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
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.
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.)
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.
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.
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')
.
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:
__main__
module. The value of the expression or
code object will be returned.
__main__
module.
__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 streans sys.stdin
,
sys.stderr
, and sys.stdout
.
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.
ImportError
exception if the module is considered unsafe.
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()
.
sys.modules
dictionary).
And their equivalents with access to restricted standard I/O streams:
ImportError
exception if the module is considered unsafe.