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<div class="section" id="module-rexec">
<span id="rexec-restricted-execution-framework"></span><h1>29.1. <a class="reference internal" href="#module-rexec" title="rexec: Basic restricted execution framework. (deprecated)"><tt class="xref py py-mod docutils literal"><span class="pre">rexec</span></tt></a> — 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 <a class="reference internal" href="#module-rexec" title="rexec: Basic restricted execution framework. (deprecated)"><tt class="xref py py-mod docutils literal"><span class="pre">rexec</span></tt></a> module has been removed in Python 3.</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 class="reference internal" href="#rexec.RExec" title="rexec.RExec"><tt class="xref py py-class docutils literal"><span class="pre">RExec</span></tt></a> class, which supports <tt class="xref py py-meth docutils literal"><span class="pre">r_eval()</span></tt>,
<tt class="xref py py-meth docutils literal"><span class="pre">r_execfile()</span></tt>, <tt class="xref py py-meth docutils literal"><span class="pre">r_exec()</span></tt>, and <tt class="xref py py-meth docutils literal"><span class="pre">r_import()</span></tt> methods, which are
restricted versions of the standard Python functions <a class="reference internal" href="functions.html#eval" title="eval"><tt class="xref py py-meth docutils literal"><span class="pre">eval()</span></tt></a>,
<a class="reference internal" href="functions.html#execfile" title="execfile"><tt class="xref py py-meth docutils literal"><span class="pre">execfile()</span></tt></a> and the <a class="reference internal" href="../reference/simple_stmts.html#exec"><tt class="xref std std-keyword docutils literal"><span class="pre">exec</span></tt></a> and <a class="reference internal" href="../reference/simple_stmts.html#import"><tt class="xref std std-keyword 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 class="reference internal" href="#rexec.RExec" title="rexec.RExec"><tt class="xref py py-class 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 <a class="reference internal" href="#module-rexec" title="rexec: Basic restricted execution framework. (deprecated)"><tt class="xref py py-mod docutils literal"><span class="pre">rexec</span></tt></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)"><tt class="xref py py-mod docutils literal"><span class="pre">rexec</span></tt></a> vulnerabilities would be
welcomed.</p>
</div>
<div class="admonition note">
<p class="first admonition-title">Note</p>
<p class="last">The <a class="reference internal" href="#rexec.RExec" title="rexec.RExec"><tt class="xref py py-class 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 class="reference internal" href="#rexec.RExec" title="rexec.RExec"><tt class="xref py py-class docutils literal"><span class="pre">RExec</span></tt></a> class.</p>
<p><em>hooks</em> is an instance of the <tt class="xref py py-class docutils literal"><span class="pre">RHooks</span></tt> class or a subclass of it. If it
is omitted or <tt class="docutils literal"><span class="pre">None</span></tt>, the default <tt class="xref py py-class docutils literal"><span class="pre">RHooks</span></tt> class is instantiated.
Whenever the <a class="reference internal" href="#module-rexec" title="rexec: Basic restricted execution framework. (deprecated)"><tt class="xref py py-mod docutils literal"><span class="pre">rexec</span></tt></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 <tt class="xref py py-class docutils literal"><span class="pre">RHooks</span></tt> instance that was passed to or
created by its constructor. (Actually, the <a class="reference internal" href="#rexec.RExec" title="rexec.RExec"><tt class="xref py py-class 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 class="reference internal" href="#rexec.RExec" title="rexec.RExec"><tt class="xref py py-class 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 internal" href="../reference/simple_stmts.html#import"><tt class="xref std std-keyword docutils literal"><span class="pre">import</span></tt></a> within the restricted
environment.)</p>
<p>By providing an alternate <tt class="xref py py-class 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 py py-class 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 class="reference internal" href="sys.html#sys.exit" title="sys.exit"><tt class="xref py py-func 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 internal" href="../reference/compound_stmts.html#try"><tt class="xref std std-keyword docutils literal"><span class="pre">try</span></tt></a>/<a class="reference internal" href="../reference/compound_stmts.html#except"><tt class="xref std std-keyword docutils literal"><span class="pre">except</span></tt></a> statement that catches the
<a class="reference internal" href="exceptions.html#exceptions.SystemExit" title="exceptions.SystemExit"><tt class="xref py py-exc docutils literal"><span class="pre">SystemExit</span></tt></a> exception. Removing the <a class="reference internal" href="sys.html#sys.exit" title="sys.exit"><tt class="xref py py-func 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 class="reference internal" href="exceptions.html#exceptions.SystemExit" title="exceptions.SystemExit"><tt class="xref py py-exc 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 <a class="reference internal" href="#module-rexec" title="rexec: Basic restricted execution framework. (deprecated)"><tt class="xref py py-mod docutils literal"><span class="pre">rexec</span></tt></a>
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 class="reference internal" href="#rexec.RExec" title="rexec.RExec"><tt class="xref py py-class 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><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."><tt class="xref py py-mod docutils literal"><span class="pre">__main__</span></tt></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">
<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><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."><tt class="xref py py-mod docutils literal"><span class="pre">__main__</span></tt></a> module.</p>
</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><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."><tt class="xref py py-mod docutils literal"><span class="pre">__main__</span></tt></a> module.</p>
</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><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">
<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><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">
<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><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"><tt class="xref py py-class 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><p>Import the module <em>modulename</em>, raising an <a class="reference internal" href="exceptions.html#exceptions.ImportError" title="exceptions.ImportError"><tt class="xref py py-exc docutils literal"><span class="pre">ImportError</span></tt></a> exception if the
module is considered unsafe.</p>
</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><p>Method called when <a class="reference internal" href="functions.html#open" title="open"><tt class="xref py py-func docutils literal"><span class="pre">open()</span></tt></a> is called in the restricted environment. The
arguments are identical to those of <a class="reference internal" href="functions.html#open" title="open"><tt class="xref py py-func 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 class="reference internal" href="#rexec.RExec" title="rexec.RExec"><tt class="xref py py-class 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 class="reference internal" href="#rexec.RExec.r_open" title="rexec.RExec.r_open"><tt class="xref py py-meth docutils literal"><span class="pre">r_open()</span></tt></a>.</p>
</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><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">
<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><p>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).</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">
<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><p>Import the module <em>modulename</em>, raising an <a class="reference internal" href="exceptions.html#exceptions.ImportError" title="exceptions.ImportError"><tt class="xref py py-exc docutils literal"><span class="pre">ImportError</span></tt></a> exception if the
module is considered unsafe.</p>
</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><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">
<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><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>29.1.2. 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"><tt class="xref py py-class docutils literal"><span class="pre">RExec</span></tt></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__"><tt class="xref py py-meth 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 class="reference internal" href="#rexec.RExec" title="rexec.RExec"><tt class="xref py py-class 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><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"><tt class="xref py py-class 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.)</p>
</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><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"><tt class="xref py py-class 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.</p>
</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><p>Contains the directories which will be searched when an <a class="reference internal" href="../reference/simple_stmts.html#import"><tt class="xref std std-keyword docutils literal"><span class="pre">import</span></tt></a> is
performed in the restricted environment. The value for <a class="reference internal" href="#rexec.RExec" title="rexec.RExec"><tt class="xref py py-class 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.</p>
</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><p>Contains the names of the functions in the <a class="reference internal" href="os.html#module-os" title="os: Miscellaneous operating system interfaces."><tt class="xref py py-mod 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 class="reference internal" href="#rexec.RExec" title="rexec.RExec"><tt class="xref py py-class 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>.</p>
</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><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."><tt class="xref py py-mod 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 class="reference internal" href="#rexec.RExec" title="rexec.RExec"><tt class="xref py py-class 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>.</p>
</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><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."><tt class="xref py py-mod docutils literal"><span class="pre">imp</span></tt></a> module. The meaningful
values are <tt class="xref py py-const docutils literal"><span class="pre">PY_SOURCE</span></tt>, <tt class="xref py py-const docutils literal"><span class="pre">PY_COMPILED</span></tt>, and <tt class="xref py py-const docutils literal"><span class="pre">C_EXTENSION</span></tt>.
The value for <a class="reference internal" href="#rexec.RExec" title="rexec.RExec"><tt class="xref py py-class 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 py py-const 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="file docutils literal"><span class="pre">.pyc</span></tt>) anywhere in your file system, for example by writing it to
<tt class="file docutils literal"><span class="pre">/tmp</span></tt> or uploading it to the <tt class="file docutils literal"><span class="pre">/incoming</span></tt> directory of your public
FTP server.</p>
</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 class="reference internal" href="#rexec.RExec" title="rexec.RExec"><tt class="xref py py-class docutils literal"><span class="pre">RExec</span></tt></a> class. For example, if we’re willing to allow files in
<tt class="file docutils literal"><span class="pre">/tmp</span></tt> to be written, we can subclass the <a class="reference internal" href="#rexec.RExec" title="rexec.RExec"><tt class="xref py py-class 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="file docutils literal"><span class="pre">/tmp/foo/../bar</span></tt>. To fix this, the <tt class="xref py py-meth docutils literal"><span class="pre">r_open()</span></tt> method would
have to simplify the filename to <tt class="file 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>
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<h3><a href="../contents.html">Table Of Contents</a></h3>
<ul>
<li><a class="reference internal" href="#">29.1. <tt class="docutils literal"><span class="pre">rexec</span></tt> — Restricted execution framework</a><ul>
<li><a class="reference internal" href="#rexec-objects">29.1.1. RExec Objects</a></li>
<li><a class="reference internal" href="#defining-restricted-environments">29.1.2. Defining restricted environments</a></li>
<li><a class="reference internal" href="#an-example">29.1.3. An example</a></li>
</ul>
</li>
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<h4>Previous topic</h4>
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title="previous chapter">29. Restricted Execution</a></p>
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<p class="topless"><a href="bastion.html"
title="next chapter">29.2. <tt class="docutils literal"><span class="pre">Bastion</span></tt> — Restricting access to objects</a></p>
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