Andrew S. Tanenbaum, Jorrit N. Herder, and Herbert Bos

Vrije Universiteit, Amsterdam

Microkernels—long discarded as unacceptable because of their lower performance

compared with monolithic kernels—might be making a comeback in operating systems

due to their potentially higher reliability,which many researchers now regard as more

important than performance.

When was the last time your TV set crashed

or implored you to download some emergency

software update from the Web? After

all, unless it is an ancient set, it is just a computer

with a CPU, a big monitor, some analog

electronics for decoding radio signals, a couple of

peculiar I/O devices—a remote control, a built-in VCR

or DVD drive—and a boatload of software in ROM.

This rhetorical question points out a nasty little secret

that we in the computer industry do not like to discuss:

Why are TV sets, DVD recorders, MP3 players, cell

phones, and other software-laden electronic devices reliable

and secure but computers are not? Of course there

are many “reasons”—computers are flexible, users can

change the software, the IT industry is immature, and

so on—but as we move to an era in which the vast

majority of computer users are nontechnical people,

increasingly these seem like lame excuses to them.

What consumers expect from a computer is what they

expect from a TV set: You buy it, you plug it in, and it

works perfectly for the next 10 years. As IT professionals,

we need to take up this challenge and make computers

as reliable and secure as TV sets.

The worst offender when it comes to reliability and

security is the operating system. Although application

programs contain many flaws, if the operating system

were bug free, bugs in application programs could do

only limited damage, so we will focus here on operating

systems.

However, before getting into the details, a few words

about the relationship between reliability and security

are in order. Problems with each of these domains often

have the same root cause: bugs in the software. A buffer

overrun error can cause a system crash (reliability problem),

but it can also allow a cleverly written virus or

worm to take over the computer (security problem).

Although we focus primarily on reliability, improving

reliability can also improve security.

WHY ARE SYSTEMS UNRELIABLE?

Current operating systems have two characteristics

that make them unreliable and insecure: They are huge

and they have very poor fault isolation. The Linux kernel

has more than 2.5 million lines of code; the Windows

XP kernel is more than twice as large.

One study of software reliability showed that code

contains between six and 16 bugs per 1,000 lines of executable

code,1 while another study put the fault density

at two to 75 bugs per 1,000 lines of executable code,2

depending on module size

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