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Fixed some logical markup nits.

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Added a pointer to Grail in the see-also section, since it's used as
an example.
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Fred Drake 1999-04-23 17:26:24 +00:00
parent deb7e096d3
commit 87b8f318b0
1 changed files with 26 additions and 21 deletions
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47
Doc/lib/librestricted.tex
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@ -1,15 +1,14 @@
\chapter{Restricted Execution}
\label{restricted}
\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
using the \function{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'',
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,
@ -38,27 +37,28 @@ 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.
may redefine the built-in \function{open()} function so that it raises
an exception whenever the \var{mode} parameter is \code{'w'}. It
might also perform a \cfunction{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 \function{open()} function in its
environment, with the same calling interface. The semantics would be
identical too, with \exception{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.
dictionary of) the standard \refmodule[builtin]{__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.
For instance, the function object attribute \member{func_globals} and
the class and instance object attribute \member{__dict__} are
unavailable.
Two modules provide the framework for setting up restricted execution
environments:
@ -66,6 +66,11 @@ environments:
\localmoduletable
\begin{seealso}
\seetext{Andrew Kuchling, ``Restricted Execution HOWTO.'' Available
online at \url{http://www.python.org/doc/howto/rexec/}.}
\seetext{Andrew Kuchling, ``Restricted Execution HOWTO.'' Available
online at \url{http://www.python.org/doc/howto/rexec/}.}
\seetext{Grail, an Internet browser written in Python, is available
at \url{http://grail.cnri.reston.va.us/grail/}. More
information on the use of Python's restricted execution
mode in Grail is available on the Web site.}
\end{seealso}