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 

    29 __all__ = ["BastionClass", "Bastion"]

    30 

    31 from types import MethodType

    32 

    33 

    34 class BastionClass:

    35 

    36     """Helper class used by the Bastion() function.

    37 

    38     You could subclass this and pass the subclass as the bastionclass

    39     argument to the Bastion() function, as long as the constructor has

    40     the same signature (a get() function and a name for the object).

    41 

    42     """

    43 

    44     def __init__(self, get, name):

    45         """Constructor.

    46 

    47         Arguments:

    48 

    49         get - a function that gets the attribute value (by name)

    50         name - a human-readable name for the original object

    51                (suggestion: use repr(object))

    52 

    53         """

    54         self._get_ = get

    55         self._name_ = name

    56 

    57     def __repr__(self):

    58         """Return a representation string.

    59 

    60         This includes the name passed in to the constructor, so that

    61         if you print the bastion during debugging, at least you have

    62         some idea of what it is.

    63 

    64         """

    65         return "<Bastion for %s>" % self._name_

    66 

    67     def __getattr__(self, name):

    68         """Get an as-yet undefined attribute value.

    69 

    70         This calls the get() function that was passed to the

    71         constructor.  The result is stored as an instance variable so

    72         that the next time the same attribute is requested,

    73         __getattr__() won't be invoked.

    74 

    75         If the get() function raises an exception, this is simply

    76         passed on -- exceptions are not cached.

    77 

    78         """

    79         attribute = self._get_(name)

    80         self.__dict__[name] = attribute

    81         return attribute

    82 

    83 

    84 def Bastion(object, filter = lambda name: name[:1] != '_',

    85             name=None, bastionclass=BastionClass):

    86     """Create a bastion for an object, using an optional filter.

    87 

    88     See the Bastion module's documentation for background.

    89 

    90     Arguments:

    91 

    92     object - the original object

    93     filter - a predicate that decides whether a function name is OK;

    94              by default all names are OK that don't start with '_'

    95     name - the name of the object; default repr(object)

    96     bastionclass - class used to create the bastion; default BastionClass

    97 

    98     """

    99 

   100     raise RuntimeError, "This code is not secure in Python 2.2 and later"

   101 

   102     # Note: we define *two* ad-hoc functions here, get1 and get2.

   103     # Both are intended to be called in the same way: get(name).

   104     # It is clear that the real work (getting the attribute

   105     # from the object and calling the filter) is done in get1.

   106     # Why can't we pass get1 to the bastion?  Because the user

   107     # would be able to override the filter argument!  With get2,

   108     # overriding the default argument is no security loophole:

   109     # all it does is call it.

   110     # Also notice that we can't place the object and filter as

   111     # instance variables on the bastion object itself, since

   112     # the user has full access to all instance variables!

   113 

   114     def get1(name, object=object, filter=filter):

   115         """Internal function for Bastion().  See source comments."""

   116         if filter(name):

   117             attribute = getattr(object, name)

   118             if type(attribute) == MethodType:

   119                 return attribute

   120         raise AttributeError, name

   121 

   122     def get2(name, get1=get1):

   123         """Internal function for Bastion().  See source comments."""

   124         return get1(name)

   125 

   126     if name is None:

   127         name = repr(object)

   128     return bastionclass(get2, name)

   129 

   130 

   131 def _test():

   132     """Test the Bastion() function."""

   133     class Original:

   134         def __init__(self):

   135             self.sum = 0

   136         def add(self, n):

   137             self._add(n)

   138         def _add(self, n):

   139             self.sum = self.sum + n

   140         def total(self):

   141             return self.sum

   142     o = Original()

   143     b = Bastion(o)

   144     testcode = """if 1:

   145     b.add(81)

   146     b.add(18)

   147     print "b.total() =", b.total()

   148     try:

   149         print "b.sum =", b.sum,

   150     except:

   151         print "inaccessible"

   152     else:

   153         print "accessible"

   154     try:

   155         print "b._add =", b._add,

   156     except:

   157         print "inaccessible"

   158     else:

   159         print "accessible"

   160     try:

   161         print "b._get_.func_defaults =", map(type, b._get_.func_defaults),

   162     except:

   163         print "inaccessible"

   164     else:

   165         print "accessible"

   166     \n"""

   167     exec testcode

   168     print '='*20, "Using rexec:", '='*20

   169     import rexec

   170     r = rexec.RExec()

   171     m = r.add_module('__main__')

   172     m.b = b

   173     r.r_exec(testcode)

   174 

   175 

   176 if __name__ == '__main__':

   177     _test()