MCS 494 midterm 2
             2004.11.17
             D. J. Bernstein

             Your answers must be based solely on your own knowledge and the information
             on this sheet. You are not permitted to use books, notes, or computers. Do not
             ask your proctor for interpretations or clarifications.
             Do not hand in this sheet. Anything that you want to have graded must appear
             in the answer booklet. Make sure that your name is on the front of the booklet.

Problem 1. A file /etc/crontab.default has the following contents:
                 0 * * * * root /usr/libexec/atrun
             A file /etc/crontab has the following contents:
                 0 * * * * root /usr/libexec/atrun-local
             Here is the C code for a program /bin/crontab-reset:
                 #include <fcntl.h>
                 #include <stdio.h>
                 int main(int argc,char **argv)
                 { char *result = "succeeded";
                      int out;
                      int in;
                      char ch;
                      int r;
                      out = open("/etc/crontab",O_WRONLY | O_TRUNC | O_CREAT,0644);
                      if (out == -1) goto failed;
                      in = open("/etc/crontab.default",O_RDONLY);
                      if (in == -1) goto failed;
                      for (;;) {
                        r = read(in,&ch,1);
                        if (r == -1) goto failed;
                        if (r == 0) goto done;
                        if (write(out,&ch,1) < 1) goto failed;
                      }failed:
                      result = "failed";
                      done:
                      fprintf(stderr,"%s %s\n",argv[0],result);
                      return 0;
                 }
             The system administrator runs /bin/crontab-reset. (1) What number does
             the first open() return? You may assume that nothing unusual happens: in
             particular, the file will be opened successfully. (2) What number does the second
             open() return? (3) What are the final contents of /etc/crontab.default? (4)
             What are the final contents of /etc/crontab? (5) What does the program
             display on the screen?

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Problem 2. Same as Problem 1, with one complication: the program receives a TERM signal
              immediately after the second open(). (1) What number does the first open()
              return? (2) What number does the second open() return? (3) What are the
              final contents of /etc/crontab.default? (4) What are the final contents of
              /etc/crontab? (5) What does the program display on the screen?

Problem 3. Same as Problem 1, with one complication: the program is started with a
              resource limit preventing it from having more than 4 files open. (1) What
              number does the first open() return? (2) What number does the second open()
              return? (3) What are the final contents of /etc/crontab.default? (4) What
              are the final contents of /etc/crontab? (5) What does the program display on
              the screen?

Problem 4. Same as Problem 1, with one complication: file descriptor 2 is closed when the
              program begins. (1) What number does the first open() return? (2) What
              number does the second open() return? (3) What are the final contents of
              /etc/crontab.default? (4) What are the final contents of /etc/crontab?
              (5) What does the program display on the screen?

Problem 5. In the situation of Problem 1, assume that /bin/crontab-reset is setuid root.
              A local user joe creates a file evil.c and runs
                 gcc -o evil evil.c
                 ./evil
              with the following unauthorized result: /etc/crontab contains the line
                 0 * * * * root /home/joe/atrun
              among other lines. What were the contents of evil.c?




















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