Information Technology Reference
In-Depth Information
Chapter 2
The Kernel Abstraction
Strong fences make good neighbors.
{17thcenturyproverb
A central role of operating systems is protection | the isolation of poten-
Definition: protection
tially misbehaving applications and users so that they do not corrupt other
applications or the operating system itself. Protection is essential to achieving
several of the goals we listed for operating systems in the previous chapter:
Reliability. Protection is needed to prevent bugs in one program from
causing crashes in other programs or in the operating system. To the
user, a system crash will appear to be the operating system's fault, even
if the root cause of the problem was some unexpected behavior by an
application or user. Thus, for high system reliability, an operating system
must bullet proof itself so that it operates correctly regardless of whatever
an application or user might do.
Security. Some users or applications on a system may be less than com-
pletely trustworthy and therefore the operating system needs to limit the
scope of what they can do. Without protection, a malicious user might
surreptitiously change application files or even the operating system itself,
leaving the user none the wiser. For example, if a malicious application is
permitted to write directly to the disk, it could modify the file containing
the operating system's code, so that the next time the system starts, the
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