Re: NULL pointer or objptr should not pass the validity check.

I need to take some more investigation on it to see when and where the NULL pointer is introduced to the heap. Thank you for explanation on it.

在 2013-5-11，下午6:48，Matthew Fluet <[email protected]> 写道：

> On Thu, May 9, 2013 at 5:27 AM, 吴佳森 <[email protected]> wrote:
>> When I was porting the MLton, I met two segment faults. And after careful
>> inspection on the trace, I made following changes to runtime/gc/objptr.c and
>> runtime/gc/pointer.c.
>> 
>> The changes are about deciding whether the given pointer or objptr is valid.
>> The original code only check the last few bits must be zero, and hence NULL
>> pointer is also considered valid. I don't think NULL pointer should pass the
>> check hence made the changes.
>> 
>> I am not sure if I'm correct on the judgement because I guess if may also be
>> some problem elsewhere that introduce the NULL pointer to GC.
>> 
>> Could someone please have a check on it? I would appreciate any feed back
>> for it. Thank you very much.
> 
> The isObjptr and isPointer functions are used by the garbage collector
> to distinguish true pointers to ML objects from other data that is
> packed into a pointer-sized field (and that could, sometimes, be a
> true pointer).  The canonical example is a type like:
> 
>  (int * int) option
> 
> The SOME variant must be represented as a pointer to the (int * int)
> tuple, but the NONE variant can be represented by a pointer-sized word
> with non-zero low bits.
> 
> This should always be the case --- either the word being inspected by
> isObjptr/isPointer is a true pointer or it has non-zero low bits.  I
> don't believe that it these functions should ever be called on NULL.
> 
> I think the better choice would be to add:
>  assert (0 != p)
> 
> If these functions are being called on NULL, then the heap has been
> corrupted at some earlier point.  Returning 'false' and treating the
> NULL as a non-pointer will simply mask the corruption for a little
> longer.
> 
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