Homework 4

Total Points: 250	Due Date: Monday, November 27, 1998

1.	(100 points) Please write a program to determine if a named user can access a named file or directory. The pro-
gram is to be called access, and the command line looks like:
        access options user file

Here, user is either a user's login name or UID,  file is a file (or directory, or other entity), and options is any com-
bination of r (for read), w (for write), and x (for execute), with a leading -. If none of r, w, or x is given, act as 
though all three had been given. Your output should say:

        user can read file

or

        user can read and write file

and so forth. If the option l is given, then every component of the path is to be checked as well as the file itself.

If the file cannot be accessed, any command line argument is malformed, or an error occurs in your code, print an 
appropriate error message and exit with an exit code of 1. Otherwise, exit with an error code of 0. Please write 
your code robustly, and comment it!

Example. The output of

       access -rwx bishop /home/cs153/public_html

should be

       bishop can read and execute /home/cs153/public_html

The output of

       access -l bishop /home/cs153/public_html/index.html

should be

       bishop can read and execute /
       bishop can read and execute /home
       bishop can read and execute /home/cs153
       bishop can read and execute /home/cs153/public_html
       bishop can read /home/cs153/public_html/index.html

Submitting your programs. If you write your programs in a compilable language (like C), you must submit a 
makefile to compile your source code.  A single make should generate the access executable.  Do not submit 
access executables unless it is written in a scripted language (such as shell script or Perl).  Your program should 
run correctly on any of the CSIF machines (that is, on the DECs, the HPs, and the SGIs).

2.	(20 points) Can the Trojan Horse attack work on a system protected by capabilities?

3.	(10 points) Some early computers protected their operating systems against destruction by placing it in memory 
locations from which all programs (including the operating system itself) could read, but into which no program 
could write.  What problem does this protection scheme create?

4.	(20 points) Prove or disprove: the power set of any set and the relation "subset" form a lattice. To prove this, 
show how to construct a greatest lower bound and a least upper bound (under "subset") for any pair of elements 
of the power set, and prove that "subset" is transitive, associative, and antisymmetric. To disprove this, exhibit a 
specific counterexample to any of these properties.

5.	(100 points) Please read the book The Shockwave Rider by John Brunner, and contrast his use of the terms 
"worm," "virus," and other references to malicious logic with the way we use those terms now. As with previous 
essays, please write well. Your essay should be at least two pages (120 lines) long.

Extra credit

6.	(20 points) Why do most UNIX systems limit the use of the chroot(2) system call to the super-user?  (Hint: think 
about protection problems.)

7.	(30 points) Determine which of the systems in the CSIF provide login/audit UIDs and GIDs, and saqved UIDs 
and GIDs. Write a program to print each of these, as well as the real and effective UIDs and GIDs.