Understanding Effective Actions

In PolicyGlass we express ARPs (Action Resource policyglass.principal.Principal) as though they are potentially infinite sets.

In reality they are finite sets because there are only a finite number of allowed actions, resources, or principals. However because actions are being constantly updated by AWS, and new resources and princiapls are being created all the time, we here treat them as infinite sets because their extent is unknowable by us when we are parsing the policy.


The difference between set x and set y is the elements that are contained in set x that are not contained in set y. In essence it’s a subtraction. Remove the elements in set y from set x and you have the difference.


Let’s say we calculate the difference between two effective actions like so.

>>> from policyglass import EffectiveAction, Action
>>> x = EffectiveAction(inclusion=Action("S3:*"))
>>> y = EffectiveAction(inclusion=Action("S3:Get*"))
>>> x.difference(y)
[EffectiveAction(inclusion=Action('S3:*'), exclusions=frozenset({Action('S3:Get*')}))]

The result is that the inclusion from y is added to the exclusions of x.


Simple Effective Difference

  • A is the inclusion from x

  • B is the inclusion from y

The inclusion from x is added as an exclusion of y is because our Actions are essentially infinite sets. The wildcard at the end of S3:* could extend to an infinitely long string for all we know, so we can’t create an Action that expresses S3:* but not S3:Get* so we must add it as an exclusion in an EffectiveAction.

This is the reason EffectiveAction s exist, so we can express the intersection of the complement of ininite set B with inifite set A.


Let’s say we have two effective actions like so:

>>> from policyglass import EffectiveAction, Action
>>> x = EffectiveAction(inclusion=Action("S3:*"))
>>> y = EffectiveAction(inclusion=Action("S3:Get*"), exclusions=frozenset({Action("S3:GetObject")}))
>>> print(x.difference(y))
[EffectiveAction(inclusion=Action('S3:*'), exclusions=frozenset({Action('S3:Get*')})),
 EffectiveAction(inclusion=Action('S3:GetObject'), exclusions=frozenset())]

Let’s unpack what happened here.

  1. We added the inclusion (S3:get*) from y to the exclusions of x

  2. We returned y unchanged.

Why? What’s happening here?


Complex Effective Difference

  • A is our inclusion from x

  • B is our exclusion from x (which we added in order to accurately refect the difference)

  • C is our inclusion from y

  • D is our exclusion from y

In the simple example we added the inclusion from y to the exclusions of x because that’s what difference means, but why did we return y unchanged in the list?

Remember that the exclusions in an EffectiveAction are negations, they are holes punched in what’s allowed. As a result, what is in the exclusion of y should not be removed from x because it’s explicitly not part of y.

Because we can’t express the fact that we want to exclude B and C but include A in our result, we have to return two separate EffectiveAction s, one which includes A but the entirety of B, and another that just includes D.