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Done with this for 1.4.

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Guido van Rossum 1996-10-24 22:14:06 +00:00
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112
Doc/lib/librexec.tex
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@ -6,7 +6,8 @@ This module contains the \code{RExec} class, which supports
\code{r_exec()}, \code{r_eval()}, \code{r_execfile()}, and
\code{r_import()} methods, which are restricted versions of the standard
Python functions \code{exec()}, \code{eval()}, \code{execfile()}, and
\code{import()}. Code executed in this restricted environment will
the \code{import} statement.
Code executed in this restricted environment will
only have access to modules and functions that are deemed safe; you
can subclass \code{RExec} to add or remove capabilities as desired.
@ -14,14 +15,13 @@ can subclass \code{RExec} to add or remove capabilities as desired.
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.
% XXX is there any protection against this?
\begin{funcdesc}{RExec}{\optional{hooks\, verbose} }
\begin{funcdesc}{RExec}{\optional{hooks\optional{\, verbose}}}
Returns an instance of the \code{RExec} class.
% XXX is ihooks.py documented? If yes, there should be a ref here
\var{hooks} is an instance of the \code{RHooks} class or a subclass of it.
If it is omitted or \code{None}, the default \code{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
@ -30,7 +30,7 @@ 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 actual
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
@ -38,12 +38,11 @@ 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.
% XXX does verbose actually do anything at the moment?
If \var{verbose} is true, additional debugging output will be sent to
If \var{verbose} is true, additional debugging output may be sent to
standard output.
\end{funcdesc}
RExec instances have the following attributes, which are used by the
The RExec class has the following class attributes, which are used by the
\code{__init__} method. Changing them on an existing instance won't
have any effect; instead, create a subclass of \code{RExec} and assign
them new values in the class definition. Instances of the new class
@ -53,23 +52,32 @@ strings.
\renewcommand{\indexsubitem}{(RExec object attribute)}
\begin{datadesc}{nok_builtin_names}
Contains the names of built-in functions which will \emph{not} be
available to programs running in the restricted environment. The
value for \code{RExec} is \code{('open',} \code{reload',}
\code{__import__')}.
available to programs running in the restricted environment. The
value for \code{RExec} is \code{('open',} \code{'reload',}
\code{'__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.)
\end{datadesc}
\begin{datadesc}{ok_builtin_modules}
Contains the names of built-in modules which can be safely imported.
The value for \code{RExec} is \code{('array',} \code{'binascii',} \code{'audioop',}
\code{'imageop',} \code{'marshal',} \code{'math',} \code{'md5',} \code{'parser',} \code{'regex',} \code{'rotor',}
\code{'select',} \code{'strop',} \code{'struct',} \code{'time')}.
The value for \code{RExec} is \code{('audioop',} \code{'array',}
\code{'binascii',} \code{'cmath',} \code{'errno',} \code{'imageop',}
\code{'marshal',} \code{'math',} \code{'md5',} \code{'operator',}
\code{'parser',} \code{'regex',} \code{'rotor',} \code{'select',}
\code{'strop',} \code{'struct',} \code{'time')}. A similar remark
about overriding this variable applies --- use the value from the base
class as a starting point.
\end{datadesc}
\begin{datadesc}{ok_path}
Contains the directories which will be searched when an \code{import}
is performed in the restricted environment.
The value for \code{RExec} is the same as \code{sys.path} for
unrestricted code.
The value for \code{RExec} is the same as \code{sys.path} (at the time
the module is loaded) for unrestricted code.
\end{datadesc}
\begin{datadesc}{ok_posix_names}
@ -84,35 +92,38 @@ value for \code{RExec} is \code{('error',} \code{'fstat',}
\end{datadesc}
\begin{datadesc}{ok_sys_names}
Contains the names of the functions and variables in the \code{sys} module which will be
available to programs running in the restricted environment. The
value for \code{RExec} is \code{('ps1',} \code{'ps2',}
\code{'copyright',} \code{'version',} \code{'platform',} \code{'exit',}
\code{'maxint')}.
Contains the names of the functions and variables in the \code{sys}
module which will be available to programs running in the restricted
environment. The value for \code{RExec} is \code{('ps1',}
\code{'ps2',} \code{'copyright',} \code{'version',} \code{'platform',}
\code{'exit',} \code{'maxint')}.
\end{datadesc}
RExec instances support the following methods:
\renewcommand{\indexsubitem}{(RExec object method)}
\begin{funcdesc}{r_eval}{code}
\var{code} must either be a string containing a Python expression, or a compiled code object, which will
be evaluated in the restricted environment. The value of the expression or code object will be returned.
\var{code} must either be a string containing a Python expression, or
a compiled code object, which will be evaluated in the restricted
environment's \code{__main__} module. The value of the expression or
code object will be returned.
\end{funcdesc}
\begin{funcdesc}{r_exec}{code}
\var{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.
\var{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 \code{__main__} module.
\end{funcdesc}
\begin{funcdesc}{r_execfile}{filename}
Execute the Python code contained in the file \var{filename} in the
restricted environment.
restricted environment's \code{__main__} module.
\end{funcdesc}
Methods whose names begin with \code{s_} are similar to the functions
beginning with \code{r_}, but the code will be granted access to
restricted versions of \code{sys.stdin}, \code{sys.stderr}, and
\code{sys.stdout}.
restricted versions of the standard I/O streans \code{sys.stdin},
\code{sys.stderr}, and \code{sys.stdout}.
\begin{funcdesc}{s_eval}{code}
\var{code} must be a string containing a Python expression, which will
@ -129,13 +140,14 @@ Execute the Python code contained in the file \var{filename} in the
restricted environment.
\end{funcdesc}
\code{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.
\code{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.
\begin{funcdesc}{r_import}{modulename\optional{\, globals, locals, fromlist}}
Import the module \var{modulename}, raising an \code{ImportError} exception
if the module is considered unsafe.
\begin{funcdesc}{r_import}{modulename\optional{\, globals\, locals\, fromlist}}
Import the module \var{modulename}, raising an \code{ImportError}
exception if the module is considered unsafe.
\end{funcdesc}
\begin{funcdesc}{r_open}{filename\optional{\, mode\optional{\, bufsize}}}
@ -144,7 +156,8 @@ environment. The arguments are identical to those of \code{open()},
and a file object (or a class instance compatible with file objects)
should be returned. \code{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 \code{r_open()}.
the example below for an implementation of a less restrictive
\code{r_open()}.
\end{funcdesc}
\begin{funcdesc}{r_reload}{module}
@ -152,13 +165,15 @@ Reload the module object \var{module}, re-parsing and re-initializing it.
\end{funcdesc}
\begin{funcdesc}{r_unload}{module}
Unload the module object \var{module}.
% XXX what are the semantics of this?
Unload the module object \var{module} (i.e., remove it from the
restricted environment's \code{sys.modules} dictionary).
\end{funcdesc}
And their equivalents with access to restricted standard I/O streams:
\begin{funcdesc}{s_import}{modulename\optional{\, globals, locals, fromlist}}
Import the module \var{modulename}, raising an \code{ImportError} exception
if the module is considered unsafe.
Import the module \var{modulename}, raising an \code{ImportError}
exception if the module is considered unsafe.
\end{funcdesc}
\begin{funcdesc}{s_reload}{module}
@ -179,13 +194,16 @@ standard RExec class. For example, if we're willing to allow files in
\bcode\begin{verbatim}
class TmpWriterRExec(rexec.RExec):
def r_open(self, file, mode='r', buf=-1):
if mode in ('r', 'rb'): pass
elif mode in ('w', 'wb'):
# check filename : must begin with /tmp/
if file[0:5]!='/tmp/':
raise IOError, "can't open files for writing outside of /tmp"
elif string.find(file, '/../')!=-1:
raise IOError, "'..' in filename; open for writing forbidden"
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)
\end{verbatim}\ecode