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<H2><A NAME="SECTION0018110000000000000000">
16.1.1 An example</A>
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<P>
Let us say that we want a slightly more relaxed policy than the
standard <tt class="class">RExec</tt> class.  For example, if we're willing to allow
files in <span class="file">/tmp</span> to be written, we can subclass the <tt class="class">RExec</tt>
class:

<P>
<dl><dd><pre class="verbatim">
class TmpWriterRExec(rexec.RExec):
    def r_open(self, file, mode='r', buf=-1):
        if mode in ('r', 'rb'):
            pass
        elif mode in ('w', 'wb', 'a', 'ab'):
            # check filename : must begin with /tmp/
            if file[:5]!='/tmp/': 
                raise IOError, "can't write outside /tmp"
            elif (string.find(file, '/../') &gt;= 0 or
                 file[:3] == '../' or file[-3:] == '/..'):
                raise IOError, "'..' in filename forbidden"
        else: raise IOError, "Illegal open() mode"
        return open(file, mode, buf)
</pre></dl>
Notice that the above code will occasionally forbid a perfectly valid
filename; for example, code in the restricted environment won't be
able to open a file called <span class="file">/tmp/foo/../bar</span>.  To fix this, the
<tt class="method">r_open()</tt> method would have to simplify the filename to
<span class="file">/tmp/bar</span>, which would require splitting apart the filename and
performing various operations on it.  In cases where security is at
stake, it may be preferable to write simple code which is sometimes
overly restrictive, instead of more general code that is also more
complex and may harbor a subtle security hole.

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