Re: [Qemu-devel] [PATCH 1/2] block: Detect multiplication overflow in bdrv_getlength

[Fam Zheng <famz@redhat.com>]

On Fri, 05/15 10:10, Markus Armbruster wrote:
> Fam Zheng <famz@redhat.com> writes:
> 
> > Bogus image may have a large total_sectors that will overflow the
> > multiplication. For cleanness, fix the return code so the error message
> > will be meaningful.
> >
> > Reported-by: Richard W.M. Jones <rjones@redhat.com>
> > Signed-off-by: Fam Zheng <famz@redhat.com>
> > ---
> >  block.c | 1 +
> >  1 file changed, 1 insertion(+)
> >
> > diff --git a/block.c b/block.c
> > index 7904098..5ee3fdf 100644
> > --- a/block.c
> > +++ b/block.c
> > @@ -2330,6 +2330,7 @@ int64_t bdrv_getlength(BlockDriverState *bs)
> >  {
> >      int64_t ret = bdrv_nb_sectors(bs);
> >  
> > +    ret = (int64_t)(ret * BDRV_SECTOR_SIZE) < 0 ? -EFBIG : ret;
> >      return ret < 0 ? ret : ret * BDRV_SECTOR_SIZE;
> >  }
> 
> Signed integer overflow is undefined behavior.  Your code works just
> fine on any remotely sane machine, *except* when the optimizer decides
> to use its undefined behavior license to mess with you.
> 
> A more prudent way to test for overflow would be something like
> 
>     ret > INT64_MAX / BDRV_SECTOR_SIZE

Yes, this is better, will fix.

Thanks,
Fam