Re: [PATCH 1/2] python: disallow python code to instanciate certain types

[Phil Muldoon <pmuldoon at redhat dot com>]

On 30/08/13 15:40, Sanimir Agovic wrote:
> I simply followed the pattern used for gdb.Type and assign NULL to tp_new
> thus the type cannot be created from python code.
> 
> before>
> (gdb) python gdb.Frame()
> (gdb)
> 
> after>
> (gdb) python gdb.Frame()
> Traceback (most recent call last):
>   File "<string>", line 1, in <module>
> TypeError: cannot create 'gdb.Frame' instances
> Error while executing Python code.
> 
> 2013-08-30  Sanimir Agovic  <sanimir.agovic@intel.com>

Thanks.


> -static PyObject *
> -objfpy_new (PyTypeObject *type, PyObject *args, PyObject *keywords)
> -{
> -  objfile_object *self = (objfile_object *) type->tp_alloc (type, 0);
> -
> -  if (self)
> -    {
> -      self->objfile = NULL;
> -
> -      self->printers = PyList_New (0);
> -      if (!self->printers)
> -	{
> -	  Py_DECREF (self);
> -	  return NULL;
> -	}
> -
> -      self->frame_filters = PyDict_New ();
> -      if (!self->frame_filters)
> -	{
> -	  Py_DECREF (self);
> -	  return NULL;
> -	}
> -
> -      self->type_printers = PyList_New (0);
> -      if (!self->type_printers)
> -	{
> -	  Py_DECREF (self);
> -	  return NULL;
> -	}
> -    }
> -  return (PyObject *) self;
> -}

In removing this function, you are removing the instantiation of
self->printers (Which is needed for pretty-printers),
self->frame-filters (for frame filters) and self->type_printers (for
type printers).  If this is the case, and I have not missed an
equivalent instantiation somewhere else, you will have an
uninitialized PyObject for each of the above and likely will result in
a crash?  I'm surprised this did not crash GDB.  Each object file
keeps a separate list or dictionary of frame filters, pretty printers
and type printers that may be populated for object file specific
scenarios (like auto-loading). Also, I suspect the corresponding
clean-up function for these objects will crash when they are freed.


> -static PyObject *
> -pspy_new (PyTypeObject *type, PyObject *args, PyObject *keywords)
> -{
> -  pspace_object *self = (pspace_object *) type->tp_alloc (type, 0);
> -
> -  if (self)
> -    {
> -      self->pspace = NULL;
> -
> -      self->printers = PyList_New (0);
> -      if (!self->printers)
> -	{
> -	  Py_DECREF (self);
> -	  return NULL;
> -	}
> -
> -      self->frame_filters = PyDict_New ();
> -      if (!self->frame_filters)
> -	{
> -	  Py_DECREF (self);
> -	  return NULL;
> -	}
> -
> -      self->type_printers = PyList_New (0);
> -      if (!self->type_printers)
> -	{
> -	  Py_DECREF (self);
> -	  return NULL;
> -	}
> -    }
> -  return (PyObject *) self;
> -}
> -

Ditto, same with object files above.

Cheers,

Phil