Modern Operating Systems by Herbert Bos ...
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Modern Operating Systems by Herbert Bos and Andrew...
Modern_Operating_Systems_by_Herbert_Bos_and_Andrew_S._Tanenbaum_4th_Ed.pdf-M ODERN O PERATING S YSTEMS
Modern Operating Systems by Herbert...
Modern_Operating_Systems_by_Herbert_Bos_and_Andrew_S._Tanenbaum_4th_Ed.pdf-M ODERN O PERATING S YSTEMS
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712
SECURITY
CHAP. 9
62.
Write a program that emulates overwriting viruses outlined in Sec. 9.9.2 under the
heading ‘‘Executable Program Viruses’’. Choosean existing executable file that you
know can be overwritten without any harm. For the virus binary, choose any harmless
executable binary.


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10
CASE STUDY 1: UNIX,
LINUX, AND ANDROID
In the previous chapters, we took a close look at many operating system prin-
ciples, abstractions, algorithms, and techniques in general. Now it is time to look at
some concrete systems to see how these principles are applied in the real world.
We will begin with Linux, a popular variant of UNIX, which runs on a wide variety
of computers.
It is one of the dominant operating systems on high-end worksta-
tions and servers, but it is also used on systems ranging from smartphones
(Android is based on Linux) to supercomputers.
Our discussion will start with its history and evolution of UNIX and Linux.
Then we will provide an overview of Linux, to give an idea of how it is used. This
overview will be of special value to readers familiar only with Windows, since the
latter hides virtually all the details of the system from its users. Although graphical
interfaces may be easy for beginners, they provide little flexibility and no insight
into how the system works.
Next we come to the heart of this chapter, an examination of processes, memo-
ry management, I/O, the file system, and security in Linux. For each topic we will
first discuss the fundamental concepts, then the system calls, and finally the imple-
mentation.
Right off the bat we should address the question: Why Linux? Linux is a vari-
ant of UNIX, but there are many other versions and variants of UNIX including
AIX, FreeBSD, HP-UX, SCO UNIX, System V, Solaris, and others. Fortunately,
the fundamental principles and system calls are pretty much the same for all of
them (by design).
Furthermore, the general implementation strategies, algorithms,
713


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CASE STUDY 1: UNIX, LINUX, AND ANDROID
CHAP. 10
and data structures are similar, but there are some differences. To make the ex-
amples concrete, it is best to choose one of them and describe it consistently. Since
most readers are more likely to have encountered Linux than any of the others, we
will use it as our running example, but again be aware that except for the infor-
mation on implementation, much of this chapter applies to all UNIX systems.
A
large number of books have been written on how to use UNIX, but there are also
some about advanced features and system internals (Love, 2013; McKusick and
Neville-Neil, 2004; Nemeth et al., 2013; Ostrowick, 2013; Sobell, 2014; Stevens
and Rago, 2013; and Vahalia, 2007).
10.1 HISTORY OF UNIX AND LINUX
UNIX and Linux have a long and interesting history, so we will begin our
study there. What started out as the pet project of one young researcher (Ken
Thompson) has become a billion-dollar industry involving universities, multina-
tional corporations, governments, and international standardization bodies.
In the
following pages we will tell how this story has unfolded.
10.1.1 UNICS
Way back in the 1940s and 1950s, all computers were personal computers in
the sense that the then-normal way to use a computer was to sign up for an hour of
time and take over the entire machine for that period.
Of course, these machines
were physically immense, but only one person (the programmer) could use them at
any given time. When batch systems took over, in the 1960s, the programmer sub-
mitted a job on punched cards by bringing it to the machine room. When enough
jobs had been assembled, the operator read them all in as a single batch.
It usually
took an hour or more after submitting a job until the output was returned. Under
these circumstances, debugging was a time-consuming process, because a single
misplaced comma might result in wasting several hours of the programmer’s time.
To get around what everyone viewed as an unsatisfactory, unproductive, and
frustrating arrangement, timesharing was invented at Dartmouth College and
M.I.T.
The Dartmouth system ran only BASIC and enjoyed a short-term commer-
cial success before vanishing. The M.I.T. system, CTSS, was general purpose and
was a big success in the scientific community. Within a short time, researchers at
M.I.T. joined forces with Bell Labs and General Electric (then a computer vendor)
and began designing a second-generation system,
MULTICS
(
MULTiplexed
Information and Computing Service
), as we discussed in Chap. 1.
Although Bell Labs was one of the founding partners in the MULTICS project,
it later pulled out, which left one of the Bell Labs researchers, Ken Thompson,
looking around for something interesting to do.
He eventually decided to write a
stripped-down MULTICS all by himself (in assembly language this time) on an old


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