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Chapter 14
Protection
by a user. Of more general importance, however, is the need to ensure that
each program component active in a system uses system resources only in
ways consistent with stated policies. This requirement is an absolute one for a
reliable system.
Protection can improve reliability by detecting latent errors at the interfaces
between component subsystems. Early detection of interface errors can often
prevent contamination of a healthy subsystem by a malfunctioning subsystem.
Also, an unprotected resource cannot defend against use (or misuse) by an
unauthorized or incompetent user. A protection-oriented system provides
means to distinguish between authorized and unauthorized usage.
The role of protection in a computer system is to provide a mechanism for
the enforcement of the policies governing resource use. These policies can be
established in a variety of ways. Some are fixed in the design of the system,
while others are formulated by the management of a system. Still others are
defined by the individual users to protect their own files and programs. A
protection system must have the flexibility to enforce a variety of policies.
Policies for resource use may vary by application, and they may change
over time. For these reasons, protection is no longer the concern solely of
the designer of an operating system. The application programmer needs to
use protection mechanisms as well, to guard resources created and supported
by an application subsystem against misuse. In this chapter, we describe the
protection mechanisms the operating system should provide, but application
designers can use them as well in designing their own protection software.
Note that
mechanisms are distinct from
policies. Mechanisms determine
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