Taller de sistemas operativos

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|Can We Make Operating Systems Reliable and Secure? |
|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 kernelsmight 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 inROM. |
|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, theIT industry is immature, and so on but as |
|we move to an era in which the vast majority of computer users are non technical people, increasingly these seem like lame excuses to|
|them. |
|What consumers expect from a computer is what they expect from a TV set: You buyit, 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 applicationprograms 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 reliabilityshowed 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. Using a |
|conservative estimate of six bugs per 1,000 lines of code, the Linux kernel probably has something like 15,000 bugs; Windows XP has |
|at least double that....
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