During the past couple of months I have been working on an object-capability subset of Python - in other words, a restricted execution scheme for sandboxing Python code. It has been influenced by other object-capability subset languages, such as Joe-E (a subset of Java [1]), Caja/Cajita (subsets of Javascript [2]) and Caperl (based on Perl [3]). I'm calling it CapPython because the name doesn't seem to have been taken yet. :-)
I believe it is now secure, so it seems like a good time to announce it here! The basic idea behind CapPython is to enforce encapsulation by restricting access to private attributes of objects. This is achieved through a combination of static checking and limiting access to unsafe builtins and modules. Private attributes may only be accessed through "self" variables. "Self" variables are defined as being the first arguments of functions defined inside class definitions, with a few restrictions intended to prevent these functions from escaping without being safely wrapped. Private attribute names are those starting with "_". Additionally, "im_self", "im_func" and some other special cases are treated as private attributes. Assignments to attributes are only allowed via "self" variables. For example, the following code is accepted by the static verifier: class Counter(object): def __init__(self): self._count = 0 def get_next(self): self._count += 1 return self._count But the following code reads a private attribute and so it is rejected as violating encapsulation: counter._count -= 1 CapPython consists of three parts: - a static verifier; - a "safe exec" function, which will check code before executing it and can run code in a safe scope; - a module loader which implements a safe __import__ function. Eventually this will be runnable as untrusted CapPython code. I am documenting CapPython via my blog at the moment, with the following posts so far: http://lackingrhoticity.blogspot.com/2008/08/introducing-cappython.html http://lackingrhoticity.blogspot.com/2008/09/dealing-with-modules-and-builtins-in.html http://lackingrhoticity.blogspot.com/2008/09/cappython-unbound-methods-and-python-30.html The code is available from a Bazaar repository on Launchpad: https://code.launchpad.net/cappython I am currently working on creating a simple example program, which will be a wsgiref-based web server with a form for executing CapPython code. This involves taming some of the standard libraries to pass the verifier. There are some design notes here - http://plash.beasts.org/wiki/CapPython - although these notes are more a list of references and problems CapPython needs to address than an explanation of the current design. There was also a thread about CapPython on the e-lang mailing list: http://www.eros-os.org/pipermail/e-lang/2008-August/012828.html Mark [1] http://code.google.com/p/joe-e/ [2] http://code.google.com/p/google-caja/ [3] http://caperl.links.org/ _______________________________________________ Python-Dev mailing list Python-Dev@python.org http://mail.python.org/mailman/listinfo/python-dev Unsubscribe: http://mail.python.org/mailman/options/python-dev/archive%40mail-archive.com