Bugs item #1174712, was opened at 2005-04-01 09:22
Message generated for change (Comment added) made by arigo
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Category: Python Interpreter Core
Group: None
>Status: Closed
>Resolution: Fixed
Priority: 5
Private: No
Submitted By: Armin Rigo (arigo)
Assigned to: Nobody/Anonymous (nobody)
Summary: subclassing ModuleType and another built-in type
Initial Comment:
class X(types.ModuleType, str): pass
X('name')
-> segfault
This buggy subclassing goes through typeobject.c's checks because
PyModuleObject looks exactly like a user-defined subclass of 'object': it has a
PyObject_HEAD followed only by the dict, as specified by tp_dictoffset.
A fix would be to forbid any subclassing to move the tp_dictoffset of a
non-heap type.
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>Comment By: Armin Rigo (arigo)
Date: 2007-05-02 19:25
Message:
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Originator: YES
Fixed by zseil's patch for #1303614, r55080.
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Comment By: Armin Rigo (arigo)
Date: 2005-12-14 14:57
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The objection to the proposed fix is not valid in light
of the bug #1303614, which gives a general way to abuse
subclassing to allow the __dict__ of an instance to be
assigned to and deleted, even when it should not be allowed. So I
wouldn't worry too much about the case I pointed up, because it should be
fixed together with #1303614 (though I don't really know how). For now I
would be happy with just checking that subclassing a non-heap type doesn't
move the dict within the structure.
The same remark probably applies to the weakref field. Both cases could
be fixed by being more careful in typeobject.c:extra_ivars(): PyModule_Type
should be considered to have extra_ivars() when compared to
PyBaseObject_Type. This could be achieved by skipping the "t_size -= ..."
part if "type" is not a heap type. Indeed, for non-heap types we should
not try to consider that an extra dict or weakref slot is not important,
because the C code probably accesses this extra slot directly, as in the
case of moduleobject.c.
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Comment By: Michael Hudson (mwh)
Date: 2005-04-03 13:39
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> This might point to the need for cleaning up typeobject.c's
> darker corners...
No kidding. Maybe Samuele, you and I can gang up on Guido at
EuroPython (is he sprinting? Perhaps we should ask).
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Comment By: Armin Rigo (arigo)
Date: 2005-04-02 12:27
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The proposed fix is not good enough. If another built-in C type similar
to PyModuleObject is defined (say by a C extension module), then it would
become possible to subclass both this new type and ModuleType. This might
lead to unwanted behavior if e.g. one parent class allows rebinding
__dict__ and the other not (and crashes if __dict__ is rebound).
This might point to the need for cleaning up typeobject.c's darker
corners...
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