CS140 Operating Systems and Systems Programming Final Exam December 10th 2001 Total time 165 minutes Total Points 165 Name please print In recognition of and in the spirit of the Stanford University Honor Code I certify that I will neither give nor receive unpermitted aid on this exam Signature This examination is close notes and close book You may not collaborate in any manner on this exam You have 165 minutes 2 hours and 45 minutes to complete the exam Before starting please check to make sure that you have all 13 pages 1 2 3 Total Name Name 1 Protection and Security 68 points a 8 points In a public key cryptosystem a person can encrypt a file with his or her own public key What might be the purpose if any of a person doing this operation b 8 points In a public key cryptosystem a person can compute a message digest of a file and then encrypt the message digest with his or her own private key The encrypted message digest could be appended to the file when it is distributed over the Internet What might be the purpose if any of a person doing this operation Name c 10 points Describe how Microsoft s Windows 2000 operating system attempts to battle password stealing Trojan Horses d 6 points What is a nonce and what is the problem it is intended to solve e 8 points Explain why someone might want to publish details of their security mechanism Name f 10 points Some systems have deployed a protection system that uses both capabilities and access control lists to protect files Describe the problem with capabilities that adding an access control list can help with Describe how it helps g 6 points Describe what is meant by the confinement problem Name h 12 points Lists the three core components of a protection system For each component give an example of a technique used to implement it Name 2 File systems 72 points a 8 points A file system designer claims that an aggressive prefetching algorithm in his file system can turn a latency problem into a bandwidth problem What does he mean by this b 6 points The original Unix file system s consistency check program fsck had an option to sort the free list into increasing disk block number order Why would someone want to do this to a file system Name c 6 points Under what conditions would a freelist be a better data structure than a bitmap for tracking free blocks in a file system d 6 points Explain why moving a disk formatted with a BSD fast file system from a faster to slower CPU machine can result in the maximum transfer speed for large files dropping to a value equal to the file system s block size times the revolution rate of the disk drive Name e 5 points Describe the mechanisms used by the BSD fast file system to get disk locality for files in the same directory f 8 points Given that CPUs are much faster than disks some people have suggested doing disk scheduling by trying all possible orderings of the requests using a disk simulator and choosing the ordering that the simulator predicts the disk could process the fastest Assuming that CPUs are fast enough to do this what would be the problem of deploying something like this in a real system Name g 8 points Describe the factors that go into deciding how large of log area is needed for a file system that does write ahead logging of metadata Name h 12 points List one advantage and one disadvantage of each of the following file descriptor techniques a contiguous allocation b linked files c multilevel index Name i 7 points Given the BSD fast file system which would be a better way for a disk designer to increase the capacity of a disk a add more cylinders or b make each cylinder bigger j 6 points Your partner has an idea at 11 45PM on the day the file system project is due and quickly modifies the file system to allow files to share the same disk block if they have the same contents He does this by detecting if a block being written already exists on the disk and updates the file descriptor to point at that block rather than allocating a new one The idea would be to use less disk space for workloads that have many copies of the same files What other changes would you need to make to the file system to get this work correct Name 3 Virtual Memory 25 points a 10 points You decide to implement a working set algorithm for your Nachos virtual memory system Your partner decides than he is going to play with you a bit by setting the working set parameter to bogus values What would be the symptoms if he set it way too large How about if he set it way too small Name b 7 points Given a machine with a 32bit virtual address and a 36bit physical address fully populated with memory would it be possible to make the machine thrash assuming that you have a reasonable virtual memory system c 8 points Would it ever make sense to use a LRU replacement algorithm with a local page replacement policy Justify your answer
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