12.1 Standard Module rexec

 

This module contains the RExec class, which supports r_exec(), r_eval(), r_execfile(), and r_import() methods, which are restricted versions of the standard Python functions exec(), eval(), execfile(), and the import statement. 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.

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 CPU time.

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, e.g. using packages.)

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.

The RExec class has the following class attributes, which are used by the __init__() method. Changing them on an existing instance won't have any effect; instead, create a subclass of RExec 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.

nok_builtin_names

Contains the names of built-in functions which will not be available to programs running in the restricted environment. The value for RExec is ('open', 'reload', '__import__'). (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.)

ok_builtin_modules

Contains the names of built-in modules which can be safely imported. The value for RExec is ('audioop', 'array', 'binascii', 'cmath', 'errno', 'imageop', 'marshal', 'math', 'md5', 'operator', 'parser', 'regex', 'rotor', 'select', 'strop', 'struct', 'time'). A similar remark about overriding this variable applies -- use the value from the base class as a starting point.

ok_path

Contains the directories which will be searched when an import is performed in the restricted environment. The value for RExec is the same as sys.path (at the time the module is loaded) for unrestricted code.

ok_posix_names

Contains the names of the functions in the os module which will be available to programs running in the restricted environment. The value for RExec is ('error', 'fstat', 'listdir', 'lstat', 'readlink', 'stat', 'times', 'uname', 'getpid', 'getppid', 'getcwd', 'getuid', 'getgid', 'geteuid', 'getegid').

ok_sys_names

Contains the names of the functions and variables in the sys module which will be available to programs running in the restricted environment. The value for RExec is ('ps1', 'ps2', 'copyright', 'version', 'platform', 'exit', 'maxint').

RExec instances support the following methods:

r_eval (code)

code must either be a string containing a Python expression, or a compiled code object, which will be evaluated in the restricted environment's __main__ module. The value of the expression or code object will be returned.

r_exec (code)

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 __main__ module.

r_execfile (filename)

Execute the Python code contained in the file filename in the restricted environment's __main__ module.

Methods whose names begin with "s_" are similar to the functions beginning with "r_", but the code will be granted access to restricted versions of the standard I/O streams sys.stdin, sys.stderr, and sys.stdout.

s_eval (code)

code must be a string containing a Python expression, which will be evaluated in the restricted environment.

s_exec (code)

code must be a string containing one or more lines of Python code, which will be executed in the restricted environment.

s_execfile (code)

Execute the Python code contained in the file filename in the restricted environment.

RExec 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.

r_import (modulename[, globals[, locals[, fromlist]]])

Import the module modulename, raising an ImportError exception if the module is considered unsafe.

r_open (filename[, mode[, bufsize]])

Method called when open() is called in the restricted environment. The arguments are identical to those of open(), and a file object (or a class instance compatible with file objects) should be returned. RExec'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 r_open().

r_reload (module)

Reload the module object module, re-parsing and re-initializing it.

r_unload (module)

Unload the module object module (i.e., remove it from the restricted environment's sys.modules dictionary).

And their equivalents with access to restricted standard I/O streams:

s_import (modulename[, globals[, locals[, fromlist]]])

Import the module modulename, raising an ImportError exception if the module is considered unsafe.

s_reload (module)

Reload the module object module, re-parsing and re-initializing it.

s_unload (module)

Unload the module object module.