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: Open
Resolution: None
Priority: 5
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: 2005-12-14 14:57
Message:
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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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