Edit File: rexec.html
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"> <html xmlns="http://www.w3.org/1999/xhtml"> <head> <meta http-equiv="Content-Type" content="text/html; charset=utf-8" /> <title>29.1. rexec — Restricted execution framework — 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="29. 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Restricted Execution</a> »</li> </ul> </div> <div class="document"> <div class="documentwrapper"> <div class="bodywrapper"> <div class="body"> <div class="section" id="module-rexec"> <h1>29.1. <tt class="xref docutils literal"><span class="pre">rexec</span></tt> — Restricted execution framework<a class="headerlink" href="#module-rexec" title="Permalink to this headline">¶</a></h1> <p class="deprecated"> <span class="versionmodified">Deprecated since version 2.6: </span>The <tt class="xref docutils literal"><span class="pre">rexec</span></tt> module has been removed in Python 3.0.</p> <p class="versionchanged"> <span class="versionmodified">Changed in version 2.3: </span>Disabled module.</p> <div class="admonition warning"> <p class="first admonition-title">Warning</p> <p class="last">The documentation has been left in place to help in reading old code that uses the module.</p> </div> <p>This module contains the <a title="rexec.RExec" class="reference internal" href="#rexec.RExec"><tt class="xref docutils literal"><span class="pre">RExec</span></tt></a> class, which supports <tt class="xref docutils literal"><span class="pre">r_eval()</span></tt>, <tt class="xref docutils literal"><span class="pre">r_execfile()</span></tt>, <tt class="xref docutils literal"><span class="pre">r_exec()</span></tt>, and <tt class="xref docutils literal"><span class="pre">r_import()</span></tt> methods, which are restricted versions of the standard Python functions <a title="eval" class="reference external" href="functions.html#eval"><tt class="xref docutils literal"><span class="pre">eval()</span></tt></a>, <a title="execfile" class="reference external" href="functions.html#execfile"><tt class="xref docutils literal"><span class="pre">execfile()</span></tt></a> and the <a class="reference external" href="../reference/simple_stmts.html#exec"><tt class="xref docutils literal"><span class="pre">exec</span></tt></a> and <a class="reference external" href="../reference/simple_stmts.html#import"><tt class="xref docutils literal"><span class="pre">import</span></tt></a> statements. Code executed in this restricted environment will only have access to modules and functions that are deemed safe; you can subclass <a title="rexec.RExec" class="reference internal" href="#rexec.RExec"><tt class="xref docutils literal"><span class="pre">RExec</span></tt></a> to add or remove capabilities as desired.</p> <div class="admonition warning"> <p class="first admonition-title">Warning</p> <p class="last">While the <tt class="xref docutils literal"><span class="pre">rexec</span></tt> module is designed to perform as described below, it does have a few known vulnerabilities which could be exploited by carefully written code. Thus it should not be relied upon in situations requiring “production ready” security. In such situations, execution via sub-processes or very careful “cleansing” of both code and data to be processed may be necessary. Alternatively, help in patching known <tt class="xref docutils literal"><span class="pre">rexec</span></tt> vulnerabilities would be welcomed.</p> </div> <div class="admonition note"> <p class="first admonition-title">Note</p> <p class="last">The <a title="rexec.RExec" class="reference internal" href="#rexec.RExec"><tt class="xref docutils literal"><span class="pre">RExec</span></tt></a> class can prevent code from performing unsafe operations like reading or writing disk files, or using TCP/IP sockets. However, it does not protect against code using extremely large amounts of memory or processor time.</p> </div> <dl class="class"> <dt id="rexec.RExec"> <em class="property">class </em><tt class="descclassname">rexec.</tt><tt class="descname">RExec</tt><big>(</big><span class="optional">[</span><em>hooks</em><span class="optional">[</span>, <em>verbose</em><span class="optional">]</span><span class="optional">]</span><big>)</big><a class="headerlink" href="#rexec.RExec" title="Permalink to this definition">¶</a></dt> <dd><p>Returns an instance of the <a title="rexec.RExec" class="reference internal" href="#rexec.RExec"><tt class="xref docutils literal"><span class="pre">RExec</span></tt></a> class.</p> <p><em>hooks</em> is an instance of the <tt class="xref docutils literal"><span class="pre">RHooks</span></tt> class or a subclass of it. If it is omitted or <tt class="xref docutils literal"><span class="pre">None</span></tt>, the default <tt class="xref docutils literal"><span class="pre">RHooks</span></tt> class is instantiated. Whenever the <tt class="xref docutils literal"><span class="pre">rexec</span></tt> module searches for a module (even a built-in one) or reads a module’s code, it doesn’t actually go out to the file system itself. Rather, it calls methods of an <tt class="xref docutils literal"><span class="pre">RHooks</span></tt> instance that was passed to or created by its constructor. (Actually, the <a title="rexec.RExec" class="reference internal" href="#rexec.RExec"><tt class="xref docutils literal"><span class="pre">RExec</span></tt></a> object doesn’t make these calls — they are made by a module loader object that’s part of the <a title="rexec.RExec" class="reference internal" href="#rexec.RExec"><tt class="xref docutils literal"><span class="pre">RExec</span></tt></a> object. This allows another level of flexibility, which can be useful when changing the mechanics of <a class="reference external" href="../reference/simple_stmts.html#import"><tt class="xref docutils literal"><span class="pre">import</span></tt></a> within the restricted environment.)</p> <p>By providing an alternate <tt class="xref docutils literal"><span class="pre">RHooks</span></tt> object, we can control the file system accesses made to import a module, without changing the actual algorithm that controls the order in which those accesses are made. For instance, we could substitute an <tt class="xref docutils literal"><span class="pre">RHooks</span></tt> object that passes all filesystem requests to a file server elsewhere, via some RPC mechanism such as ILU. Grail’s applet loader uses this to support importing applets from a URL for a directory.</p> <p>If <em>verbose</em> is true, additional debugging output may be sent to standard output.</p> </dd></dl> <p>It is important to be aware that code running in a restricted environment can still call the <a title="sys.exit" class="reference external" href="sys.html#sys.exit"><tt class="xref docutils literal"><span class="pre">sys.exit()</span></tt></a> function. To disallow restricted code from exiting the interpreter, always protect calls that cause restricted code to run with a <a class="reference external" href="../reference/compound_stmts.html#try"><tt class="xref docutils literal"><span class="pre">try</span></tt></a>/<a class="reference external" href="../reference/compound_stmts.html#except"><tt class="xref docutils literal"><span class="pre">except</span></tt></a> statement that catches the <a title="exceptions.SystemExit" class="reference external" href="exceptions.html#exceptions.SystemExit"><tt class="xref docutils literal"><span class="pre">SystemExit</span></tt></a> exception. Removing the <a title="sys.exit" class="reference external" href="sys.html#sys.exit"><tt class="xref docutils literal"><span class="pre">sys.exit()</span></tt></a> function from the restricted environment is not sufficient — the restricted code could still use <tt class="docutils literal"><span class="pre">raise</span> <span class="pre">SystemExit</span></tt>. Removing <a title="exceptions.SystemExit" class="reference external" href="exceptions.html#exceptions.SystemExit"><tt class="xref docutils literal"><span class="pre">SystemExit</span></tt></a> is not a reasonable option; some library code makes use of this and would break were it not available.</p> <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 is a Web browser written entirely in Python. It uses the <tt class="xref docutils literal"><span class="pre">rexec</span></tt> module as a foundation for supporting Python applets, and can be used as an example usage of this module.</dd> </dl> </div> <div class="section" id="rexec-objects"> <span id="id1"></span><h2>29.1.1. RExec Objects<a class="headerlink" href="#rexec-objects" title="Permalink to this headline">¶</a></h2> <p><a title="rexec.RExec" class="reference internal" href="#rexec.RExec"><tt class="xref docutils literal"><span class="pre">RExec</span></tt></a> instances support the following methods:</p> <dl class="method"> <dt id="rexec.RExec.r_eval"> <tt class="descclassname">RExec.</tt><tt class="descname">r_eval</tt><big>(</big><em>code</em><big>)</big><a class="headerlink" href="#rexec.RExec.r_eval" title="Permalink to this definition">¶</a></dt> <dd><em>code</em> must either be a string containing a Python expression, or a compiled code object, which will be evaluated in the restricted environment’s <a title="The environment where the top-level script is run." class="reference external" href="__main__.html#module-__main__"><tt class="xref docutils literal"><span class="pre">__main__</span></tt></a> module. The value of the expression or code object will be returned.</dd></dl> <dl class="method"> <dt id="rexec.RExec.r_exec"> <tt class="descclassname">RExec.</tt><tt class="descname">r_exec</tt><big>(</big><em>code</em><big>)</big><a class="headerlink" href="#rexec.RExec.r_exec" title="Permalink to this definition">¶</a></dt> <dd><em>code</em> must either be a string containing one or more lines of Python code, or a compiled code object, which will be executed in the restricted environment’s <a title="The environment where the top-level script is run." class="reference external" href="__main__.html#module-__main__"><tt class="xref docutils literal"><span class="pre">__main__</span></tt></a> module.</dd></dl> <dl class="method"> <dt id="rexec.RExec.r_execfile"> <tt class="descclassname">RExec.</tt><tt class="descname">r_execfile</tt><big>(</big><em>filename</em><big>)</big><a class="headerlink" href="#rexec.RExec.r_execfile" title="Permalink to this definition">¶</a></dt> <dd>Execute the Python code contained in the file <em>filename</em> in the restricted environment’s <a title="The environment where the top-level script is run." class="reference external" href="__main__.html#module-__main__"><tt class="xref docutils literal"><span class="pre">__main__</span></tt></a> module.</dd></dl> <p>Methods whose names begin with <tt class="docutils literal"><span class="pre">s_</span></tt> are similar to the functions beginning with <tt class="docutils literal"><span class="pre">r_</span></tt>, but the code will be granted access to restricted versions of the standard I/O streams <tt class="docutils literal"><span class="pre">sys.stdin</span></tt>, <tt class="docutils literal"><span class="pre">sys.stderr</span></tt>, and <tt class="docutils literal"><span class="pre">sys.stdout</span></tt>.</p> <dl class="method"> <dt id="rexec.RExec.s_eval"> <tt class="descclassname">RExec.</tt><tt class="descname">s_eval</tt><big>(</big><em>code</em><big>)</big><a class="headerlink" href="#rexec.RExec.s_eval" title="Permalink to this definition">¶</a></dt> <dd><em>code</em> must be a string containing a Python expression, which will be evaluated in the restricted environment.</dd></dl> <dl class="method"> <dt id="rexec.RExec.s_exec"> <tt class="descclassname">RExec.</tt><tt class="descname">s_exec</tt><big>(</big><em>code</em><big>)</big><a class="headerlink" href="#rexec.RExec.s_exec" title="Permalink to this definition">¶</a></dt> <dd><em>code</em> must be a string containing one or more lines of Python code, which will be executed in the restricted environment.</dd></dl> <dl class="method"> <dt id="rexec.RExec.s_execfile"> <tt class="descclassname">RExec.</tt><tt class="descname">s_execfile</tt><big>(</big><em>code</em><big>)</big><a class="headerlink" href="#rexec.RExec.s_execfile" title="Permalink to this definition">¶</a></dt> <dd>Execute the Python code contained in the file <em>filename</em> in the restricted environment.</dd></dl> <p><a title="rexec.RExec" class="reference internal" href="#rexec.RExec"><tt class="xref docutils literal"><span class="pre">RExec</span></tt></a> objects must also support various methods which will be implicitly called by code executing in the restricted environment. Overriding these methods in a subclass is used to change the policies enforced by a restricted environment.</p> <dl class="method"> <dt id="rexec.RExec.r_import"> <tt class="descclassname">RExec.</tt><tt class="descname">r_import</tt><big>(</big><em>modulename</em><span class="optional">[</span>, <em>globals</em><span class="optional">[</span>, <em>locals</em><span class="optional">[</span>, <em>fromlist</em><span class="optional">]</span><span class="optional">]</span><span class="optional">]</span><big>)</big><a class="headerlink" href="#rexec.RExec.r_import" title="Permalink to this definition">¶</a></dt> <dd>Import the module <em>modulename</em>, raising an <a title="exceptions.ImportError" class="reference external" href="exceptions.html#exceptions.ImportError"><tt class="xref docutils literal"><span class="pre">ImportError</span></tt></a> exception if the module is considered unsafe.</dd></dl> <dl class="method"> <dt id="rexec.RExec.r_open"> <tt class="descclassname">RExec.</tt><tt class="descname">r_open</tt><big>(</big><em>filename</em><span class="optional">[</span>, <em>mode</em><span class="optional">[</span>, <em>bufsize</em><span class="optional">]</span><span class="optional">]</span><big>)</big><a class="headerlink" href="#rexec.RExec.r_open" title="Permalink to this definition">¶</a></dt> <dd>Method called when <a title="open" class="reference external" href="functions.html#open"><tt class="xref docutils literal"><span class="pre">open()</span></tt></a> is called in the restricted environment. The arguments are identical to those of <a title="open" class="reference external" href="functions.html#open"><tt class="xref docutils literal"><span class="pre">open()</span></tt></a>, and a file object (or a class instance compatible with file objects) should be returned. <a title="rexec.RExec" class="reference internal" href="#rexec.RExec"><tt class="xref docutils literal"><span class="pre">RExec</span></tt></a>‘s default behaviour is allow opening any file for reading, but forbidding any attempt to write a file. See the example below for an implementation of a less restrictive <a title="rexec.RExec.r_open" class="reference internal" href="#rexec.RExec.r_open"><tt class="xref docutils literal"><span class="pre">r_open()</span></tt></a>.</dd></dl> <dl class="method"> <dt id="rexec.RExec.r_reload"> <tt class="descclassname">RExec.</tt><tt class="descname">r_reload</tt><big>(</big><em>module</em><big>)</big><a class="headerlink" href="#rexec.RExec.r_reload" title="Permalink to this definition">¶</a></dt> <dd>Reload the module object <em>module</em>, re-parsing and re-initializing it.</dd></dl> <dl class="method"> <dt id="rexec.RExec.r_unload"> <tt class="descclassname">RExec.</tt><tt class="descname">r_unload</tt><big>(</big><em>module</em><big>)</big><a class="headerlink" href="#rexec.RExec.r_unload" title="Permalink to this definition">¶</a></dt> <dd>Unload the module object <em>module</em> (remove it from the restricted environment’s <tt class="docutils literal"><span class="pre">sys.modules</span></tt> dictionary).</dd></dl> <p>And their equivalents with access to restricted standard I/O streams:</p> <dl class="method"> <dt id="rexec.RExec.s_import"> <tt class="descclassname">RExec.</tt><tt class="descname">s_import</tt><big>(</big><em>modulename</em><span class="optional">[</span>, <em>globals</em><span class="optional">[</span>, <em>locals</em><span class="optional">[</span>, <em>fromlist</em><span class="optional">]</span><span class="optional">]</span><span class="optional">]</span><big>)</big><a class="headerlink" href="#rexec.RExec.s_import" title="Permalink to this definition">¶</a></dt> <dd>Import the module <em>modulename</em>, raising an <a title="exceptions.ImportError" class="reference external" href="exceptions.html#exceptions.ImportError"><tt class="xref docutils literal"><span class="pre">ImportError</span></tt></a> exception if the module is considered unsafe.</dd></dl> <dl class="method"> <dt id="rexec.RExec.s_reload"> <tt class="descclassname">RExec.</tt><tt class="descname">s_reload</tt><big>(</big><em>module</em><big>)</big><a class="headerlink" href="#rexec.RExec.s_reload" title="Permalink to this definition">¶</a></dt> <dd>Reload the module object <em>module</em>, re-parsing and re-initializing it.</dd></dl> <dl class="method"> <dt id="rexec.RExec.s_unload"> <tt class="descclassname">RExec.</tt><tt class="descname">s_unload</tt><big>(</big><em>module</em><big>)</big><a class="headerlink" href="#rexec.RExec.s_unload" title="Permalink to this definition">¶</a></dt> <dd>Unload the module object <em>module</em>.</dd></dl> </div> <div class="section" id="defining-restricted-environments"> <span id="rexec-extension"></span><h2>29.1.2. Defining restricted environments<a class="headerlink" href="#defining-restricted-environments" title="Permalink to this headline">¶</a></h2> <p>The <a title="rexec.RExec" class="reference internal" href="#rexec.RExec"><tt class="xref docutils literal"><span class="pre">RExec</span></tt></a> class has the following class attributes, which are used by the <a title="object.__init__" class="reference external" href="../reference/datamodel.html#object.__init__"><tt class="xref docutils literal"><span class="pre">__init__()</span></tt></a> method. Changing them on an existing instance won’t have any effect; instead, create a subclass of <a title="rexec.RExec" class="reference internal" href="#rexec.RExec"><tt class="xref docutils literal"><span class="pre">RExec</span></tt></a> and assign them new values in the class definition. Instances of the new class will then use those new values. All these attributes are tuples of strings.</p> <dl class="attribute"> <dt id="rexec.RExec.nok_builtin_names"> <tt class="descclassname">RExec.</tt><tt class="descname">nok_builtin_names</tt><a class="headerlink" href="#rexec.RExec.nok_builtin_names" title="Permalink to this definition">¶</a></dt> <dd>Contains the names of built-in functions which will <em>not</em> be available to programs running in the restricted environment. The value for <a title="rexec.RExec" class="reference internal" href="#rexec.RExec"><tt class="xref docutils literal"><span class="pre">RExec</span></tt></a> is <tt class="docutils literal"><span class="pre">('open',</span> <span class="pre">'reload',</span> <span class="pre">'__import__')</span></tt>. (This gives the exceptions, because by far the majority of built-in functions are harmless. A subclass that wants to override this variable should probably start with the value from the base class and concatenate additional forbidden functions — when new dangerous built-in functions are added to Python, they will also be added to this module.)</dd></dl> <dl class="attribute"> <dt id="rexec.RExec.ok_builtin_modules"> <tt class="descclassname">RExec.</tt><tt class="descname">ok_builtin_modules</tt><a class="headerlink" href="#rexec.RExec.ok_builtin_modules" title="Permalink to this definition">¶</a></dt> <dd>Contains the names of built-in modules which can be safely imported. The value for <a title="rexec.RExec" class="reference internal" href="#rexec.RExec"><tt class="xref docutils literal"><span class="pre">RExec</span></tt></a> is <tt class="docutils literal"><span class="pre">('audioop',</span> <span class="pre">'array',</span> <span class="pre">'binascii',</span> <span class="pre">'cmath',</span> <span class="pre">'errno',</span> <span class="pre">'imageop',</span> <span class="pre">'marshal',</span> <span class="pre">'math',</span> <span class="pre">'md5',</span> <span class="pre">'operator',</span> <span class="pre">'parser',</span> <span class="pre">'regex',</span> <span class="pre">'select',</span> <span class="pre">'sha',</span> <span class="pre">'_sre',</span> <span class="pre">'strop',</span> <span class="pre">'struct',</span> <span class="pre">'time')</span></tt>. A similar remark about overriding this variable applies — use the value from the base class as a starting point.</dd></dl> <dl class="attribute"> <dt id="rexec.RExec.ok_path"> <tt class="descclassname">RExec.</tt><tt class="descname">ok_path</tt><a class="headerlink" href="#rexec.RExec.ok_path" title="Permalink to this definition">¶</a></dt> <dd>Contains the directories which will be searched when an <a class="reference external" href="../reference/simple_stmts.html#import"><tt class="xref docutils literal"><span class="pre">import</span></tt></a> is performed in the restricted environment. The value for <a title="rexec.RExec" class="reference internal" href="#rexec.RExec"><tt class="xref docutils literal"><span class="pre">RExec</span></tt></a> is the same as <tt class="docutils literal"><span class="pre">sys.path</span></tt> (at the time the module is loaded) for unrestricted code.</dd></dl> <dl class="attribute"> <dt id="rexec.RExec.ok_posix_names"> <tt class="descclassname">RExec.</tt><tt class="descname">ok_posix_names</tt><a class="headerlink" href="#rexec.RExec.ok_posix_names" title="Permalink to this definition">¶</a></dt> <dd>Contains the names of the functions in the <a title="Miscellaneous operating system interfaces." class="reference external" href="os.html#module-os"><tt class="xref docutils literal"><span class="pre">os</span></tt></a> module which will be available to programs running in the restricted environment. The value for <a title="rexec.RExec" class="reference internal" href="#rexec.RExec"><tt class="xref docutils literal"><span class="pre">RExec</span></tt></a> is <tt class="docutils literal"><span class="pre">('error',</span> <span class="pre">'fstat',</span> <span class="pre">'listdir',</span> <span class="pre">'lstat',</span> <span class="pre">'readlink',</span> <span class="pre">'stat',</span> <span class="pre">'times',</span> <span class="pre">'uname',</span> <span class="pre">'getpid',</span> <span class="pre">'getppid',</span> <span class="pre">'getcwd',</span> <span class="pre">'getuid',</span> <span class="pre">'getgid',</span> <span class="pre">'geteuid',</span> <span class="pre">'getegid')</span></tt>.</dd></dl> <dl class="attribute"> <dt id="rexec.RExec.ok_sys_names"> <tt class="descclassname">RExec.</tt><tt class="descname">ok_sys_names</tt><a class="headerlink" href="#rexec.RExec.ok_sys_names" title="Permalink to this definition">¶</a></dt> <dd>Contains the names of the functions and variables in the <a title="Access system-specific parameters and functions." class="reference external" href="sys.html#module-sys"><tt class="xref docutils literal"><span class="pre">sys</span></tt></a> module which will be available to programs running in the restricted environment. The value for <a title="rexec.RExec" class="reference internal" href="#rexec.RExec"><tt class="xref docutils literal"><span class="pre">RExec</span></tt></a> is <tt class="docutils literal"><span class="pre">('ps1',</span> <span class="pre">'ps2',</span> <span class="pre">'copyright',</span> <span class="pre">'version',</span> <span class="pre">'platform',</span> <span class="pre">'exit',</span> <span class="pre">'maxint')</span></tt>.</dd></dl> <dl class="attribute"> <dt id="rexec.RExec.ok_file_types"> <tt class="descclassname">RExec.</tt><tt class="descname">ok_file_types</tt><a class="headerlink" href="#rexec.RExec.ok_file_types" title="Permalink to this definition">¶</a></dt> <dd>Contains the file types from which modules are allowed to be loaded. Each file type is an integer constant defined in the <a title="Access the implementation of the import statement." class="reference external" href="imp.html#module-imp"><tt class="xref docutils literal"><span class="pre">imp</span></tt></a> module. The meaningful values are <tt class="xref docutils literal"><span class="pre">PY_SOURCE</span></tt>, <tt class="xref docutils literal"><span class="pre">PY_COMPILED</span></tt>, and <tt class="xref docutils literal"><span class="pre">C_EXTENSION</span></tt>. The value for <a title="rexec.RExec" class="reference internal" href="#rexec.RExec"><tt class="xref docutils literal"><span class="pre">RExec</span></tt></a> is <tt class="docutils literal"><span class="pre">(C_EXTENSION,</span> <span class="pre">PY_SOURCE)</span></tt>. Adding <tt class="xref docutils literal"><span class="pre">PY_COMPILED</span></tt> in subclasses is not recommended; an attacker could exit the restricted execution mode by putting a forged byte-compiled file (<tt class="docutils literal"><span class="pre">.pyc</span></tt>) anywhere in your file system, for example by writing it to <tt class="docutils literal"><span class="pre">/tmp</span></tt> or uploading it to the <tt class="docutils literal"><span class="pre">/incoming</span></tt> directory of your public FTP server.</dd></dl> </div> <div class="section" id="an-example"> <h2>29.1.3. An example<a class="headerlink" href="#an-example" title="Permalink to this headline">¶</a></h2> <p>Let us say that we want a slightly more relaxed policy than the standard <a title="rexec.RExec" class="reference internal" href="#rexec.RExec"><tt class="xref docutils literal"><span class="pre">RExec</span></tt></a> class. For example, if we’re willing to allow files in <tt class="docutils literal"><span class="pre">/tmp</span></tt> to be written, we can subclass the <a title="rexec.RExec" class="reference internal" href="#rexec.RExec"><tt class="xref docutils literal"><span class="pre">RExec</span></tt></a> class:</p> <div class="highlight-python"><div class="highlight"><pre><span class="k">class</span> <span class="nc">TmpWriterRExec</span><span class="p">(</span><span class="n">rexec</span><span class="o">.</span><span class="n">RExec</span><span class="p">):</span> <span class="k">def</span> <span class="nf">r_open</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="nb">file</span><span class="p">,</span> <span class="n">mode</span><span class="o">=</span><span class="s">'r'</span><span class="p">,</span> <span class="n">buf</span><span class="o">=-</span><span class="mi">1</span><span class="p">):</span> <span class="k">if</span> <span class="n">mode</span> <span class="ow">in</span> <span class="p">(</span><span class="s">'r'</span><span class="p">,</span> <span class="s">'rb'</span><span class="p">):</span> <span class="k">pass</span> <span class="k">elif</span> <span class="n">mode</span> <span class="ow">in</span> <span class="p">(</span><span class="s">'w'</span><span class="p">,</span> <span class="s">'wb'</span><span class="p">,</span> <span class="s">'a'</span><span class="p">,</span> <span class="s">'ab'</span><span class="p">):</span> <span class="c"># check filename : must begin with /tmp/</span> <span class="k">if</span> <span class="nb">file</span><span class="p">[:</span><span class="mi">5</span><span class="p">]</span><span class="o">!=</span><span class="s">'/tmp/'</span><span class="p">:</span> <span class="k">raise</span> <span class="ne">IOError</span><span class="p">(</span><span class="s">"can't write outside /tmp"</span><span class="p">)</span> <span class="k">elif</span> <span class="p">(</span><span class="n">string</span><span class="o">.</span><span class="n">find</span><span class="p">(</span><span class="nb">file</span><span class="p">,</span> <span class="s">'/../'</span><span class="p">)</span> <span class="o">>=</span> <span class="mi">0</span> <span class="ow">or</span> <span class="nb">file</span><span class="p">[:</span><span class="mi">3</span><span class="p">]</span> <span class="o">==</span> <span class="s">'../'</span> <span class="ow">or</span> <span class="nb">file</span><span class="p">[</span><span class="o">-</span><span class="mi">3</span><span class="p">:]</span> <span class="o">==</span> <span class="s">'/..'</span><span class="p">):</span> <span class="k">raise</span> <span class="ne">IOError</span><span class="p">(</span><span class="s">"'..' in filename forbidden"</span><span class="p">)</span> <span class="k">else</span><span class="p">:</span> <span class="k">raise</span> <span class="ne">IOError</span><span class="p">(</span><span class="s">"Illegal open() mode"</span><span class="p">)</span> <span class="k">return</span> <span class="nb">open</span><span class="p">(</span><span class="nb">file</span><span class="p">,</span> <span class="n">mode</span><span class="p">,</span> <span class="n">buf</span><span class="p">)</span> </pre></div> </div> <p>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 <tt class="docutils literal"><span class="pre">/tmp/foo/../bar</span></tt>. To fix this, the <tt class="xref docutils literal"><span class="pre">r_open()</span></tt> method would have to simplify the filename to <tt class="docutils literal"><span class="pre">/tmp/bar</span></tt>, 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.</p> </div> </div> </div> </div> </div> <div class="sphinxsidebar"> <div class="sphinxsidebarwrapper"> <h3><a href="../contents.html">Table Of Contents</a></h3> <ul> <li><a class="reference external" href="#">29.1. <tt class="docutils literal"><span class="pre">rexec</span></tt> — Restricted execution framework</a><ul> <li><a class="reference external" href="#rexec-objects">29.1.1. RExec Objects</a></li> <li><a class="reference external" href="#defining-restricted-environments">29.1.2. Defining restricted environments</a></li> <li><a class="reference external" href="#an-example">29.1.3. An example</a></li> </ul> </li> </ul> <h4>Previous topic</h4> <p class="topless"><a href="restricted.html" title="previous chapter">29. Restricted Execution</a></p> <h4>Next topic</h4> <p class="topless"><a href="bastion.html" title="next chapter">29.2. <tt class="docutils literal docutils literal docutils literal"><span class="pre">Bastion</span></tt> — Restricting access to objects</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/rexec.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="bastion.html" title="29.2. 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