[Python-Dev] Capabilities (original) (raw)

Paul Prescod paul@prescod.net
Sun, 30 Mar 2003 10:43:12 -0800

Ka-Ping Yee wrote:

...

Specifically, the only introspective attributes we have to disallow, in order for these objects to enforce their intended restrictions, are imself and funcglobals. Of course, we still have to hide import and sys.modules if we want to prevent code from obtaining access to the filesystem in other ways.

It wouldn't have hurt for you to describe how the code achieves security by using lexical closure namespaces instead of dictionary-backed namespaces. ;) Part of the trick is that the external names are irrelevant to the functioning of the object.

I don't understand one thing.

The immutability imposed by the "ImmutableNamespace" trick is easy to turn off. But once I turn it off, I couldn't figure out any way to violate the security because the closure's variables are invisible to any code that is not defined within its block. Why bother with the ImmutableNamespace bit at all?

x = DirectoryReader(".", "foo") print x.getfiles() del x.class.setattr x.foo = 5 del x.getfiles del x.getdirs x.getfiles()

Traceback (most recent call last): File "../foo.py", line 64, in ? x.getfiles() AttributeError: ImmutableNamespace instance has no attribute 'getfiles'

But I couldn't figure out how to use this to get access to the file system because as I said before, the external names are irrelevant to the object's implementation. They are early bound.

 def FileReader(path, name):
       ...
       def open2():
         print "open2"
         return open()

direct = DirectoryReader(".", "foo") file = direct.getfiles()[0] print file.open2()

FileReaderClass = file.class del FileReaderClass.setattr

del file.open print file.open2()

"open2" binds to open at definition time, not at runtime. I can't see in this model how to implement what C++ calls a "friend" class. Even C++ and Java have ways that related classes can poke around each others internals. So perhaps this is part of what would need to change in Python to have a first-class capabilities feature.

If this technique became widespread, Python's restrictions on assigning to lexically inherited variables would probably become annoying.

Paul Prescod