Lib/Bastion.py

   1 """Bastionification utility.
   2
   3 A bastion (for another object -- the 'original') is an object that has
   4 the same methods as the original but does not give access to its
   5 instance variables.  Bastions have a number of uses, but the most
   6 obvious one is to provide code executing in restricted mode with a
   7 safe interface to an object implemented in unrestricted mode.
   8
   9 The bastionification routine has an optional second argument which is
  10 a filter function.  Only those methods for which the filter method
  11 (called with the method name as argument) returns true are accessible.
  12 The default filter method returns true unless the method name begins
  13 with an underscore.
  14
  15 There are a number of possible implementations of bastions.  We use a
  16 'lazy' approach where the bastion's __getattr__() discipline does all
  17 the work for a particular method the first time it is used.  This is
  18 usually fastest, especially if the user doesn't call all available
  19 methods.  The retrieved methods are stored as instance variables of
  20 the bastion, so the overhead is only occurred on the first use of each
  21 method.
  22
  23 Detail: the bastion class has a __repr__() discipline which includes
  24 the repr() of the original object.  This is precomputed when the
  25 bastion is created.
  26
  27 """
  28 from warnings import warnpy3k
  29 warnpy3k("the Bastion module has been removed in Python 3.0", stacklevel=2)
  30 del warnpy3k
  31
  32 __all__ = ["BastionClass", "Bastion"]
  33
  34 from types import MethodType
  35
  36
  37 class BastionClass:
  38
  39     """Helper class used by the Bastion() function.
  40
  41     You could subclass this and pass the subclass as the bastionclass
  42     argument to the Bastion() function, as long as the constructor has
  43     the same signature (a get() function and a name for the object).
  44
  45     """
  46
  47     def __init__(self, get, name):
  48         """Constructor.
  49
  50         Arguments:
  51
  52         get - a function that gets the attribute value (by name)
  53         name - a human-readable name for the original object
  54                (suggestion: use repr(object))
  55
  56         """
  57         self._get_ = get
  58         self._name_ = name
  59
  60     def __repr__(self):
  61         """Return a representation string.
  62
  63         This includes the name passed in to the constructor, so that
  64         if you print the bastion during debugging, at least you have
  65         some idea of what it is.
  66
  67         """
  68         return "<Bastion for %s>" % self._name_
  69
  70     def __getattr__(self, name):
  71         """Get an as-yet undefined attribute value.
  72
  73         This calls the get() function that was passed to the
  74         constructor.  The result is stored as an instance variable so
  75         that the next time the same attribute is requested,
  76         __getattr__() won't be invoked.
  77
  78         If the get() function raises an exception, this is simply
  79         passed on -- exceptions are not cached.
  80
  81         """
  82         attribute = self._get_(name)
  83         self.__dict__[name] = attribute
  84         return attribute
  85
  86
  87 def Bastion(object, filter = lambda name: name[:1] != '_',
  88             name=None, bastionclass=BastionClass):
  89     """Create a bastion for an object, using an optional filter.
  90
  91     See the Bastion module's documentation for background.
  92
  93     Arguments:
  94
  95     object - the original object
  96     filter - a predicate that decides whether a function name is OK;
  97              by default all names are OK that don't start with '_'
  98     name - the name of the object; default repr(object)
  99     bastionclass - class used to create the bastion; default BastionClass
 100
 101     """
 102
 103     raise RuntimeError, "This code is not secure in Python 2.2 and later"
 104
 105     # Note: we define *two* ad-hoc functions here, get1 and get2.
 106     # Both are intended to be called in the same way: get(name).
 107     # It is clear that the real work (getting the attribute
 108     # from the object and calling the filter) is done in get1.
 109     # Why can't we pass get1 to the bastion?  Because the user
 110     # would be able to override the filter argument!  With get2,
 111     # overriding the default argument is no security loophole:
 112     # all it does is call it.
 113     # Also notice that we can't place the object and filter as
 114     # instance variables on the bastion object itself, since
 115     # the user has full access to all instance variables!
 116
 117     def get1(name, object=object, filter=filter):
 118         """Internal function for Bastion().  See source comments."""
 119         if filter(name):
 120             attribute = getattr(object, name)
 121             if type(attribute) == MethodType:
 122                 return attribute
 123         raise AttributeError, name
 124
 125     def get2(name, get1=get1):
 126         """Internal function for Bastion().  See source comments."""
 127         return get1(name)
 128
 129     if name is None:
 130         name = repr(object)
 131     return bastionclass(get2, name)
 132
 133
 134 def _test():
 135     """Test the Bastion() function."""
 136     class Original:
 137         def __init__(self):
 138             self.sum = 0
 139         def add(self, n):
 140             self._add(n)
 141         def _add(self, n):
 142             self.sum = self.sum + n
 143         def total(self):
 144             return self.sum
 145     o = Original()
 146     b = Bastion(o)
 147     testcode = """if 1:
 148     b.add(81)
 149     b.add(18)
 150     print "b.total() =", b.total()
 151     try:
 152         print "b.sum =", b.sum,
 153     except:
 154         print "inaccessible"
 155     else:
 156         print "accessible"
 157     try:
 158         print "b._add =", b._add,
 159     except:
 160         print "inaccessible"
 161     else:
 162         print "accessible"
 163     try:
 164         print "b._get_.func_defaults =", map(type, b._get_.func_defaults),
 165     except:
 166         print "inaccessible"
 167     else:
 168         print "accessible"
 169     \n"""
 170     exec testcode
 171     print '='*20, "Using rexec:", '='*20
 172     import rexec
 173     r = rexec.RExec()
 174     m = r.add_module('__main__')
 175     m.b = b
 176     r.r_exec(testcode)
 177
 178
 179 if __name__ == '__main__':
 180     _test()