Re: Obscure crashes due to gcc 4.9 -O2 => -fisolate-erroneous-paths-dereference

[Florian Weimer <fweimer@redhat.com>]

On 02/20/2015 10:30 AM, Andrew Haley wrote:
> I doubt that such a thing is ever going to be safe.  The idea that a
> null pointer points to nothing is so hard-baked into the design of C
> that you can't get away from it.  Also, almost every C programmer and
> especially library writer "knows" that a null pointer points to
> nothing.

NULL pointer dereferences (or NULL pointer with small offsets) were
common programming idioms in the DOS days because the interrupt vector
table was located at this address.  Quite a few systems once had a
readable page zero, and (manual, I assume) optimizations for list
traversal (p != NULL && p->next != NULL → p->next != NULL) were commonly
used on these systems.

I think the treatment of pointers not as addresses, but something that
has type information and provenience associated with it, came much
later, when most of the design was already settled.

-- 
Florian Weimer / Red Hat Product Security