Re: [PATCH] hw/ide: Don't divide by zero if guest specifies 0 sectors

Ping^2 ?

thanks
-- PMM

On Mon, 20 Jul 2026 at 09:42, Peter Maydell <[email protected]> wrote:
>
> Ping -- would anybody on the qemu-block side like to review this?
>
> thanks
> -- PMM
>
> On Tue, 30 Jun 2026 at 11:42, Peter Maydell <[email protected]> wrote:
> >
> > In ide_set_sector() if we're using CHS addressing the way we update
> > the drive state involves dividing by (s->heads * s->sectors).  These
> > values can be set by the guest using the INITIALIZE DRIVE PARAMETERS
> > command, which means that s->sectors can be 0.  (s->heads can't be 0
> > because the command passes a heads-1 value.) INITIALIZE DRIVE
> > PARAMETERS is specified to not check the input values for validity;
> > instead no error is posted until some other command makes an illegal
> > access. So we have to cope with a 0 divisor here.
> >
> > Special-case s->sectors being zero.
> >
> > Cc: [email protected]
> > Resolves: https://gitlab.com/qemu-project/qemu/-/work_items/2399
> > Signed-off-by: Peter Maydell <[email protected]>
> > ---
> > We don't seem to ever actually check that the guest's read/write ops
> > are within the CHS limits it sets with INITIALIZE DRIVE PARAMETERS --
> > ide_sect_range_ok() only checks against the underlying block device
> > geometry.  Maybe we should?  Then a zero-sectors misconfig would
> > result in our reporting the error on the read/write as the spec
> > suggests we should.  This patch fixes the division-by-zero, at least,
> > though.
> > ---
> >  hw/ide/core.c | 28 +++++++++++++++++++++-------
> >  1 file changed, 21 insertions(+), 7 deletions(-)
> >
> > diff --git a/hw/ide/core.c b/hw/ide/core.c
> > index f78b00220b..b030531534 100644
> > --- a/hw/ide/core.c
> > +++ b/hw/ide/core.c
> > @@ -646,13 +646,27 @@ void ide_set_sector(IDEState *s, int64_t sector_num)
> >          }
> >      } else {
> >          /* CHS */
> > -        cyl = sector_num / (s->heads * s->sectors);
> > -        r = sector_num % (s->heads * s->sectors);
> > -        s->hcyl = cyl >> 8;
> > -        s->lcyl = cyl;
> > -        s->select = (s->select & ~(ATA_DEV_HS)) |
> > -            ((r / s->sectors) & (ATA_DEV_HS));
> > -        s->sector = (r % s->sectors) + 1;
> > +        if (s->sectors == 0) {
> > +            /*
> > +             * s->sectors is under guest control via INITIALIZE DRIVE
> > +             * PARAMETERS and can be 0; this is an error but can't
> > +             * be reported at INITIALIZE DRIVE PARAMETERS time so we
> > +             * have to cope with the bad value here.
> > +             * (s->heads is always at least 1.)
> > +             */
> > +            s->hcyl = 0;
> > +            s->lcyl = 0;
> > +            s->select = s->select & ~(ATA_DEV_HS);
> > +            s->sector = 1;
> > +        } else {
> > +            cyl = sector_num / (s->heads * s->sectors);
> > +            r = sector_num % (s->heads * s->sectors);
> > +            s->hcyl = cyl >> 8;
> > +            s->lcyl = cyl;
> > +            s->select = (s->select & ~(ATA_DEV_HS)) |
> > +                ((r / s->sectors) & (ATA_DEV_HS));
> > +            s->sector = (r % s->sectors) + 1;
> > +        }
> >      }
> >  }