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.
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.
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