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Add documentation for the help() built-in; contributed by Ka-Ping Yee. This is part of SF patch #494622.
author Fred Drake <fdrake@acm.org>
date Tue, 18 Dec 2001 16:31:08 +0000
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1 \chapter{Restricted Execution \label{restricted}}
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2
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3 In general, Python programs have complete access to the underlying
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4 operating system throug the various functions and classes, For
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5 example, a Python program can open any file for reading and writing by
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6 using the \function{open()} built-in function (provided the underlying
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7 operating system gives you permission!). This is exactly what you want
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8 for most applications.
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9
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10 There exists a class of applications for which this ``openness'' is
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11 inappropriate. Take Grail: a Web browser that accepts ``applets,''
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12 snippets of Python code, from anywhere on the Internet for execution
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13 on the local system. This can be used to improve the user interface
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14 of forms, for instance. Since the originator of the code is unknown,
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15 it is obvious that it cannot be trusted with the full resources of the
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16 local machine.
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17
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18 \emph{Restricted execution} is the basic framework in Python that allows
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19 for the segregation of trusted and untrusted code. It is based on the
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20 notion that trusted Python code (a \emph{supervisor}) can create a
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21 ``padded cell' (or environment) with limited permissions, and run the
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22 untrusted code within this cell. The untrusted code cannot break out
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23 of its cell, and can only interact with sensitive system resources
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24 through interfaces defined and managed by the trusted code. The term
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25 ``restricted execution'' is favored over ``safe-Python''
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26 since true safety is hard to define, and is determined by the way the
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27 restricted environment is created. Note that the restricted
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28 environments can be nested, with inner cells creating subcells of
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29 lesser, but never greater, privilege.
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30
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31 An interesting aspect of Python's restricted execution model is that
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32 the interfaces presented to untrusted code usually have the same names
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33 as those presented to trusted code. Therefore no special interfaces
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34 need to be learned to write code designed to run in a restricted
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35 environment. And because the exact nature of the padded cell is
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36 determined by the supervisor, different restrictions can be imposed,
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37 depending on the application. For example, it might be deemed
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38 ``safe'' for untrusted code to read any file within a specified
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39 directory, but never to write a file. In this case, the supervisor
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40 may redefine the built-in \function{open()} function so that it raises
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41 an exception whenever the \var{mode} parameter is \code{'w'}. It
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42 might also perform a \cfunction{chroot()}-like operation on the
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43 \var{filename} parameter, such that root is always relative to some
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44 safe ``sandbox'' area of the filesystem. In this case, the untrusted
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45 code would still see an built-in \function{open()} function in its
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46 environment, with the same calling interface. The semantics would be
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47 identical too, with \exception{IOError}s being raised when the
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48 supervisor determined that an unallowable parameter is being used.
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49
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50 The Python run-time determines whether a particular code block is
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51 executing in restricted execution mode based on the identity of the
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52 \code{__builtins__} object in its global variables: if this is (the
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53 dictionary of) the standard \refmodule[builtin]{__builtin__} module,
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54 the code is deemed to be unrestricted, else it is deemed to be
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55 restricted.
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56
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57 Python code executing in restricted mode faces a number of limitations
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58 that are designed to prevent it from escaping from the padded cell.
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59 For instance, the function object attribute \member{func_globals} and
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60 the class and instance object attribute \member{__dict__} are
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61 unavailable.
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62
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63 Two modules provide the framework for setting up restricted execution
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64 environments:
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65
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66 \localmoduletable
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67
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68 \begin{seealso}
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69 \seetitle[http://www.python.org/doc/howto/rexec/]
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70 {Restricted Execution HOWTO}
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71 {Andrew Kuchling's tutorial on the use of the restricted
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72 execution facilities in Python.}
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73 \seetitle[http://grail.sourceforge.net/]{Grail Home Page}
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74 {Grail, an Internet browser written in Python, uses these
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75 modules to support Python applets. More
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76 information on the use of Python's restricted execution
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77 mode in Grail is available on the Web site.}
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78 \end{seealso}