116.html

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<Pre>

<b><I>r</I></B><B>.s_execfile(</b><b><i>code</i></B><B>)</B> </Pre>

<p>Like <tT CLAss="monofont">r_execfile()</tt>  except that access to <TT CLass="monofont">sys.stdin</tt>, <tt class="monofont">sys.stdout</tt>, and <tt clasS="monofont">sys.stderr</tt> is allowed.</P>

<p>The following methods are called implicitly by code executing in the restricted environment and can be redefined in subclasses of <tt Class="monofont">RExec</Tt>:</p>

<pRE>

<B><I>r</i></b><b>.r_import(</b><B><I>modulename</I></B> <b>[,</b> <b><i>globals</I></B> <B>[,</B> <b><i>locals</i></b> <B>[,</B> <B><I>fromlist</i></b><b>]]])</b> </pre>

<p>Imports a module <i><tt class="monofont">modulename</tt></i>
. An <tT clAss="monofont">ImportError</tT> exception should be raised if the module is unsafe.</p>

<pre>

<B><i>r</i></b><B>.r_open(</B><B><I>filename</i></b> <b>[,</b> <B><I>mode</I></B> <b>[,</b> <b><i>bufsize</I></B><B>]])</B> </pre>

<p>Opens a file in the restricted environment. The arguments are the same as the built-in <TT CLass="monofont">open()</tt> function. By default, files can be opened for reading, but not for writing.</p>

<pre>

<b><i>r</i></b><b>.r_reload(</b><b><i>module</i></b><b>)</B> </prE>

<p>Reloads the module object <i><tT clasS="monofont">module</tt></i>
.</P>

<PRE>

<b><i>r</i></b><B>.r_unload(</B><B><I>module</i></b><b>)</b> </PRE>

<P>Unloads the module object <i><tt cLASS="monofont">module</tt></i>
.</p>

<pre>

<b><i>r</i></b><b>.s_import(</b><b><i>modulename</i></b> <b>[,</b> <b><I>globals</i></b> <B>[,</b> <b><i>locals</I></b> <b>[,</b> <b><I>fromlist</i></b><b>]]])</B> </PRE>

<p>Like <tt cLASS="monofont">r_import()</tt>, but with access to standard I/O streams.</p>

<pRE>

<B><I>r</i></b><b>.s_reload(</b><B><I>module</I></B><b>)</b> </pre>

<p>Like <tt class="monofont">r_reload()</tt>, but with access to standard I/O streams.</p>

<pre>

<b><I>r</i></b><B>.s_unload(</b><b><i>module</I></b><b>)</b> </pRe>

<p>Like <tT CLAss="monofont">r_unload()</tt>, but with access to standard I/O streams.</P>


<H5>Example</H5>
<P>The following program executes Python code submitted through a CGI script in a restricted environment along with limits on CPU and memory usage:</p>

<pre>

#!/usr/local/bin/python 
import rexec 
import cgi, StringIO, sys, string, resource 
form = cgi.FieldStorage() 
code = form["code"].value         # Get some arbitrary code to execute 
code = string.replace(code,"\015","") 
sys.stderr = sys.stdout    # Make error messages appear 

print "Content-type: text/plain\n\n" 
print "The output of your program is : \n\n" 
class CGIExec(rexec.RExec): 
      def r_open(*args): 
           raise SystemError, "open not supported" 
r = CGIExec()              # Create sandbox 
# Restrict memory usage to 4 Mbytes 
resource.setrlimit(resource.RLIMIT_DATA,(4000000,4000000)) 
# Set CPU time limit to 10 seconds 
resource.setrlimit(resource.RLIMIT_CPU,(10,10)) 
# Go run the code 
r.s_exec(code)             # Execute the untrusted code </PRE>

<P>? <b>See Also</b> <a hREF="116#6.html">Bastion</A> (283).</p>

<a name="6"></a>
<h4><tt class="monofont">Bastion</tt></h4>
<p>The <tT clAss="monofont">Bastion</tT>  module restricts access to attributes of objects. It抯 primarily used in conjunction with the <tt clAss="monofont">rexec</tT>  module when a privileged program wants to allow restricted programs to access attributes of unrestricted objects. The idea behind a <TT Class="monofont">Bastion</TT>  is simple梐 wrapper is placed around an object, causing every method access to be redirected through a filter function that抯 responsible for accepting or rejecting the access. Furthermore, all access to data attributes (non-methods) is prohibited.</P>

<Pre>

<b>Bastion(</b><B><I>object</I></B> <b>[,</b> <b><i>filter</I></B> <B>[,</B> <b><i>name</i></b> <b>[,</b> <b><i>class</i></b><b>]]])</b> </pre>

<p>Returns a bastion for the object <i><tt cLasS="monofont">object</tt></i>
. <I><tt clAss="monofont">filter</tT></I>
 is a function that accepts a string containing a method name and returns true or false if access to the method is permitted or denied, respectively. <I><Tt claSS="monofont">name</TT></i>
 is the name of the object that抯 printed by the bastion抯 <tt cLASS="monofont">str()</tt>  method. <i><tT CLAss="monofont">class</tt></i>
 is the class object that implements Bastion objects and is not described here (it抯 rarely necessary to supply this).</p>


<h5>Example</h5>
<p>In this example, you want to restrict access to a <tt class="monofont">StringIO</tt>  object so that only read operations are permitted (see the <a hRef="110#18.html">StringIO</A> module):</p>

<prE>

import StringIO, Bastion 

str = StringIO("") 
... 
strbast = Bastion.Bastion(str, lambda x: x in ['read','readline','readlines']) 
strbast.readline()           # Okay 
strbast.write("Ha ha")       # Fails. AttributeError : write </pre>


<h5>Notes</H5>
<ul>
<lI>
<P>If the <I><Tt claSS="monofont">filter</TT></i>
 function is omitted, a bastion limits access to all methods beginning with an underscore.</p>
</li>
<LI>
<P>Bastions cannot be placed around built-in types such as files and sockets.</P>
</li>
</ul></FONT>
<P><TABLE width="100%" border=0><TR valign="top"><TD><font size=1 color="#C0C0C0"><br></font></TD><TD align=right><font size=1 color="#C0C0C0">Last updated on 3/28/2002<br>Python Essential Reference, Second Edition, &copy;&nbsp;2002 New Riders Publishing</font></TD></TR></TABLE></P>
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