rexec
— 限定执行框架
¶
Deprecated since version 2.6:
The
rexec
module has been removed in Python 3.
Changed in version 2.3: Disabled module.
警告
The documentation has been left in place to help in reading old code that uses the module.
This module contains the
RExec
class, which supports
r_eval()
,
r_execfile()
,
r_exec()
，和
r_import()
methods, which are restricted versions of the standard Python functions
eval()
,
execfile()
和
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.
警告
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.
注意
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.
rexec.
RExec
(
[
hooks
[
,
verbose
]
]
)
¶
Returns an instance of the
RExec
类。
hooks
is an instance of the
RHooks
class or a subclass of it. If it is omitted or
None
，默认
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.
若 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.
另请参阅
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.
RExec
instances support the following methods:
RExec.
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.
RExec.
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__
模块。
RExec.
r_execfile
(
filename
)
¶
Execute the Python code contained in the file
filename
in the restricted environment’s
__main__
模块。
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
，和
sys.stdout
.
RExec.
s_eval
(
code
)
¶
code must be a string containing a Python expression, which will be evaluated in the restricted environment.
RExec.
s_exec
(
code
)
¶
code must be a string containing one or more lines of Python code, which will be executed in the restricted environment.
RExec.
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.
RExec.
r_import
(
modulename
[
,
globals
[
,
locals
[
,
fromlist
]
]
]
)
¶
Import the module
modulename
, raising an
ImportError
exception if the module is considered unsafe.
RExec.
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()
.
RExec.
r_reload
(
模块
)
¶
Reload the module object module , re-parsing and re-initializing it.
RExec.
r_unload
(
模块
)
¶
Unload the module object
module
(remove it from the restricted environment’s
sys.modules
dictionary).
And their equivalents with access to restricted standard I/O streams:
RExec.
s_import
(
modulename
[
,
globals
[
,
locals
[
,
fromlist
]
]
]
)
¶
Import the module
modulename
, raising an
ImportError
exception if the module is considered unsafe.
RExec.
s_reload
(
模块
)
¶
Reload the module object module , re-parsing and re-initializing it.
RExec.
s_unload
(
模块
)
¶
Unload the module object module .
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.
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.)
RExec.
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',
'select',
'sha',
'_sre',
'strop',
'struct',
'time')
. A similar remark about overriding this variable applies — use the value from the base class as a starting point.
RExec.
ok_path
¶
Contains the directories which will be searched when an
import
is performed in the restricted environment. The value for
RExec
如同
sys.path
(at the time the module is loaded) for unrestricted code.
RExec.
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')
.
RExec.
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.
ok_file_types
¶
Contains the file types from which modules are allowed to be loaded. Each file type is an integer constant defined in the
imp
module. The meaningful values are
PY_SOURCE
,
PY_COMPILED
，和
C_EXTENSION
。值对于
RExec
is
(C_EXTENSION, PY_SOURCE)
. Adding
PY_COMPILED
in subclasses is not recommended; an attacker could exit the restricted execution mode by putting a forged byte-compiled file (
.pyc
) anywhere in your file system, for example by writing it to
/tmp
or uploading it to the
/incoming
directory of your public FTP server.
Let us say that we want a slightly more relaxed policy than the standard
RExec
class. For example, if we’re willing to allow files in
/tmp
to be written, we can subclass the
RExec
类：
class TmpWriterRExec(rexec.RExec): def r_open(self, file, mode='r', buf=-1): if mode in ('r', 'rb'): pass elif mode in ('w', 'wb', 'a', 'ab'): # check filename: must begin with /tmp/ if file[:5]!='/tmp/': raise IOError("can't write outside /tmp") elif (string.find(file, '/../') >= 0 or file[:3] == '../' or file[-3:] == '/..'): raise IOError("'..' in filename forbidden") else: raise IOError("Illegal open() mode") return open(file, mode, buf)
Notice that the above code will occasionally forbid a perfectly valid filename; for example, code in the restricted environment won’t be able to open a file called
/tmp/foo/../bar
. To fix this, the
r_open()
method would have to simplify the filename to
/tmp/bar
, which would require splitting apart the filename and performing various operations on it. In cases where security is at stake, it may be preferable to write simple code which is sometimes overly restrictive, instead of more general code that is also more complex and may harbor a subtle security hole.