17.1 rexec -- Restricted execution framework

This module contains the RExec class, which supports r_eval(), r_execfile(), r_exec(), and r_import() methods, which are restricted versions of the standard Python functions eval(), execfile() and the exec and import statements. Code executed in this restricted environment will only have access to modules and functions that are deemed safe; you can subclass RExec to add or remove capabilities as desired.

Warning: While the rexec 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 rexec vulnerabilities would be welcomed.

Note: The RExec 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.

class RExec([hooks[, verbose]])
Returns an instance of the RExec class.

hooks is an instance of the RHooks class or a subclass of it. If it is omitted or None, the default RHooks class is instantiated. Whenever the rexec 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 RHooks instance that was passed to or created by its constructor. (Actually, the RExec object doesn't make these calls -- they are made by a module loader object that's part of the RExec object. This allows another level of flexibility, which can be useful when changing the mechanics of import within the restricted environment.)

By providing an alternate RHooks 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 RHooks 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.

It is important to be aware that code running in a restricted environment can still call the sys.exit() function. To disallow restricted code from exiting the interpreter, always protect calls that cause restricted code to run with a try/except statement that catches the SystemExit exception. Removing the sys.exit() function from the restricted environment is not sufficient -- the restricted code could still use raise SystemExit. Removing SystemExit is not a reasonable option; some library code makes use of this and would break were it not available.

See Also:

Grail Home Page
Grail is a Web browser written entirely in Python. It uses the rexec module as a foundation for supporting Python applets, and can be used as an example usage of this module.

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