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 <section id="module-rexec">
<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">rexec</code></a> — Restricted execution framework<a class="headerlink" href="#module-rexec" title="Permalink to this headline">¶</a></h1>
<div class="deprecated">
Deprecated since version 2.6: The <a class="reference internal" href="#module-rexec" title="rexec: Basic restricted execution framework. (deprecated)"><code class="xref py py-mod docutils literal notranslate">rexec</code></a> module has been removed in Python 3.
</div>
<div class="versionchanged">
Changed in version 2.3: Disabled module.
</div>
<div class="admonition warning">
Warning
The documentation has been left in place to help in reading old code that uses
the module.
</div>
This module contains the <a class="reference internal" href="#rexec.RExec" title="rexec.RExec"><code class="xref py py-class docutils literal notranslate">RExec</code></a> class, which supports <code class="xref py py-meth docutils literal notranslate">r_eval()</code>,
<code class="xref py py-meth docutils literal notranslate">r_execfile()</code>, <code class="xref py py-meth docutils literal notranslate">r_exec()</code>, and <code class="xref py py-meth docutils literal notranslate">r_import()</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">eval()</code></a>,
<a class="reference internal" href="functions.html#execfile" title="execfile"><code class="xref py py-meth docutils literal notranslate">execfile()</code></a> and the <a class="reference internal" href="../reference/simple_stmts.html#exec"><code class="xref std std-keyword docutils literal notranslate">exec</code></a> and <a class="reference internal" href="../reference/simple_stmts.html#import"><code class="xref std std-keyword docutils literal notranslate">import</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">RExec</code></a> to add or remove
capabilities as desired.
<div class="admonition warning">
Warning
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">rexec</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">rexec</code></a> vulnerabilities would be
welcomed.
</div>
<div class="admonition note">
Note
The <a class="reference internal" href="#rexec.RExec" title="rexec.RExec"><code class="xref py py-class docutils literal notranslate">RExec</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.
</div>
<dl class="py class">
<dt class="sig sig-object py" id="rexec.RExec">
class rexec.RExec([hooks[, verbose]])<a class="headerlink" href="#rexec.RExec" title="Permalink to this definition">¶</a></dt>
<dd>Returns an instance of the <a class="reference internal" href="#rexec.RExec" title="rexec.RExec"><code class="xref py py-class docutils literal notranslate">RExec</code></a> class.
hooks is an instance of the <code class="xref py py-class docutils literal notranslate">RHooks</code> class or a subclass of it. If it
is omitted or <code class="docutils literal notranslate">None</code>, the default <code class="xref py py-class docutils literal notranslate">RHooks</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">rexec</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">RHooks</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">RExec</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">RExec</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">import</code></a> within the restricted
environment.)
By providing an alternate <code class="xref py py-class docutils literal notranslate">RHooks</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">RHooks</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.
If verbose is true, additional debugging output may be sent to standard
output.
</dd></dl>

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">sys.exit()</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">try</code></a>/<a class="reference internal" href="../reference/compound_stmts.html#except"><code class="xref std std-keyword docutils literal notranslate">except</code></a> statement that catches the
<code class="xref py py-exc docutils literal notranslate">SystemExit</code> exception. Removing the <a class="reference internal" href="sys.html#sys.exit" title="sys.exit"><code class="xref py py-func docutils literal notranslate">sys.exit()</code></a> function from the
restricted environment is not sufficient — the restricted code could still use
<code class="docutils literal notranslate">raise SystemExit</code>. Removing <code class="xref py py-exc docutils literal notranslate">SystemExit</code> is not a reasonable option;
some library code makes use of this and would break were it not available.
<div class="admonition seealso">
See also
<dl class="simple">
<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)"><code class="xref py py-mod docutils literal notranslate">rexec</code></a>
module as a foundation for supporting Python applets, and can be used as an
example usage of this module.
</dd>
</dl>
</div>
<section id="rexec-objects">
<h2>30.1.1. RExec Objects<a class="headerlink" href="#rexec-objects" title="Permalink to this headline">¶</a></h2>
<a class="reference internal" href="#rexec.RExec" title="rexec.RExec"><code class="xref py py-class docutils literal notranslate">RExec</code></a> instances support the following methods:
<dl class="py method">
<dt class="sig sig-object py" id="rexec.RExec.r_eval">
RExec.r_eval(code)<a class="headerlink" href="#rexec.RExec.r_eval" title="Permalink to this definition">¶</a></dt>
<dd>code 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">__main__</code></a> module. The value of the expression or code object will be
returned.
</dd></dl>

<dl class="py method">
<dt class="sig sig-object py" id="rexec.RExec.r_exec">
RExec.r_exec(code)<a class="headerlink" href="#rexec.RExec.r_exec" title="Permalink to this definition">¶</a></dt>
<dd>code 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">__main__</code></a> module.
</dd></dl>

<dl class="py method">
<dt class="sig sig-object py" id="rexec.RExec.r_execfile">
RExec.r_execfile(filename)<a class="headerlink" href="#rexec.RExec.r_execfile" title="Permalink to this definition">¶</a></dt>
<dd>Execute the Python code contained in the file filename 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">__main__</code></a> module.
</dd></dl>

Methods whose names begin with <code class="docutils literal notranslate">s_</code> are similar to the functions beginning
with <code class="docutils literal notranslate">r_</code>, but the code will be granted access to restricted versions of the
standard I/O streams <code class="docutils literal notranslate">sys.stdin</code>, <code class="docutils literal notranslate">sys.stderr</code>, and <code class="docutils literal notranslate">sys.stdout</code>.
<dl class="py method">
<dt class="sig sig-object py" id="rexec.RExec.s_eval">
RExec.s_eval(code)<a class="headerlink" href="#rexec.RExec.s_eval" title="Permalink to this definition">¶</a></dt>
<dd>code must be a string containing a Python expression, which will be evaluated
in the restricted environment.
</dd></dl>

<dl class="py method">
<dt class="sig sig-object py" id="rexec.RExec.s_exec">
RExec.s_exec(code)<a class="headerlink" href="#rexec.RExec.s_exec" title="Permalink to this definition">¶</a></dt>
<dd>code must be a string containing one or more lines of Python code, which will
be executed in the restricted environment.
</dd></dl>

<dl class="py method">
<dt class="sig sig-object py" id="rexec.RExec.s_execfile">
RExec.s_execfile(code)<a class="headerlink" href="#rexec.RExec.s_execfile" title="Permalink to this definition">¶</a></dt>
<dd>Execute the Python code contained in the file filename in the restricted
environment.
</dd></dl>

<a class="reference internal" href="#rexec.RExec" title="rexec.RExec"><code class="xref py py-class docutils literal notranslate">RExec</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.
<dl class="py method">
<dt class="sig sig-object py" id="rexec.RExec.r_import">
RExec.r_import(modulename[, globals[, locals[, fromlist]]])<a class="headerlink" href="#rexec.RExec.r_import" title="Permalink to this definition">¶</a></dt>
<dd>Import the module modulename, raising an <code class="xref py py-exc docutils literal notranslate">ImportError</code> exception if the
module is considered unsafe.
</dd></dl>

<dl class="py method">
<dt class="sig sig-object py" id="rexec.RExec.r_open">
RExec.r_open(filename[, mode[, bufsize]])<a class="headerlink" href="#rexec.RExec.r_open" title="Permalink to this definition">¶</a></dt>
<dd>Method called when <a class="reference internal" href="functions.html#open" title="open"><code class="xref py py-func docutils literal notranslate">open()</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">open()</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">RExec</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">r_open()</code></a>.
</dd></dl>

<dl class="py method">
<dt class="sig sig-object py" id="rexec.RExec.r_reload">
RExec.r_reload(module)<a class="headerlink" href="#rexec.RExec.r_reload" title="Permalink to this definition">¶</a></dt>
<dd>Reload the module object module, re-parsing and re-initializing it.
</dd></dl>

<dl class="py method">
<dt class="sig sig-object py" id="rexec.RExec.r_unload">
RExec.r_unload(module)<a class="headerlink" href="#rexec.RExec.r_unload" title="Permalink to this definition">¶</a></dt>
<dd>Unload the module object module (remove it from the restricted environment’s
<code class="docutils literal notranslate">sys.modules</code> dictionary).
</dd></dl>

And their equivalents with access to restricted standard I/O streams:
<dl class="py method">
<dt class="sig sig-object py" id="rexec.RExec.s_import">
RExec.s_import(modulename[, globals[, locals[, fromlist]]])<a class="headerlink" href="#rexec.RExec.s_import" title="Permalink to this definition">¶</a></dt>
<dd>Import the module modulename, raising an <code class="xref py py-exc docutils literal notranslate">ImportError</code> exception if the
module is considered unsafe.
</dd></dl>

<dl class="py method">
<dt class="sig sig-object py" id="rexec.RExec.s_reload">
RExec.s_reload(module)<a class="headerlink" href="#rexec.RExec.s_reload" title="Permalink to this definition">¶</a></dt>
<dd>Reload the module object module, re-parsing and re-initializing it.
</dd></dl>

<dl class="py method">
<dt class="sig sig-object py" id="rexec.RExec.s_unload">
RExec.s_unload(module)<a class="headerlink" href="#rexec.RExec.s_unload" title="Permalink to this definition">¶</a></dt>
<dd>Unload the module object module.
</dd></dl>

</section>
<section id="defining-restricted-environments">
<h2>30.1.2. Defining restricted environments<a class="headerlink" href="#defining-restricted-environments" title="Permalink to this headline">¶</a></h2>
The <a class="reference internal" href="#rexec.RExec" title="rexec.RExec"><code class="xref py py-class docutils literal notranslate">RExec</code></a> class has the following class attributes, which are used by
the <code class="xref py py-meth docutils literal notranslate">__init__()</code> 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">RExec</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.
<dl class="py attribute">
<dt class="sig sig-object py" id="rexec.RExec.nok_builtin_names">
RExec.nok_builtin_names<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 not 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">RExec</code></a> is
<code class="docutils literal notranslate">('open', 'reload', '__import__')</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.)
</dd></dl>

<dl class="py attribute">
<dt class="sig sig-object py" id="rexec.RExec.ok_builtin_modules">
RExec.ok_builtin_modules<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 class="reference internal" href="#rexec.RExec" title="rexec.RExec"><code class="xref py py-class docutils literal notranslate">RExec</code></a> is <code class="docutils literal notranslate">('audioop', 'array', 'binascii', 'cmath', 'errno',
'imageop', 'marshal', 'math', 'md5', 'operator', 'parser', 'regex', 'select',
'sha', '_sre', 'strop', 'struct', 'time')</code>. A similar remark about overriding
this variable applies — use the value from the base class as a starting point.
</dd></dl>

<dl class="py attribute">
<dt class="sig sig-object py" id="rexec.RExec.ok_path">
RExec.ok_path<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 internal" href="../reference/simple_stmts.html#import"><code class="xref std std-keyword docutils literal notranslate">import</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">RExec</code></a> is the
same as <code class="docutils literal notranslate">sys.path</code> (at the time the module is loaded) for unrestricted code.
</dd></dl>

<dl class="py attribute">
<dt class="sig sig-object py" id="rexec.RExec.ok_posix_names">
RExec.ok_posix_names<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 class="reference internal" href="os.html#module-os" title="os: Miscellaneous operating system interfaces."><code class="xref py py-mod docutils literal notranslate">os</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">RExec</code></a> is <code class="docutils literal notranslate">('error', 'fstat', 'listdir', 'lstat', 'readlink', 'stat',
'times', 'uname', 'getpid', 'getppid', 'getcwd', 'getuid', 'getgid', 'geteuid',
'getegid')</code>.
</dd></dl>

<dl class="py attribute">
<dt class="sig sig-object py" id="rexec.RExec.ok_sys_names">
RExec.ok_sys_names<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 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">sys</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">RExec</code></a> is <code class="docutils literal notranslate">('ps1', 'ps2', 'copyright', 'version', 'platform',
'exit', 'maxint')</code>.
</dd></dl>

<dl class="py attribute">
<dt class="sig sig-object py" id="rexec.RExec.ok_file_types">
RExec.ok_file_types<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 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">imp</code></a> module. The meaningful
values are <code class="xref py py-const docutils literal notranslate">PY_SOURCE</code>, <code class="xref py py-const docutils literal notranslate">PY_COMPILED</code>, and <code class="xref py py-const docutils literal notranslate">C_EXTENSION</code>.
The value for <a class="reference internal" href="#rexec.RExec" title="rexec.RExec"><code class="xref py py-class docutils literal notranslate">RExec</code></a> is <code class="docutils literal notranslate">(C_EXTENSION, PY_SOURCE)</code>. Adding
<code class="xref py py-const docutils literal notranslate">PY_COMPILED</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">.pyc</code>) anywhere in your file system, for example by writing it to
<code class="file docutils literal notranslate">/tmp</code> or uploading it to the <code class="file docutils literal notranslate">/incoming</code> directory of your public
FTP server.
</dd></dl>

</section>
<section id="an-example">
<h2>30.1.3. An example<a class="headerlink" href="#an-example" title="Permalink to this headline">¶</a></h2>
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">RExec</code></a> class. For example, if we’re willing to allow files in
<code class="file docutils literal notranslate">/tmp</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">RExec</code></a> class:
<div class="highlight-default notranslate"><div class="highlight"><pre>class TmpWriterRExec(rexec.RExec):
 def r_open(self, file, mode=&#39;r&#39;, buf=-1):
 if mode in (&#39;r&#39;, &#39;rb&#39;):
 pass
 elif mode in (&#39;w&#39;, &#39;wb&#39;, &#39;a&#39;, &#39;ab&#39;):
 # check filename: must begin with /tmp/
 if file[:5]!=&#39;/tmp/&#39;:
 raise IOError(&quot;can&#39;t write outside /tmp&quot;)
 elif (string.find(file, &#39;/../&#39;) &gt;= 0 or
 file[:3] == &#39;../&#39; or file[-3:] == &#39;/..&#39;):
 raise IOError(&quot;&#39;..&#39; in filename forbidden&quot;)
 else: raise IOError(&quot;Illegal open() mode&quot;)
 return open(file, mode, buf)
</pre></div>
</div>
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">/tmp/foo/../bar</code>. To fix this, the <code class="xref py py-meth docutils literal notranslate">r_open()</code> method would
have to simplify the filename to <code class="file docutils literal notranslate">/tmp/bar</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.
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