Edit File: restricted.html
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Restricted Execution — Python v2.6.6 documentation</title> <link rel="stylesheet" href="../_static/default.css" type="text/css" /> <link rel="stylesheet" href="../_static/pygments.css" type="text/css" /> <script type="text/javascript"> var DOCUMENTATION_OPTIONS = { URL_ROOT: '../', VERSION: '2.6.6', COLLAPSE_MODINDEX: false, FILE_SUFFIX: '.html', HAS_SOURCE: true }; </script> <script type="text/javascript" src="../_static/jquery.js"></script> <script type="text/javascript" src="../_static/doctools.js"></script> <link rel="search" type="application/opensearchdescription+xml" title="Search within Python v2.6.6 documentation" href="../_static/opensearch.xml"/> <link rel="author" title="About these documents" href="../about.html" /> <link rel="copyright" title="Copyright" href="../copyright.html" /> <link rel="top" title="Python v2.6.6 documentation" href="../index.html" /> <link rel="up" title="The Python Standard Library" href="index.html" /> <link rel="next" title="29.1. rexec — Restricted execution framework" href="rexec.html" /> <link rel="prev" title="28.2. codeop — Compile Python code" href="codeop.html" /> <link rel="shortcut icon" type="image/png" href="../_static/py.png" /> </head> <body> <div class="related"> <h3>Navigation</h3> <ul> <li class="right" style="margin-right: 10px"> <a href="../genindex.html" title="General Index" accesskey="I">index</a></li> <li class="right" > <a href="../modindex.html" title="Global Module Index" accesskey="M">modules</a> |</li> <li class="right" > <a href="rexec.html" title="29.1. rexec — Restricted execution framework" accesskey="N">next</a> |</li> <li class="right" > <a href="codeop.html" title="28.2. codeop — Compile Python code" accesskey="P">previous</a> |</li> <li><img src="../_static/py.png" alt="" style="vertical-align: middle; margin-top: -1px"/></li> <li><a href="../index.html">Python v2.6.6 documentation</a> »</li> <li><a href="index.html" accesskey="U">The Python Standard Library</a> »</li> </ul> </div> <div class="document"> <div class="documentwrapper"> <div class="bodywrapper"> <div class="body"> <div class="section" id="restricted-execution"> <span id="restricted"></span><h1>29. Restricted Execution<a class="headerlink" href="#restricted-execution" title="Permalink to this headline">¶</a></h1> <div class="admonition warning"> <p class="first admonition-title">Warning</p> <p class="last">In Python 2.3 these modules have been disabled due to various known and not readily fixable security holes. The modules are still documented here to help in reading old code that uses the <a title="Basic restricted execution framework. (deprecated)" class="reference external" href="rexec.html#module-rexec"><tt class="xref docutils literal"><span class="pre">rexec</span></tt></a> and <a title="Providing restricted access to objects. (deprecated)" class="reference external" href="bastion.html#module-Bastion"><tt class="xref docutils literal"><span class="pre">Bastion</span></tt></a> modules.</p> </div> <p><em>Restricted execution</em> is the basic framework in Python that allows for the segregation of trusted and untrusted code. The framework is based on the notion that trusted Python code (a <em>supervisor</em>) can create a “padded cell’ (or environment) with limited permissions, and run the untrusted code within this cell. The untrusted code cannot break out of its cell, and can only interact with sensitive system resources through interfaces defined and managed by the trusted code. The term “restricted execution” is favored over “safe-Python” since true safety is hard to define, and is determined by the way the restricted environment is created. Note that the restricted environments can be nested, with inner cells creating subcells of lesser, but never greater, privilege.</p> <p>An interesting aspect of Python’s restricted execution model is that the interfaces presented to untrusted code usually have the same names as those presented to trusted code. Therefore no special interfaces need to be learned to write code designed to run in a restricted environment. And because the exact nature of the padded cell is determined by the supervisor, different restrictions can be imposed, depending on the application. For example, it might be deemed “safe” for untrusted code to read any file within a specified directory, but never to write a file. In this case, the supervisor may redefine the built-in <a title="open" class="reference external" href="functions.html#open"><tt class="xref docutils literal"><span class="pre">open()</span></tt></a> function so that it raises an exception whenever the <em>mode</em> parameter is <tt class="docutils literal"><span class="pre">'w'</span></tt>. It might also perform a <tt class="xref docutils literal"><span class="pre">chroot()</span></tt>-like operation on the <em>filename</em> parameter, such that root is always relative to some safe “sandbox” area of the filesystem. In this case, the untrusted code would still see an built-in <a title="open" class="reference external" href="functions.html#open"><tt class="xref docutils literal"><span class="pre">open()</span></tt></a> function in its environment, with the same calling interface. The semantics would be identical too, with <a title="exceptions.IOError" class="reference external" href="exceptions.html#exceptions.IOError"><tt class="xref docutils literal"><span class="pre">IOError</span></tt></a>s being raised when the supervisor determined that an unallowable parameter is being used.</p> <p>The Python run-time determines whether a particular code block is executing in restricted execution mode based on the identity of the <tt class="docutils literal"><span class="pre">__builtins__</span></tt> object in its global variables: if this is (the dictionary of) the standard <a title="The module that provides the built-in namespace." class="reference external" href="__builtin__.html#module-__builtin__"><tt class="xref docutils literal"><span class="pre">__builtin__</span></tt></a> module, the code is deemed to be unrestricted, else it is deemed to be restricted.</p> <p>Python code executing in restricted mode faces a number of limitations that are designed to prevent it from escaping from the padded cell. For instance, the function object attribute <tt class="xref docutils literal"><span class="pre">func_globals</span></tt> and the class and instance object attribute <tt class="xref docutils literal"><span class="pre">__dict__</span></tt> are unavailable.</p> <p>Two modules provide the framework for setting up restricted execution environments:</p> <ul> <li class="toctree-l1"><a class="reference external" href="rexec.html">29.1. <tt class="docutils literal"><span class="pre">rexec</span></tt> — Restricted execution framework</a><ul> <li class="toctree-l2"><a class="reference external" href="rexec.html#rexec-objects">29.1.1. RExec Objects</a></li> <li class="toctree-l2"><a class="reference external" href="rexec.html#defining-restricted-environments">29.1.2. Defining restricted environments</a></li> <li class="toctree-l2"><a class="reference external" href="rexec.html#an-example">29.1.3. An example</a></li> </ul> </li> <li class="toctree-l1"><a class="reference external" href="bastion.html">29.2. <tt class="docutils literal"><span class="pre">Bastion</span></tt> — Restricting access to objects</a></li> </ul> <div class="admonition-see-also admonition seealso"> <p class="first admonition-title">See also</p> <dl class="last docutils"> <dt><a class="reference external" href="http://grail.sourceforge.net/">Grail Home Page</a></dt> <dd>Grail, an Internet browser written in Python, uses these modules to support Python applets. More information on the use of Python’s restricted execution mode in Grail is available on the Web site.</dd> </dl> </div> </div> </div> </div> </div> <div class="sphinxsidebar"> <div class="sphinxsidebarwrapper"> <h4>Previous topic</h4> <p class="topless"><a href="codeop.html" title="previous chapter">28.2. <tt class="docutils literal docutils literal docutils literal"><span class="pre">codeop</span></tt> — Compile Python code</a></p> <h4>Next topic</h4> <p class="topless"><a href="rexec.html" title="next chapter">29.1. <tt class="docutils literal docutils literal"><span class="pre">rexec</span></tt> — Restricted execution framework</a></p> <h3>This Page</h3> <ul class="this-page-menu"> <li><a href="../bugs.html">Report a Bug</a></li> <li><a href="../_sources/library/restricted.txt" rel="nofollow">Show Source</a></li> </ul> <div id="searchbox" style="display: none"> <h3>Quick search</h3> <form class="search" action="../search.html" method="get"> <input type="text" name="q" size="18" /> <input type="submit" value="Go" /> <input type="hidden" name="check_keywords" value="yes" /> <input type="hidden" name="area" value="default" /> </form> <p class="searchtip" style="font-size: 90%"> Enter search terms or a module, class or function name. </p> </div> <script type="text/javascript">$('#searchbox').show(0);</script> </div> </div> <div class="clearer"></div> </div> <div class="related"> <h3>Navigation</h3> <ul> <li class="right" style="margin-right: 10px"> <a href="../genindex.html" title="General Index" >index</a></li> <li class="right" > <a href="../modindex.html" title="Global Module Index" >modules</a> |</li> <li class="right" > <a href="rexec.html" title="29.1. rexec — Restricted execution framework" >next</a> |</li> <li class="right" > <a href="codeop.html" title="28.2. codeop — Compile Python code" >previous</a> |</li> <li><img src="../_static/py.png" alt="" style="vertical-align: middle; margin-top: -1px"/></li> <li><a href="../index.html">Python v2.6.6 documentation</a> »</li> <li><a href="index.html" >The Python Standard Library</a> »</li> </ul> </div> <div class="footer"> © <a href="../copyright.html">Copyright</a> 1990-2011, Python Software Foundation. <br /> The Python Software Foundation is a non-profit corporation. <a href="http://www.python.org/psf/donations/">Please donate.</a> <br /> Last updated on Jul 20, 2011. <a href="../bugs.html">Found a bug</a>? <br /> Created using <a href="http://sphinx.pocoo.org/">Sphinx</a> 0.6.6. </div> </body> </html>
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