CSCI 350 Fall 2001 Quiz 6

Statistics:

mean     42.154 (548.000/13)
stddev   3.900
median   42.000
midrange 38.500-45.750

#   avg
1   1    / 5
2   5.85 / 9
3   8.77 / 9
4   9    / 9
5   9    / 9
6   8.54 / 9

1. [5 pts] Unix does not permit two directory entries (filenames) in different filesystems to link to the same file. That is, a file on one disk cannot link to a file on another. (It does permit a symbolic link between files, but a true link (known as a hard link) is impossible.) Explain why Unix cannot permit this.

   A Unix directory entry consists of only a filename and an i-node number. I-nodes are numbered sequentially, starting at 0, for each disk. There's no way that a file can link to another, since there's no way to indicate in the i-node number which filesystem the i-node lies in.

2. [9 pts] Describe three pieces of data that can be found in a typical Unix i-node.

   Date of last modification, number of links to file, size of file, file permissions, owner ID, group ID, block ID of first several block in file, block ID of indirect block.

3. [9 pts] Alice and Bob have just met in a Internet chatroom, and now Alice wants to send Bob her telephone number via an encrypted e-mail using public-key cryptography. What are the steps that Alice and Bob must accomplish in order for Alice's number to reach Bob?

   Bob thinks up corresponding private and public keys, and tells Alice his public key. Alice encrypts here telephone number with the public key, and sends the encryption to Bob. Bob inverts the encryption using his private key to obtain the telephone number.

4. [9 pts] How does classical Unix attempt to protect passwords from people trying to read them from the disk?

   It hashes them using a hard-to-invert hash function, so that the passwords are never actually stored on the disk.

5. [9 pts] Suppose the following occurs at the command prompt.

   % ls -l quiz6
   -rwxr-xr--   1 cburch   cs          1833 Dec  5 23:50 quiz6
   

   Who has permission to read the file quiz6? Who can modify it? Who can execute it?

   Everybody in the world has the right to read the file. Only the owner (cburch) can alter the file. Both the owner and the members of the cs group can execute the file.

6. [9 pts] Explain how RAID can improve system performance.

   RAID improves the apparent disk performance by using several disks within the same filesystem, distributing data across the disks so that several pieces of data can be accessed in parallel.