This is my patch #876198 plus a NEWS entry and a header frob.

Remove the ability to use (from C) arbitrary objects supporting the
read buffer interface as the co_code member of code objects.

Include/compile.h

@@ -81,11 +81,6 @@ PyAPI_FUNC(PyCodeObject *) PyNode_CompileFlags(struct _node *, const char *,
 #define FUTURE_GENERATORS "generators"
 #define FUTURE_DIVISION "division"
 
-/* for internal use only */
-#define _PyCode_GETCODEPTR(co, pp) \
-	((*(co)->co_code->ob_type->tp_as_buffer->bf_getreadbuffer) \
-	 ((co)->co_code, 0, (void **)(pp)))
-
 #ifdef __cplusplus
 }
 #endif

Misc/NEWS

@@ -12,6 +12,10 @@ What's New in Python 2.4 alpha 1?
 Core and builtins
 -----------------
 
+- Support for arbitrary objects supporting the read-only buffer
+  interface as the co_code field of code objects (something that was
+  only possible to create from C code) has been removed.
+
 - Made omitted callback and None equivalent for weakref.ref() and
   weakref.proxy(); the None case wasn't handled correctly in all
   cases.

Python/ceval.c

@@ -754,7 +754,7 @@ eval_frame(PyFrameObject *f)
 	consts = co->co_consts;
 	fastlocals = f->f_localsplus;
 	freevars = f->f_localsplus + f->f_nlocals;
-	_PyCode_GETCODEPTR(co, &first_instr);
+	first_instr = PyString_AS_STRING(co->co_code);
 	/* An explanation is in order for the next line.
 
 	   f->f_lasti now refers to the index of the last instruction