titan IVb, Sea Launch, pacemakers, Therac 25 and in Los Alamos inadequate testing was a proximate cause. We know testing is hard. Yet it's usually deferred till near the end of the project, so gets shortchanged in favor of shipping now.

Tired programmers make mistakes. Well, duh. Mars Polar Lander, Deep Space 2 and the Mars Expedition Rover were lost or compromised from this well-known and preventable problem.

Crummy exception handlers were one of proximate causes of problems with the Mars Expedition Rover, Los Alamos and plenty of other disasters.

Had a defined software process, including decent inspections, been used no one would have been killed by the Therac 25. I estimate about 2% of embedded developers inspect all new code. Yet properly-performed inspections are a silver bullet that accelerates the schedule and yields far better code.

I have a large collection of embedded disasters. Some of the stories are tragic; others enlightening, and some merely funny. What's striking is that most of the failures stem from just a handful of causes. Remember the Tacoma Narrows bridge failure I described last month? Leon Moisseiff was unable to learn from his profession's series of bridge failures from wind-induced torsional flutter, or even from his own previous encounters with the same phenomena, so the bridge collapsed just four months after it opened.

I fear too many of us in the embedded field are 21^st century Leon Moisseiffs, ignoring the screams of agony from our customers, the crashed code and buggy products that are, well, everywhere. We do have a lore of disaster. It's up to us to draw the appropriate lessons.