Software Reliability

20 February 2008

Simplified Model of a Computer

retrieves the instruction directs data movement Control Unit processor Arithmetic Logic Unit Performs the operations defines an algorithm instructions MEMORY data the information that it works on

Points to Remember

 Computers access information by location and doesn’t know the value  Computers store numbers in fixed size packets, which means that they can not grow indefinitely  Computers do not distinguish between different types of data (e.g., instructions or text or numbers)

        

Where are Computers Used?

Finance: banking; stock market; commerce Medical: diagnostics; life support; medical devices Communications: television; radio; news; networks Transportation: traffic signals; air traffic control; air craft; space craft; trains; cars Military: weapons systems; intelligence gathering Energy: power plants; toxic chemical plants; oil & gas Water: sewer Buildings: HVAC; security; lights Personal & household items

But Things Go Wrong

 Usability  Bad Design  Reliability  Programming Mistakes   Why is it so hard?

Why can’t we get it right?

Is this a Problem?

“Our civilization runs on software, yet the art of creating it continues to be a dark mystery, even to the experts.

And the greater our ambitions, the more spectacular we seem to fail.” Scott Rosenberg, “ Dreaming in Code ”

Usability

Computer

as a

tool

 A

useful

tool should

do WHAT

?

 Help you achieve in

less WHAT

time, with

minimal

you want, effort….

 Do

ALL

that you need …  Having a Bad Day

Characteristics of a Useful Tool



Easy to learn ( intuitive )



Easy to remember



Standardized

Remote controls with a zillion same-sized buttons

Try plugging in this hair dryer…

What’s wrong here?

Lots of “Great” Examples

–

Great Interface Disasters

–

Bad Designs

–

Usability in the movies… yeah, right!

Reliability

 What is it?

  Correct output – every time How often it fails  Reasons that it fails 1.

2.

3.

4.

5.

6.

7.

Top management commitment User commitment Misunderstood requirements Inadequate user involvement Mismanaged user expectations Scope creep Lack of knowledge or skill Keil et al, “A Framework for Identifying Software Project Risks,” CACM 41:11, November 1998

80% of software projects fail

  50%

challenged

   2x budget 2x completion time 2/3 planned function 30%

impaired

 Scrapped Standish Group, 1995

Is the Problem Overstated?

 More recently: Sauer et al “delivered close to budget, schedule, and scope expectations” claim 67%  NIST estimates cost to US economy from inadequate software testing > $59 billion/yr. NIST Planning Report 02-3

What is a Bug?

Bug

 Problems in code that cause it to behave in an unintended, unanticipated or unpredictable manner  Origin   Grace Hopper (1947): moth in a relay

"First actual case of bug being found."

1906-1992

Thomas Edison used the term in 1878

"Bugs"—as such little faults and difficulties are called—

First Computer Bug

Why are bugs hard to find?

 The error can appear in another program  Device drivers, memory management  The error may only occur occasionally  May require multiple conditions to occur

Classes of Problems

 Poorly designed software  Poorly understood requirements  Poorly designed user interfaces  Improper use  Data entry problems  Simple coding errors

Can’t We Test Out the Problems?

 In order to establish that the probability of failure of software is less than 10 -9 in 10 hours, testing required with one computer is

greater than 1 million years

Butler and Finelli, “The Infeasibility of Experimental Quantification of Life-Critical Software Reliability”

Simple Problems

  Tampa couple was billed $4,062,599.57 for a month’s electricity   Correct bill was $146.76

Input error – clearly not good enough check for reasonable values High School freshman banned from football because of drug use in middle school  Actual offense was chewing gum and being tardy  Different codes not properly translated - systems are only as good as their weakest links

User Interface Bug

   Usability Issue Afghanistan War (December 2001)  Friendly fire kills 3 injures 20 when satellite-guided bomb landed on a battalion command post Use of GPS Receiver to determine coordinators   Change battery What should come up?

 www.washingtonpost.com/ac2/wp-dyn/A8853-2002Mar23

Denver Airport Baggage System (1995)

How stuff work s

Denver Airport Baggage System (1995)

   4 years in development at cost of $193M Massively complex system  4000 cars, 21 miles of track, scanners, photocells, 300 computers Cars misrouted and crashed, baggage lost and damaged  Delayed opening cost $1.1M/day  When airport opened a year late only one airline used it www.cis.gsu.edu/~mmoore/CIS3300/handouts/SciAmSept1994.html

Denver Airport System

  Examples of bugs:    Photocell could not detect bags on the belt and therefore didn’t stop system System had lost track of state of carts during jams Timing between conveyor belts and carts not properly synchronized Overall   Not just software glitches very complex, poorly engineered system

Ariane 5 (1996)

Software error Integer overflow

External view

Only about 40 seconds after initiation of the flight sequence, at an altitude of about 3700 m, the launcher veered off its flight path, broke up and exploded

External view

Cost

Development cost $7 Billion Delay of more than one year

+

One set of four identical, uninsured scientific satellites One rocket $500,000,000

What Happened?

 Overflow: tried to put too big a number into too small a space  Even worse – the feature that caused the problem wasn’t needed! It was only needed to set up the launch!

archive.eiffel.com/doc/manuals/technology/contract/ariane/page.html

Bank of New York: Nov 20, 1985

 BoNY: Nation’s largest clearer of Govt securities.

 Software to track Federal securities transactions wrote new information on top of old.

 Feds debited the bank for each transaction but bank did not know who owed it how much.

 90 minutes => $32 Billion overdraft!

Cost of Bug

     Bank had to borrow $24 billion from federal reserves. Interest paid ~$5 million for 1 day. (Annual earnings of bank ~120 million) BoNY share prices dropped by 25¢ Federal funds rate dropped from 8.4% to 5.5% System down for 28 hours.

Fear of financial crisis caused increase in price of platinum!

Cause of bug

 Message buffer counter at BoNY system was 16-bit long.  Counters at Fed (and other banks) 32 bit.  More than 32,000 transactions that morning! =>Counter overflow  Securities database corrupted.

The Drama continues…

 Trying to correct it – they copied corrupted data over the backup.  Lost a few hours because of this.  Reference: Wiener,

Digital Woes

, 1993

And then there is …



Therac 25

Therac-25

    Landmark case of how things can go terribly wrong Medical linear accelerator: radiation therapy for cancer patients  Used to zap tumors with high energy beams   Electron beams for shallow tissue X-ray photons for deeper tissue Eleven Therac-25s were installed:  Six in Canada  Five in the United States Developed by Atomic Energy of Canada Limited (AECL).

Therac-25

  Improvements over Therac-20:  Uses new “double pass” technique to accelerate electrons.

 Machine itself takes up less space.

Other differences from the Therac-20:   Software now coupled to the rest of the system and responsible for safety checks.

 Hardware safety interlocks removed.

“Easier to use.”

Understanding Therac – Two treatment modes – Why

> 1

mode in

one

machine?

Therac-25 Turntable

Field Light Mirror Counterweight Turntable Beam Flattener (X-ray Mode) Scan Magnet (Electron Mode)

1985-1987: Six known accidents

  

Jun 1985

: Patient at

Mareitta GA

received overdose

July 1985: Hamilton, Ontario

: patient severely burned, died that November.

December 1985:

 overdose Patient in

Yakima, WA

Vernon Kidd (4

th

case)

   Early March 1986, Tyler, Tx: receives dose > 100 times too high Complained he felt burned…..

 Engineer: It’s

not

overdose.

possible for Therac-25 to give an  Engineering firm: Machine does

not

appear capable of giving a patient an electrical shock...

  Died 5 months later Put back in use late March

3 Weeks Later: Ray Cox

 Second accident in Tyler, Tx  Same operator  Patient died 1 month later  This time they were able to reproduce

What Went Wrong?

 User Interface    Operator entered code for high energy rather than low energy “Malfunction message” Operator entered “Proceed” because system was known to give quirky errors  Result  Turntable was in the wrong position

What would cause that to happen?

      Race conditions.

 Several different race condition bugs.

Overflow error.

 The turntable position was not checked every 256th time the “Class3” variable is incremented.

No hardware safety interlocks.

Wrong information on the console.

Non-descriptive error messages.

  “Malfunction 54” “H-tilt” User-override-able error modes.

• One of the software design errors – SOFTWARE included a

set-up test

each treatment…. before • Tested various components ….

• Variable

incremented

with each part of test:

X = X + 1

• 8 bits….

• Can store values from 0 thru

255

8-bit code:

256

128 64 32 16 8 4 2 1 place value 1 1 1 1 1 1 1 1 = 255

1

0 0 0 0 0 0 0 0 =

0

IF X = 0 then

PROCEED with treatment

Source of the Bug

 Incompetent engineering.

 Safety analysis excluded the software!

 No usability testing.



Sources

Leveson, N., Turner, C. S., An Investigation of the Therac-25

Accidents. IEEE Computer , Vol. 26, No. 7, July 1993, pp. 18-41. http://courses.cs.vt.edu/~cs3604/lib/Therac_25/Therac_1.html

The authors: Nancy Leveson Clark S. Turner

“

The ethical dimensions of computer reliability are bound up with the

nature

of software, and the

complexity

systems.” of such

Lots more stories

http://www5.in.tum.de/~huckle/bugse.html

Slideshow of famous IT Failures http://www.cs.tau.ac.il/~nachumd/horror.html

You can read about these famous cases…

Dec 1995: Plane crashes into mountain; 159 people killed. Bad user interface in Flight Management System blamed …

Mar 2000:

malfunction blamed on software glitch Sea Launch

Oct 2005:

Russian rocket falls into ocean shortly after liftoff.

A few more recent events

AUG 2007: Computer glitch holds up 20,000 at LAX AUG 2007: Error in Skype Software shuts down ph svc AUG 2007: Wells Fargo recovering from computer crash JUN 2007: Computer failure stalls United flight JUN 2007: Computer crash hits space station FEB 2007: Software glitch forces stealth fighters to cancel flight

Final Discussion

 Should Microsoft be held responsible for the business problems and viruses caused by security holes in their software?