The processes in an operating system must be protected from one another’s
tải về 252.86 Kb. Chế độ xem pdf
|
| Exercises
651 14.6 Consider a computing environment where a unique number is associ- ated with each process and each object in the system. Suppose that we allow a process with number n to access an object with number m only if n > m. What type of protection structure do we have? 14.7 Consider a computing environment where a process is given the privilege of accessing an object only n times. Suggest a scheme for implementing this policy. 14.8 If all the access rights to an object are deleted, the object can no longer be accessed. At this point, the object should also be deleted, and the space it occupies should be returned to the system. Suggest an efficient implementation of this scheme. 14.9 Why is it difficult to protect a system in which users are allowed to do their own I/O ? 14.10 Capability lists are usually kept within the address space of the user. How does the system ensure that the user cannot modify the contents of the list? Exercises 14.11 Consider the ring-protection scheme in MULTICS . If we were to imple- ment the system calls of a typical operating system and store them in a segment associated with ring 0, what should be the values stored in the ring field of the segment descriptor? What happens during a system call when a process executing in a higher-numbered ring invokes a procedure in ring 0? 14.12 The access-control matrix can be used to determine whether a process can switch from, say, domain A to domain B and enjoy the access privileges of domain B. Is this approach equivalent to including the access privileges of domain B in those of domain A? 14.13 Consider a computer system in which computer games can be played by students only between 10 P.M. and 6 A.M. , by faculty members between 5 P.M. and 8 A.M. , and by the computer center staff at all times. Suggest a scheme for implementing this policy efficiently. 14.14 What hardware features does a computer system need for efficient capability manipulation? Can these features be used for memory protection? tải về 252.86 Kb. Chia sẻ với bạn bè của bạn:
|