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<title>17.1 rexec -- Restricted execution framework
</title>
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<H1><A NAME=
"SECTION0019100000000000000000">
17.1 <tt class=
"module">rexec
</tt> --
Restricted execution framework
</A><A NAME=
"module-rexec"></A>
<span class=
"versionnote">Changed in version
2.3:
<div class=
"warning"><b class=
"label">Warning:
</b>
The documentation has been left in place to help in reading old code
This module contains the
<tt class=
"class">RExec
</tt> class, which supports
<tt class=
"method">r_eval()
</tt>,
<tt class=
"method">r_execfile()
</tt>,
<tt class=
"method">r_exec()
</tt>, and
<tt class=
"method">r_import()
</tt> methods, which are restricted versions of the standard
Python functions
<tt class=
"method">eval()
</tt>,
<tt class=
"method">execfile()
</tt> and
the
<tt class=
"keyword">exec
</tt> and
<tt class=
"keyword">import
</tt> statements.
Code executed in this restricted environment will
only have access to modules and functions that are deemed safe; you
can subclass
<tt class=
"class">RExec
</tt> to add or remove capabilities as desired.
<div class=
"warning"><b class=
"label">Warning:
</b>
While the
<tt class=
"module">rexec
</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=
"module">rexec
</tt> vulnerabilities would be welcomed.
<div class=
"note"><b class=
"label">Note:
</b>
The
<tt class=
"class">RExec
</tt> 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.
<dl><dt><table cellpadding=
"0" cellspacing=
"0"><tr valign=
"baseline">
<td><nobr><b><span class=
"typelabel">class
</span> <tt id='l2h-
4913' xml:id='l2h-
4913'
class=
"class">RExec
</tt></b>(
</nobr></td>
<td><var></var><big>[
</big><var>hooks
</var><big>[
</big><var>, verbose
</var><big>]
</big><var></var><big>]
</big><var></var>)
</td></tr></table></dt>
Returns an instance of the
<tt class=
"class">RExec
</tt> class.
<var>hooks
</var> is an instance of the
<tt class=
"class">RHooks
</tt> class or a subclass of it.
If it is omitted or
<code>None
</code>, the default
<tt class=
"class">RHooks
</tt> class is
Whenever the
<tt class=
"module">rexec
</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=
"class">RHooks
</tt>instance that was passed to or created by its constructor. (Actually,
the
<tt class=
"class">RExec
</tt> object doesn't make these calls -- they are made by
a module loader object that's part of the
<tt class=
"class">RExec
</tt> object. This
allows another level of flexibility, which can be useful when changing
the mechanics of
<tt class=
"keyword">import
</tt> within the restricted environment.)
By providing an alternate
<tt class=
"class">RHooks
</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=
"class">RHooks
</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.
If
<var>verbose
</var> is true, additional debugging output may be sent to
It is important to be aware that code running in a restricted
environment can still call the
<tt class=
"function">sys.exit()
</tt> function. To
disallow restricted code from exiting the interpreter, always protect
calls that cause restricted code to run with a
<tt class=
"keyword">try
</tt>/
<tt class=
"keyword">except
</tt> statement that catches the
<tt class=
"exception">SystemExit
</tt> exception. Removing the
<tt class=
"function">sys.exit()
</tt>
function from the restricted environment is not sufficient -- the
restricted code could still use
<code>raise SystemExit
</code>. Removing
<tt class=
"exception">SystemExit
</tt> is not a reasonable option; some library code
makes use of this and would break were it not available.
<p class=
"heading">See Also:
</p>
<dl compact=
"compact" class=
"seetitle">
<dt><em class=
"citetitle"><a href=
"http://grail.sourceforge.net/"
>Grail Home Page
</a></em></dt>
Web browser written entirely in Python. It uses the
<tt class=
"module">rexec
</tt> module as a foundation for supporting
Python applets, and can be used as an example usage of
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<!--Table of Child-Links-->
<A NAME=
"CHILD_LINKS"><STRONG>Subsections
</STRONG></a>
<LI><A href=
"rexec-objects.html">17.1.1 RExec Objects
</a>
<LI><A href=
"rexec-extension.html">17.1.2 Defining restricted environments
</a>
<LI><A href=
"node763.html">17.1.3 An example
</a>
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<span class=
"release-info">Release
2.4.2, documentation updated on
28 September
2005.
</span>
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