Done with this for 1.4.

Doc/lib/librestricted.tex

@@ -1,34 +1,35 @@
 \chapter{Restricted Execution}
 
-In general, executing Python programs have complete access to the
-underlying operating system through the various functions and classes
-contained in Python's modules.  For example, a Python program can open
-any file\footnote{Provided the underlying OS gives you permission!}
-for reading and writing by using the
-\code{open()} built-in function.  This is exactly what you want for
-most applications.
+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 is a class of applications for which this ``openness'' is
-inappropriate.  Imagine a web browser that accepts ``applets'', snippets of
-Python code, from anywhere on the Internet for execution on the local
-system.  Since the originator of the code is unknown, it is obvious that it
-cannot be trusted with the full resources of the local machine.
+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 Python framework that allows
+\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) of limited permissions, and run the
+``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 the term ``safe-Python''
+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, privledge.
+lesser, but never greater, privilege.
 
 An interesting aspect of Python's restricted execution model is that
-the attributes presented to untrusted code usually have the same names
+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
@@ -42,11 +43,22 @@ may redefine the built-in
 \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
-\code{open()} function in its \code{__builtin__} module, with the same
+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: