The processes in an operating system must be protected from one another’s
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domain , which specifies the resources that the process may access. Each domain defines a set of objects and the types of operations that may be invoked on each object. The ability to execute an operation on an object is an access right . A domain is a collection of access rights, each of which is an ordered pair < object-name, rights-set > . For example, if domain D has the access right < file F , {read,write}>, then a process executing in domain D can both read and write file F. It cannot, however, perform any other operation on that object. Domains may share access rights. For example, in Figure 14.1, we have three domains: D 1 , D 2 , and D 3 . The access right <O 4 , {print}> is shared by D 2 and D 3 , implying that a process executing in either of these two domains can print object O 4 . Note that a process must be executing in domain D 1 to read and write object O 1 , while only processes in domain D 3 may execute object O 1 . The association between a process and a domain may be either static , if the set of resources available to the process is fixed throughout the process’s lifetime, or dynamic . As might be expected, establishing dynamic protection domains is more complicated than establishing static protection domains. If the association between processes and domains is fixed, and we want to adhere to the need-to-know principle, then a mechanism must be available to change the content of a domain. The reason stems from the fact that a process may execute in two different phases and may, for example, need read access in one phase and write access in another. If a domain is static, we must define the domain to include both read and write access. However, this arrangement provides more rights than are needed in each of the two phases, since we have read access in the phase where we need only write access, and vice versa. D 1 ( O 3 , {read, write} ) ( O 1 , {read, write} ) ( O 2 , {execute} ) ( O 1 , {execute} ) ( O 3 , {read} ) ( O 2 , {write} ) ( O 4 , {print} ) D 2 D 3 tải về 252.86 Kb. Chia sẻ với bạn bè của bạn:
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