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

Denis, would you be interested?

Peter Maydell <[email protected]> writes:

> 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;
>> > +        }
>> >      }
>> >  }