Re: [OT] Testing PROT_NONE and other protections, and a surprise

Matt Mackall
2004-07-01T16:37:40Z

On Thu, Jul 01, 2004 at 11:01:52AM -0400, Kyle Moffett wrote:
> On Jul 01, 2004, at 10:50, Jamie Lokier wrote:
> >Kyle Moffett wrote:
> >>>The error code is -1, aka. MAP_FAILED.
> >>Oops! I guess I was just lucky that part didn't fail :-D On the
> >>other hand, it couldn't legally return 0 anyway, could it?
> >
> >Yes it could -- if you request a mapping at address 0 with MAP_FIXED.
> >A few OSes won't do that, but Linux and many others will.
>
> That allows untrapped dereferencing of a NULL pointer. IMHO, that
> would be a very unintelligent thing for a program to do, to deny itself
> the bug-catching features provided therein, but it's interesting to see
> that it is possible.

A typical use is vm86-based emulation of 16-bit DOS where there's data
in the immediate vicinity of NULL.

--
Mathematics is the supreme nostalgia of our time.