[ python-Bugs-1303614 ] Bypassing __dict__ readonlyness

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Bugs item #1303614, was opened at 2005-09-24 23:40
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: Armin Rigo (arigo)
Summary: Bypassing __dict__ readonlyness

Initial Comment:
The __dict__ attribute of some objects is read-only,
e.g. for type objects.  However, there is a generic
way to still access and modify it (without hacking with
gc.get_referrents(), that is).  This can lead to
obscure crashes.  Attached is an example that shows
a potential "problem" involving putting strange keys
in the __dict__ of a type.

This is probably very minor anyway.  If we wanted to
fix this, we would need a __dict__ descriptor that
looks more cleverly at the object to which it is
applied.

BTW the first person who understand why the attached
program crashes gets a free coffee.

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>Comment By: Armin Rigo (arigo)
Date: 2006-06-29 21:19

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Brett: I think you're approaching the problem from the wrong
angle.  The problem is being allowed to freely tamper with
the dict stored in objects.  Getting NULL errors here and
there is only a symptom.  As I explain, the '__dict__'
descriptor object needs to do some more checks, and to be
fully safe some Py_INCREF/Py_DECREF are needed in some
critical places.

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Comment By: Brett Cannon (bcannon)
Date: 2006-06-29 17:45

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For the deldict.py crasher, if you look at the traceback
there is no good place to do a check that tp_dict is sane
except in type_module() or PyDict_GetItem().  Now adding the
NULL check in type_module() will fix this specific problem,
but that seems like an ad-hoc patch.

Why don't we check for NULL in PyDict_GetItem() and return
NULL just like the PyDict_Check() test?  I am sure the
answer is "performance", but this is not PyDict_GETITEM()and
thus already is not the performance-critical version anyway.
 So why shouldn't we check for NULL there and possibly catch
other errors?

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Comment By: Brett Cannon (bcannon)
Date: 2006-06-29 17:41

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Simple patch for the loosing_dict_ref.py crasher is
attached.  Just checked for possible NULL values in what is
needed by _Py_ForgetReference().  Let me know what you
think, Armin.

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Comment By: Michael Hudson (mwh)
Date: 2005-12-14 15:36

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Yikes!

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Comment By: Armin Rigo (arigo)
Date: 2005-12-14 15:32

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A proposed fix for the later crash: the __dict__ descriptor of user-defined classes should verify if there is another __dict__ descriptor along the solid path of the type (i.e. following "tp_base" pointers).  If so, it should delegate to it.

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Comment By: Armin Rigo (arigo)
Date: 2005-12-14 14:49

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Uh oh.  There is a much simpler crash.  The technique outlined in deldict.py can be used to rebind or even delete the __dict__ attribute of instances where it should normally not be allowed.

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Comment By: Armin Rigo (arigo)
Date: 2005-09-25 08:55

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The bug is related to code like PyObject_GenericGetAttribute and _PyType_Lookup which are not careful about the reference counters of the objects they operate on.  This allows a much simpler crash (test67.py): the __dict__ of an object can be decrefed away while lookdict() is looking at it.  This has a simple fix -- drop some amount of Py_INCREF/Py_DECREF in core routines like PyObject_GenericGetAttr.  We probably need to measure if it has a performance impact, though.

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