57 \item A Unix directory might look as follows:

    58 

    59 \begin{center}

    60 \begin{verbatim}

    61 $ ls -ld . * */*

    62 drwxr-xr-x 1 ping staff  32768 Apr  2 2010 .

    63 -rw----r-- 1 ping students  31359 Jul 24 2011 manual.txt

    64 -r--rw--w- 1 bob students   4359 Jul 24 2011 report.txt

    65 -rwsr--r-x 1 bob students 141359 Jun  1 2013 microedit

    66 dr--r-xr-x 1 bob staff  32768 Jul 23 2011 src

    67 -rw-r--r-- 1 bob staff  81359 Feb 28 2012 src/code.c

    68 -r--rw---- 1 emma students    959 Jan 23 2012 src/code.h

    69 \end{verbatim}

    70 \end{center}

    71 

    72 with group memberships assigned as follows:

    73 \begin{center}

    74 \begin{tabular}{ll}

    75 Members of group staff: & ping, bob, emma\\ 

    76 Members of group students: & emma\\

    77 \end{tabular}

    78 \end{center}

    79 

    80 The file microedit is a text editor, which allows its users to open, edit and 

    81 save files. Note carefully that microedit has set its setuid flag. 

    82 Fill in the access control matrix below that shows for each of the above five files, 

    83 whether ping, bob, or emma are able to obtain the right to read (R) or replace (W) its 

    84 contents using the editor microedit.\bigskip

    85 

    86 \begin{center}

    87 \begin{tabular}{r|c|c|c|c|c}

    88         & manual.txt & report.txt & microedit & src/code.c & src/code.h \\\hline

    89 ping & & & & &\\\hline

    90 bob & & & & &\\\hline

    91 emma   & & & & &\\

    92 \end{tabular}

    93 \end{center}

    94