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<div class="section" id="module-rexec">
<span id="rexec-restricted-execution-framework"></span><h1>30.1. <a class="reference internal" href="#module-rexec" title="rexec: Basic restricted execution framework. (deprecated)"><code class="xref py py-mod docutils literal notranslate"><span class="pre">rexec</span></code></a> — Restricted execution framework<a class="headerlink" href="#module-rexec" title="Permalink to this headline">¶</a></h1>
<div class="deprecated">
<p><span class="versionmodified deprecated">Deprecated since version 2.6: </span>The <a class="reference internal" href="#module-rexec" title="rexec: Basic restricted execution framework. (deprecated)"><code class="xref py py-mod docutils literal notranslate"><span class="pre">rexec</span></code></a> module has been removed in Python 3.</p>
</div>
<div class="versionchanged">
<p><span class="versionmodified changed">Changed in version 2.3: </span>Disabled module.</p>
</div>
<div class="admonition warning">
<p class="admonition-title">Warning</p>
<p>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 class="reference internal" href="#rexec.RExec" title="rexec.RExec"><code class="xref py py-class docutils literal notranslate"><span class="pre">RExec</span></code></a> class, which supports <code class="xref py py-meth docutils literal notranslate"><span class="pre">r_eval()</span></code>,
<code class="xref py py-meth docutils literal notranslate"><span class="pre">r_execfile()</span></code>, <code class="xref py py-meth docutils literal notranslate"><span class="pre">r_exec()</span></code>, and <code class="xref py py-meth docutils literal notranslate"><span class="pre">r_import()</span></code> methods, which are
restricted versions of the standard Python functions <a class="reference internal" href="functions.html#eval" title="eval"><code class="xref py py-meth docutils literal notranslate"><span class="pre">eval()</span></code></a>,
<a class="reference internal" href="functions.html#execfile" title="execfile"><code class="xref py py-meth docutils literal notranslate"><span class="pre">execfile()</span></code></a> and the <a class="reference internal" href="../reference/simple_stmts.html#exec"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">exec</span></code></a> and <a class="reference internal" href="../reference/simple_stmts.html#import"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">import</span></code></a> statements. Code
executed in this restricted environment will only have access to modules and
functions that are deemed safe; you can subclass <a class="reference internal" href="#rexec.RExec" title="rexec.RExec"><code class="xref py py-class docutils literal notranslate"><span class="pre">RExec</span></code></a> to add or remove
capabilities as desired.</p>
<div class="admonition warning">
<p class="admonition-title">Warning</p>
<p>While the <a class="reference internal" href="#module-rexec" title="rexec: Basic restricted execution framework. (deprecated)"><code class="xref py py-mod docutils literal notranslate"><span class="pre">rexec</span></code></a> 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 <a class="reference internal" href="#module-rexec" title="rexec: Basic restricted execution framework. (deprecated)"><code class="xref py py-mod docutils literal notranslate"><span class="pre">rexec</span></code></a> vulnerabilities would be
welcomed.</p>
</div>
<div class="admonition note">
<p class="admonition-title">Note</p>
<p>The <a class="reference internal" href="#rexec.RExec" title="rexec.RExec"><code class="xref py py-class docutils literal notranslate"><span class="pre">RExec</span></code></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><code class="descclassname">rexec.</code><code class="descname">RExec</code><span class="sig-paren">(</span><span class="optional">[</span><em>hooks</em><span class="optional">[</span>, <em>verbose</em><span class="optional">]</span><span class="optional">]</span><span class="sig-paren">)</span><a class="headerlink" href="#rexec.RExec" title="Permalink to this definition">¶</a></dt>
<dd><p>Returns an instance of the <a class="reference internal" href="#rexec.RExec" title="rexec.RExec"><code class="xref py py-class docutils literal notranslate"><span class="pre">RExec</span></code></a> class.</p>
<p><em>hooks</em> is an instance of the <code class="xref py py-class docutils literal notranslate"><span class="pre">RHooks</span></code> class or a subclass of it. If it
is omitted or <code class="docutils literal notranslate"><span class="pre">None</span></code>, the default <code class="xref py py-class docutils literal notranslate"><span class="pre">RHooks</span></code> class is instantiated.
Whenever the <a class="reference internal" href="#module-rexec" title="rexec: Basic restricted execution framework. (deprecated)"><code class="xref py py-mod docutils literal notranslate"><span class="pre">rexec</span></code></a> 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 <code class="xref py py-class docutils literal notranslate"><span class="pre">RHooks</span></code> instance that was passed to or
created by its constructor. (Actually, the <a class="reference internal" href="#rexec.RExec" title="rexec.RExec"><code class="xref py py-class docutils literal notranslate"><span class="pre">RExec</span></code></a> object doesn’t make
these calls — they are made by a module loader object that’s part of the
<a class="reference internal" href="#rexec.RExec" title="rexec.RExec"><code class="xref py py-class docutils literal notranslate"><span class="pre">RExec</span></code></a> object. This allows another level of flexibility, which can be
useful when changing the mechanics of <a class="reference internal" href="../reference/simple_stmts.html#import"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">import</span></code></a> within the restricted
environment.)</p>
<p>By providing an alternate <code class="xref py py-class docutils literal notranslate"><span class="pre">RHooks</span></code> 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 <code class="xref py py-class docutils literal notranslate"><span class="pre">RHooks</span></code> 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 class="reference internal" href="sys.html#sys.exit" title="sys.exit"><code class="xref py py-func docutils literal notranslate"><span class="pre">sys.exit()</span></code></a> function. To disallow restricted code from
exiting the interpreter, always protect calls that cause restricted code to run
with a <a class="reference internal" href="../reference/compound_stmts.html#try"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">try</span></code></a>/<a class="reference internal" href="../reference/compound_stmts.html#except"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">except</span></code></a> statement that catches the
<a class="reference internal" href="exceptions.html#exceptions.SystemExit" title="exceptions.SystemExit"><code class="xref py py-exc docutils literal notranslate"><span class="pre">SystemExit</span></code></a> exception. Removing the <a class="reference internal" href="sys.html#sys.exit" title="sys.exit"><code class="xref py py-func docutils literal notranslate"><span class="pre">sys.exit()</span></code></a> function from the
restricted environment is not sufficient — the restricted code could still use
<code class="docutils literal notranslate"><span class="pre">raise</span> <span class="pre">SystemExit</span></code>. Removing <a class="reference internal" href="exceptions.html#exceptions.SystemExit" title="exceptions.SystemExit"><code class="xref py py-exc docutils literal notranslate"><span class="pre">SystemExit</span></code></a> is not a reasonable option;
some library code makes use of this and would break were it not available.</p>
<div class="admonition seealso">
<p class="admonition-title">See also</p>
<dl class="simple">
<dt><a class="reference external" href="http://grail.sourceforge.net/">Grail Home Page</a></dt><dd><p>Grail is a Web browser written entirely in Python. It uses the <a class="reference internal" href="#module-rexec" title="rexec: Basic restricted execution framework. (deprecated)"><code class="xref py py-mod docutils literal notranslate"><span class="pre">rexec</span></code></a>
module as a foundation for supporting Python applets, and can be used as an
example usage of this module.</p>
</dd>
</dl>
</div>
<div class="section" id="rexec-objects">
<span id="id1"></span><h2>30.1.1. RExec Objects<a class="headerlink" href="#rexec-objects" title="Permalink to this headline">¶</a></h2>
<p><a class="reference internal" href="#rexec.RExec" title="rexec.RExec"><code class="xref py py-class docutils literal notranslate"><span class="pre">RExec</span></code></a> instances support the following methods:</p>
<dl class="method">
<dt id="rexec.RExec.r_eval">
<code class="descclassname">RExec.</code><code class="descname">r_eval</code><span class="sig-paren">(</span><em>code</em><span class="sig-paren">)</span><a class="headerlink" href="#rexec.RExec.r_eval" title="Permalink to this definition">¶</a></dt>
<dd><p><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 class="reference internal" href="__main__.html#module-__main__" title="__main__: The environment where the top-level script is run."><code class="xref py py-mod docutils literal notranslate"><span class="pre">__main__</span></code></a> module. The value of the expression or code object will be
returned.</p>
</dd></dl>
<dl class="method">
<dt id="rexec.RExec.r_exec">
<code class="descclassname">RExec.</code><code class="descname">r_exec</code><span class="sig-paren">(</span><em>code</em><span class="sig-paren">)</span><a class="headerlink" href="#rexec.RExec.r_exec" title="Permalink to this definition">¶</a></dt>
<dd><p><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 class="reference internal" href="__main__.html#module-__main__" title="__main__: The environment where the top-level script is run."><code class="xref py py-mod docutils literal notranslate"><span class="pre">__main__</span></code></a> module.</p>
</dd></dl>
<dl class="method">
<dt id="rexec.RExec.r_execfile">
<code class="descclassname">RExec.</code><code class="descname">r_execfile</code><span class="sig-paren">(</span><em>filename</em><span class="sig-paren">)</span><a class="headerlink" href="#rexec.RExec.r_execfile" title="Permalink to this definition">¶</a></dt>
<dd><p>Execute the Python code contained in the file <em>filename</em> in the restricted
environment’s <a class="reference internal" href="__main__.html#module-__main__" title="__main__: The environment where the top-level script is run."><code class="xref py py-mod docutils literal notranslate"><span class="pre">__main__</span></code></a> module.</p>
</dd></dl>
<p>Methods whose names begin with <code class="docutils literal notranslate"><span class="pre">s_</span></code> are similar to the functions beginning
with <code class="docutils literal notranslate"><span class="pre">r_</span></code>, but the code will be granted access to restricted versions of the
standard I/O streams <code class="docutils literal notranslate"><span class="pre">sys.stdin</span></code>, <code class="docutils literal notranslate"><span class="pre">sys.stderr</span></code>, and <code class="docutils literal notranslate"><span class="pre">sys.stdout</span></code>.</p>
<dl class="method">
<dt id="rexec.RExec.s_eval">
<code class="descclassname">RExec.</code><code class="descname">s_eval</code><span class="sig-paren">(</span><em>code</em><span class="sig-paren">)</span><a class="headerlink" href="#rexec.RExec.s_eval" title="Permalink to this definition">¶</a></dt>
<dd><p><em>code</em> must be a string containing a Python expression, which will be evaluated
in the restricted environment.</p>
</dd></dl>
<dl class="method">
<dt id="rexec.RExec.s_exec">
<code class="descclassname">RExec.</code><code class="descname">s_exec</code><span class="sig-paren">(</span><em>code</em><span class="sig-paren">)</span><a class="headerlink" href="#rexec.RExec.s_exec" title="Permalink to this definition">¶</a></dt>
<dd><p><em>code</em> must be a string containing one or more lines of Python code, which will
be executed in the restricted environment.</p>
</dd></dl>
<dl class="method">
<dt id="rexec.RExec.s_execfile">
<code class="descclassname">RExec.</code><code class="descname">s_execfile</code><span class="sig-paren">(</span><em>code</em><span class="sig-paren">)</span><a class="headerlink" href="#rexec.RExec.s_execfile" title="Permalink to this definition">¶</a></dt>
<dd><p>Execute the Python code contained in the file <em>filename</em> in the restricted
environment.</p>
</dd></dl>
<p><a class="reference internal" href="#rexec.RExec" title="rexec.RExec"><code class="xref py py-class docutils literal notranslate"><span class="pre">RExec</span></code></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">
<code class="descclassname">RExec.</code><code class="descname">r_import</code><span class="sig-paren">(</span><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><span class="sig-paren">)</span><a class="headerlink" href="#rexec.RExec.r_import" title="Permalink to this definition">¶</a></dt>
<dd><p>Import the module <em>modulename</em>, raising an <a class="reference internal" href="exceptions.html#exceptions.ImportError" title="exceptions.ImportError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">ImportError</span></code></a> exception if the
module is considered unsafe.</p>
</dd></dl>
<dl class="method">
<dt id="rexec.RExec.r_open">
<code class="descclassname">RExec.</code><code class="descname">r_open</code><span class="sig-paren">(</span><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><span class="sig-paren">)</span><a class="headerlink" href="#rexec.RExec.r_open" title="Permalink to this definition">¶</a></dt>
<dd><p>Method called when <a class="reference internal" href="functions.html#open" title="open"><code class="xref py py-func docutils literal notranslate"><span class="pre">open()</span></code></a> is called in the restricted environment. The
arguments are identical to those of <a class="reference internal" href="functions.html#open" title="open"><code class="xref py py-func docutils literal notranslate"><span class="pre">open()</span></code></a>, and a file object (or a class
instance compatible with file objects) should be returned. <a class="reference internal" href="#rexec.RExec" title="rexec.RExec"><code class="xref py py-class docutils literal notranslate"><span class="pre">RExec</span></code></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 class="reference internal" href="#rexec.RExec.r_open" title="rexec.RExec.r_open"><code class="xref py py-meth docutils literal notranslate"><span class="pre">r_open()</span></code></a>.</p>
</dd></dl>
<dl class="method">
<dt id="rexec.RExec.r_reload">
<code class="descclassname">RExec.</code><code class="descname">r_reload</code><span class="sig-paren">(</span><em>module</em><span class="sig-paren">)</span><a class="headerlink" href="#rexec.RExec.r_reload" title="Permalink to this definition">¶</a></dt>
<dd><p>Reload the module object <em>module</em>, re-parsing and re-initializing it.</p>
</dd></dl>
<dl class="method">
<dt id="rexec.RExec.r_unload">
<code class="descclassname">RExec.</code><code class="descname">r_unload</code><span class="sig-paren">(</span><em>module</em><span class="sig-paren">)</span><a class="headerlink" href="#rexec.RExec.r_unload" title="Permalink to this definition">¶</a></dt>
<dd><p>Unload the module object <em>module</em> (remove it from the restricted environment’s
<code class="docutils literal notranslate"><span class="pre">sys.modules</span></code> dictionary).</p>
</dd></dl>
<p>And their equivalents with access to restricted standard I/O streams:</p>
<dl class="method">
<dt id="rexec.RExec.s_import">
<code class="descclassname">RExec.</code><code class="descname">s_import</code><span class="sig-paren">(</span><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><span class="sig-paren">)</span><a class="headerlink" href="#rexec.RExec.s_import" title="Permalink to this definition">¶</a></dt>
<dd><p>Import the module <em>modulename</em>, raising an <a class="reference internal" href="exceptions.html#exceptions.ImportError" title="exceptions.ImportError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">ImportError</span></code></a> exception if the
module is considered unsafe.</p>
</dd></dl>
<dl class="method">
<dt id="rexec.RExec.s_reload">
<code class="descclassname">RExec.</code><code class="descname">s_reload</code><span class="sig-paren">(</span><em>module</em><span class="sig-paren">)</span><a class="headerlink" href="#rexec.RExec.s_reload" title="Permalink to this definition">¶</a></dt>
<dd><p>Reload the module object <em>module</em>, re-parsing and re-initializing it.</p>
</dd></dl>
<dl class="method">
<dt id="rexec.RExec.s_unload">
<code class="descclassname">RExec.</code><code class="descname">s_unload</code><span class="sig-paren">(</span><em>module</em><span class="sig-paren">)</span><a class="headerlink" href="#rexec.RExec.s_unload" title="Permalink to this definition">¶</a></dt>
<dd><p>Unload the module object <em>module</em>.</p>
</dd></dl>
</div>
<div class="section" id="defining-restricted-environments">
<span id="rexec-extension"></span><h2>30.1.2. Defining restricted environments<a class="headerlink" href="#defining-restricted-environments" title="Permalink to this headline">¶</a></h2>
<p>The <a class="reference internal" href="#rexec.RExec" title="rexec.RExec"><code class="xref py py-class docutils literal notranslate"><span class="pre">RExec</span></code></a> class has the following class attributes, which are used by
the <a class="reference internal" href="../reference/datamodel.html#object.__init__" title="object.__init__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__init__()</span></code></a> method. Changing them on an existing instance won’t have
any effect; instead, create a subclass of <a class="reference internal" href="#rexec.RExec" title="rexec.RExec"><code class="xref py py-class docutils literal notranslate"><span class="pre">RExec</span></code></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">
<code class="descclassname">RExec.</code><code class="descname">nok_builtin_names</code><a class="headerlink" href="#rexec.RExec.nok_builtin_names" title="Permalink to this definition">¶</a></dt>
<dd><p>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 class="reference internal" href="#rexec.RExec" title="rexec.RExec"><code class="xref py py-class docutils literal notranslate"><span class="pre">RExec</span></code></a> is
<code class="docutils literal notranslate"><span class="pre">('open',</span> <span class="pre">'reload',</span> <span class="pre">'__import__')</span></code>. (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.)</p>
</dd></dl>
<dl class="attribute">
<dt id="rexec.RExec.ok_builtin_modules">
<code class="descclassname">RExec.</code><code class="descname">ok_builtin_modules</code><a class="headerlink" href="#rexec.RExec.ok_builtin_modules" title="Permalink to this definition">¶</a></dt>
<dd><p>Contains the names of built-in modules which can be safely imported. The value
for <a class="reference internal" href="#rexec.RExec" title="rexec.RExec"><code class="xref py py-class docutils literal notranslate"><span class="pre">RExec</span></code></a> is <code class="docutils literal notranslate"><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></code>. A similar remark about overriding
this variable applies — use the value from the base class as a starting point.</p>
</dd></dl>
<dl class="attribute">
<dt id="rexec.RExec.ok_path">
<code class="descclassname">RExec.</code><code class="descname">ok_path</code><a class="headerlink" href="#rexec.RExec.ok_path" title="Permalink to this definition">¶</a></dt>
<dd><p>Contains the directories which will be searched when an <a class="reference internal" href="../reference/simple_stmts.html#import"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">import</span></code></a> is
performed in the restricted environment. The value for <a class="reference internal" href="#rexec.RExec" title="rexec.RExec"><code class="xref py py-class docutils literal notranslate"><span class="pre">RExec</span></code></a> is the
same as <code class="docutils literal notranslate"><span class="pre">sys.path</span></code> (at the time the module is loaded) for unrestricted code.</p>
</dd></dl>
<dl class="attribute">
<dt id="rexec.RExec.ok_posix_names">
<code class="descclassname">RExec.</code><code class="descname">ok_posix_names</code><a class="headerlink" href="#rexec.RExec.ok_posix_names" title="Permalink to this definition">¶</a></dt>
<dd><p>Contains the names of the functions in the <a class="reference internal" href="os.html#module-os" title="os: Miscellaneous operating system interfaces."><code class="xref py py-mod docutils literal notranslate"><span class="pre">os</span></code></a> module which will be
available to programs running in the restricted environment. The value for
<a class="reference internal" href="#rexec.RExec" title="rexec.RExec"><code class="xref py py-class docutils literal notranslate"><span class="pre">RExec</span></code></a> is <code class="docutils literal notranslate"><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></code>.</p>
</dd></dl>
<dl class="attribute">
<dt id="rexec.RExec.ok_sys_names">
<code class="descclassname">RExec.</code><code class="descname">ok_sys_names</code><a class="headerlink" href="#rexec.RExec.ok_sys_names" title="Permalink to this definition">¶</a></dt>
<dd><p>Contains the names of the functions and variables in the <a class="reference internal" href="sys.html#module-sys" title="sys: Access system-specific parameters and functions."><code class="xref py py-mod docutils literal notranslate"><span class="pre">sys</span></code></a> module which
will be available to programs running in the restricted environment. The value
for <a class="reference internal" href="#rexec.RExec" title="rexec.RExec"><code class="xref py py-class docutils literal notranslate"><span class="pre">RExec</span></code></a> is <code class="docutils literal notranslate"><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></code>.</p>
</dd></dl>
<dl class="attribute">
<dt id="rexec.RExec.ok_file_types">
<code class="descclassname">RExec.</code><code class="descname">ok_file_types</code><a class="headerlink" href="#rexec.RExec.ok_file_types" title="Permalink to this definition">¶</a></dt>
<dd><p>Contains the file types from which modules are allowed to be loaded. Each file
type is an integer constant defined in the <a class="reference internal" href="imp.html#module-imp" title="imp: Access the implementation of the import statement."><code class="xref py py-mod docutils literal notranslate"><span class="pre">imp</span></code></a> module. The meaningful
values are <code class="xref py py-const docutils literal notranslate"><span class="pre">PY_SOURCE</span></code>, <code class="xref py py-const docutils literal notranslate"><span class="pre">PY_COMPILED</span></code>, and <code class="xref py py-const docutils literal notranslate"><span class="pre">C_EXTENSION</span></code>.
The value for <a class="reference internal" href="#rexec.RExec" title="rexec.RExec"><code class="xref py py-class docutils literal notranslate"><span class="pre">RExec</span></code></a> is <code class="docutils literal notranslate"><span class="pre">(C_EXTENSION,</span> <span class="pre">PY_SOURCE)</span></code>. Adding
<code class="xref py py-const docutils literal notranslate"><span class="pre">PY_COMPILED</span></code> in subclasses is not recommended; an attacker could exit
the restricted execution mode by putting a forged byte-compiled file
(<code class="file docutils literal notranslate"><span class="pre">.pyc</span></code>) anywhere in your file system, for example by writing it to
<code class="file docutils literal notranslate"><span class="pre">/tmp</span></code> or uploading it to the <code class="file docutils literal notranslate"><span class="pre">/incoming</span></code> directory of your public
FTP server.</p>
</dd></dl>
</div>
<div class="section" id="an-example">
<h2>30.1.3. 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 class="reference internal" href="#rexec.RExec" title="rexec.RExec"><code class="xref py py-class docutils literal notranslate"><span class="pre">RExec</span></code></a> class. For example, if we’re willing to allow files in
<code class="file docutils literal notranslate"><span class="pre">/tmp</span></code> to be written, we can subclass the <a class="reference internal" href="#rexec.RExec" title="rexec.RExec"><code class="xref py py-class docutils literal notranslate"><span class="pre">RExec</span></code></a> class:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><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="n">file</span><span class="p">,</span> <span class="n">mode</span><span class="o">=</span><span class="s1">'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="s1">'r'</span><span class="p">,</span> <span class="s1">'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="s1">'w'</span><span class="p">,</span> <span class="s1">'wb'</span><span class="p">,</span> <span class="s1">'a'</span><span class="p">,</span> <span class="s1">'ab'</span><span class="p">):</span>
<span class="c1"># check filename: must begin with /tmp/</span>
<span class="k">if</span> <span class="n">file</span><span class="p">[:</span><span class="mi">5</span><span class="p">]</span><span class="o">!=</span><span class="s1">'/tmp/'</span><span class="p">:</span>
<span class="k">raise</span> <span class="ne">IOError</span><span class="p">(</span><span class="s2">"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="n">file</span><span class="p">,</span> <span class="s1">'/../'</span><span class="p">)</span> <span class="o">>=</span> <span class="mi">0</span> <span class="ow">or</span>
<span class="n">file</span><span class="p">[:</span><span class="mi">3</span><span class="p">]</span> <span class="o">==</span> <span class="s1">'../'</span> <span class="ow">or</span> <span class="n">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="s1">'/..'</span><span class="p">):</span>
<span class="k">raise</span> <span class="ne">IOError</span><span class="p">(</span><span class="s2">"'..' 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="s2">"Illegal open() mode"</span><span class="p">)</span>
<span class="k">return</span> <span class="nb">open</span><span class="p">(</span><span class="n">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 <code class="file docutils literal notranslate"><span class="pre">/tmp/foo/../bar</span></code>. To fix this, the <code class="xref py py-meth docutils literal notranslate"><span class="pre">r_open()</span></code> method would
have to simplify the filename to <code class="file docutils literal notranslate"><span class="pre">/tmp/bar</span></code>, 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>
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<h3><a href="../contents.html">Table of Contents</a></h3>
<ul>
<li><a class="reference internal" href="#">30.1. <code class="xref py py-mod docutils literal notranslate"><span class="pre">rexec</span></code> — Restricted execution framework</a><ul>
<li><a class="reference internal" href="#rexec-objects">30.1.1. RExec Objects</a></li>
<li><a class="reference internal" href="#defining-restricted-environments">30.1.2. Defining restricted environments</a></li>
<li><a class="reference internal" href="#an-example">30.1.3. An example</a></li>
</ul>
</li>
</ul>
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<p class="topless"><a href="restricted.html"
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title="next chapter">30.2. <code class="xref py py-mod docutils literal notranslate"><span class="pre">Bastion</span></code> — Restricting access to objects</a></p>
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