Re: [OpenBIOS] [PATCH 9/10] Divide by zero check

[Mark Cave-Ayland]

On 09/08/11 22:55, William Hahne wrote:
> It is possible for BootX to purposefully divide by zero expecting 0 as
> the result.
> Index: kernel/forth.c
>
> --- kernel/forth.c (revision 1041)
> +++ kernel/forth.c (working copy)
> @@ -1157,6 +1157,12 @@
>   {
> const ucell b = POP();
> const ducell a = DPOP();
> if (b == 0) { // can't divide by zero
>        PUSH(0);
>        DPUSH(0);
>        return;
> }
> #ifdef NEED_FAKE_INT128_T
>         if (a.hi != 0) {
>             fprintf(stderr, "mudivmod called (0x%016llx %016llx /
>
> 0x%016llx)\n",

There is something wrong here: nothing will purposefully divide by zero 
as it will invoke a trap somewhere. Again, I think this is hiding 
another bug somewhere and is not necessarily the correct fix.

ATB,

Mark.

[PATCH 9/10] Divide by zero check

[William Hahne]

On Wed, Aug 10, 2011 at 6:00 AM, Mark Cave-Ayland <
[e-mail]> wrote:
> On 09/08/11 22:55, William Hahne wrote:
>
> It is possible for BootX to purposefully divide by zero expecting 0 as
> > the result.
> >
> > Index: kernel/forth.c
> > ==============================**==============================**=======
> > --- kernel/forth.c (revision 1041)
> > +++ kernel/forth.c (working copy)
> > @@ -1157,6 +1157,12 @@
> >  {
> > const ucell b = POP();
> > const ducell a = DPOP();
> > if (b == 0) { // can't divide by zero
> >        PUSH(0);
> >        DPUSH(0);
> >        return;
> > }
> >
> > #ifdef NEED_FAKE_INT128_T
> >          if (a.hi != 0) {
> >              fprintf(stderr, "mudivmod called (0x%016llx %016llx /
> > 0x%016llx)\n",
>
> There is something wrong here: nothing will purposefully divide by zero as
> it will invoke a trap somewhere. Again, I think this is hiding another bug
> somewhere and is not necessarily the correct fix.

You would be surprised then, if you type "0 0 /" into a OF mac you will get
0. I thought it was a bug as well at first but after carefully looking at
the source code for BootX I found that it wasn't. This is probably just a
peculiarity of Apple's OF implementation.
> ATB,
>
> Mark.
>
> Sirius Labs: http://www.siriusit.co.uk/labs

William Hahne