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-\chapter{Restricted Execution}
-\label{restricted}
-
-In general, Python programs have complete access to the underlying
-operating system throug the various functions and classes, For
-example, a Python program can open any file for reading and writing by
-using the \code{open()} built-in function (provided the underlying OS
-gives you permission!).  This is exactly what you want for most
-applications.
-
-There exists a class of applications for which this ``openness'' is
-inappropriate.  Take Grail: a web browser that accepts ``applets'',
-snippets of Python code, from anywhere on the Internet for execution
-on the local system.  This can be used to improve the user interface
-of forms, for instance.  Since the originator of the code is unknown,
-it is obvious that it cannot be trusted with the full resources of the
-local machine.
-
-\emph{Restricted execution} is the basic framework in Python that allows
-for the segregation of trusted and untrusted code.  It is based on the
-notion that trusted Python code (a \emph{supervisor}) can create a
-``padded cell' (or environment) with limited permissions, and run the
-untrusted code within this cell.  The untrusted code cannot break out
-of its cell, and can only interact with sensitive system resources
-through interfaces defined and managed by the trusted code.  The term
-``restricted execution'' is favored over ``safe-Python''
-since true safety is hard to define, and is determined by the way the
-restricted environment is created.  Note that the restricted
-environments can be nested, with inner cells creating subcells of
-lesser, but never greater, privilege.
-
-An interesting aspect of Python's restricted execution model is that
-the interfaces presented to untrusted code usually have the same names
-as those presented to trusted code.  Therefore no special interfaces
-need to be learned to write code designed to run in a restricted
-environment.  And because the exact nature of the padded cell is
-determined by the supervisor, different restrictions can be imposed,
-depending on the application.  For example, it might be deemed
-``safe'' for untrusted code to read any file within a specified
-directory, but never to write a file.  In this case, the supervisor
-may redefine the built-in
-\code{open()} function so that it raises an exception whenever the
-\var{mode} parameter is \code{'w'}.  It might also perform a
-\code{chroot()}-like operation on the \var{filename} parameter, such
-that root is always relative to some safe ``sandbox'' area of the
-filesystem.  In this case, the untrusted code would still see an
-built-in \code{open()} function in its environment, with the same
-calling interface.  The semantics would be identical too, with
-\code{IOError}s being raised when the supervisor determined that an
-unallowable parameter is being used.
-
-The Python run-time determines whether a particular code block is
-executing in restricted execution mode based on the identity of the
-\code{__builtins__} object in its global variables: if this is (the
-dictionary of) the standard \code{__builtin__} module, the code is
-deemed to be unrestricted, else it is deemed to be restricted.
-
-Python code executing in restricted mode faces a number of limitations
-that are designed to prevent it from escaping from the padded cell.
-For instance, the function object attribute \code{func_globals} and the
-class and instance object attribute \code{__dict__} are unavailable.
-
-Two modules provide the framework for setting up restricted execution
-environments:
-
-\begin{description}
-
-\item[rexec]
---- Basic restricted execution framework.
-
-\item[Bastion]
---- Providing restricted access to objects.
-
-\end{description}
-
-\begin{seealso}
-\seetext{Andrew Kuchling, ``Restricted Execution HOWTO.''  Available
-online at \url{http://www.python.org/doc/howto/rexec/}.}
-\end{seealso}