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Tue Jul 11 08:29:10 CEST 2006 · https://mail.python.org/pipermail/python-dev/2006-July/067228.html

Brett Cannon wrote:
> Using a factory method callback, one could store the PyCodeObject in a C
> proxy object that just acts as a complete delegate, forwarding all method
> calls to the internally stored PyCodeObject.  That would work.
> 
> 
> For this initial implementation, though, I am not going to try to support
> this.   We can always add support like this later since it doesn't
> fundamentally break or change anything that is already planned.  Let's 
> focus
> on getting even more basic stuff working before we start to get too fancy.
> 
> -Brett

Thinking about this some more, I've come up with a simpler idea for a 
generic wrapper class. The wrapper consists of two parts: a decorator to 
indicate that a given method is 'public', and a C 'guard' wrapper that 
insures that only 'public' members can be accessed.

So for example:

    from Sandbox import guard, public

    class FileProxy:
       # Public method, no decorator
       @public
       def read( length ):
          ...

       @public
       def seek( offset ):
          ...

       # Private method, no decorator
       def open( path ):
          ...

    # Construct an instance of FileProxy, and wrap a
    # guard object around it. Any attempt to access a non-public
    # attribute will raise an exception (or perhaps simply report
    # that the attribute doesn't exist.)
    fh = guard( FileProxy() )

Now, from my point of view this *is* 'the basic stuff'. In other words, 
this right here is the fundamental sandbox mechanism, and everything 
else is built on top of it.

Now, the C 'guard' function is only a low-level means to insure that 
no-one can mess with the object; It is not intended to be the actual 
restriction policy itself. Those are placed in the wrapper classes, just 
as in your proposed scheme.

(This goes back to my basic premise, that a simple "yes/no" security 
feature can be used to build up much more complex and subtle security 
features.)

The only real complexity of this, as I see it, is that methods to 
references will have to be themselves wrapped.

In other words, if I say 'fh.read' without the (), what I get back can't 
be the actual read function object - that would be too easy to fiddle 
with. What I'd have to get is a wrapped version of the method that is a 
callable.

One relatively simply way to deal with this is to have the 'public' 
decorator create a C wrapper for the function object, and store it as an 
attribute of the function. The class wrapper then simply looks up the 
attribute, and if it has an attribute wrapper, returns that, otherwise 
it fails.

(Although, I've often wished for Python to have a variant of __call__ 
that could be used to override individual methods, i.e.:

     __call_method__( self, methodname, *args )

This would make the guard wrapper much easier to construct, since we 
could restrict the methods only to being called, and not allow 
references to methods to be taken.)

-- Talin

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[Python-Dev] In defense of Capabilities [was: doc for new restricted execution design for Python]