implementing sandboxes with capabilities
Ted.Kaehler at disney.com
Wed Apr 12 05:20:33 UTC 2000
Here is another cut at the whole thing, trying to make it simpler.
All objects are divided into these four groups:
* Regular Squeak objects.
* Sandbox objects. These are instances of classes defined in the Sandbox.
The code in these classes is under the control of evil terrorists.
* Mum wrappers. When a regular Squeak object ventures into the Sandbox, it
is always wrapped in a Mum wrapper. A Mum wrapper does not pass on any
message that would do harm. A Mum wrapper never divulges the oop of any
regular Squeak object. (Think of there being one Mum wrapper class for
every Squeak class that needs to be known in the Sandbox. It defines every
message that its target understands. Harmful ones are stopped with an
* Ear wrappers. When a Sandbox object is passed around as an argument
among the regular objects, it is wrapped in an Ear wrapper. Its purpose is
to keep the Sandbox object from *hearing* about regular oops. An Ear
wrapper passes all messages on to the Sandbox object is represents, but it
wraps every argument object in a Mum wrapper before sending it in. (A
Sandbox object that is passed around as an argument may be sent a message
with an argument by an unsuspecting regular object. If the Ear wrapper did
not intervene, the Sandbox object could send arbitrary bad messages to the
The goal: No regular object every receives a message from a Sandbox object
that would cause harm. No Sandbox object every learns the oop of any
A Sandbox object is free to send any message to any object it can get the
On the Sandbox side, the only oops available are of Sandbox objects and Mum
On the regular Squeak side, the oops available are of regular Squeak
objects, Ear wrappers and Mum wrappers. (Oops of Sandbox objects are not
visible here. Part of the job of a Mum wrapper is to prevent any regular
Squeak object from learning the oop of a Sandbox object. We are protecting
ordinary Squeak code from ever dealing with a dangerous object.
Programmers writing code for regular Squeak classes do not have to be
careful about anything.)
When a regular Squeak object receivers a Mum wrapper as an argument, it
The pure implementation, using real wrapper objects is very clean. Now
let's consider SmallIntegers and Characters. Each of these is a very smart
a Mum wrapper. It is so smart, it does not need a real object behind it.
Mum wrappers can speak no evil to their target. Ear wrappers prevent
Sandbox objects from hearing any regular oops.
I think this could work very well. Even though the boudary between the
Sandbox and the main system is complex, the principles are clear.
Ted Kaehler, Walt Disney Imagineering, R&D
(home) 3415 Cork Oak Way, Palo Alto, CA 94303. voice (650) 424-1070
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