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Re: Obscure crashes due to gcc 4.9 -O2 => -fisolate-erroneous-paths-dereference
- From: Florian Weimer <fweimer at redhat dot com>
- To: Andrew Haley <aph at redhat dot com>, Jeff Prothero <jprother at altera dot com>, gcc at gcc dot gnu dot org
- Date: Fri, 20 Feb 2015 12:45:14 +0100
- Subject: Re: Obscure crashes due to gcc 4.9 -O2 => -fisolate-erroneous-paths-dereference
- Authentication-results: sourceware.org; auth=none
- References: <pdf61azt48b dot fsf at sj-interactive3 dot altera dot com> <54E6FEA4 dot 8000104 at redhat dot 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 â 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