答复: [PATCH] qcow2-bitmap: Fix uint64_t left-shift overflow

[Tuguoyi <tu.guoyi@h3c.com>]

> -----邮件原件-----
> 发件人: Vladimir Sementsov-Ogievskiy [mailto:vsementsov@virtuozzo.com]
> 发送时间: 2019年10月27日 0:50
> 收件人: tuguoyi (Cloud) <tu.guoyi@h3c.com>; kwolf@redhat.com;
> mreitz@redhat.com; qemu-block@nongnu.org
> 抄送: chengchiwen (Cloud) <chengchiwen@h3c.com>;
> qemu-devel@nongnu.org; wangyongqing (Cloud) <w_yongqing@h3c.com>;
> changlimin (Cloud) <changlimin@h3c.com>; gaoliang (Cloud)
> <liang_gao@h3c.com>; wangyong (Cloud) <wang.yongD@h3c.com>
> 主题: Re: [PATCH] qcow2-bitmap: Fix uint64_t left-shift overflow
> 
> 26.10.2019 12:19, Tuguoyi wrote:
> > In check_constraints_on_bitmap(), the sanity check on the granularity
> > will cause uint64_t integer left-shift overflow when cluster_size is
> > 2M and the granularity is bigger than 32K which is even smaller than
> > the default value for a qcow2 disk with cluster_size set to 64k or
> > bigger. This patch fix the issue by right-shift @len instead.
> >
> > Signed-off-by: Guoyi Tu <tu.guoyi@h3c.com>
> > ---
> >   block/qcow2-bitmap.c | 4 ++--
> >   1 file changed, 2 insertions(+), 2 deletions(-)
> >
> > diff --git a/block/qcow2-bitmap.c b/block/qcow2-bitmap.c index
> > 98294a7..2a1d789 100644
> > --- a/block/qcow2-bitmap.c
> > +++ b/block/qcow2-bitmap.c
> > @@ -172,8 +172,8 @@ static int
> check_constraints_on_bitmap(BlockDriverState *bs,
> >       }
> >
> >       if ((len > (uint64_t)BME_MAX_PHYS_SIZE << granularity_bits) ||
> > -        (len > (uint64_t)BME_MAX_TABLE_SIZE * s->cluster_size <<
> > -               granularity_bits))
> 
> Hmm.
> BME_MAX_TABLE_SIZE = 0x8000000
> 
> 0x8000000 * 1024 * 1024 * 2 << 16 = 2 ** 64, so for 64k granularity it
> owerflows..
> But for 32k doesn't. Or am I wrong?

You are right, it doesn't overflow for 32K.

> 
> Anyway, thanks for fixing!
> 
> > +        ((len >> granularity_bits) > (uint64_t)BME_MAX_TABLE_SIZE *
> > +                s->cluster_size))
> 
> It's a bit incorrect, as len may be unaligned, we need ((len + granularity - 1) >>
> granularity_bits) on the left, or better DIV_ROUNTD_UP(len, granularity).

Yes, @len should be ROUND-UP, thanks for pointing it out, and I'll fix it and send another patch

> 
> >       {
> >           error_setg(errp, "Too much space will be occupied by the
> bitmap. "
> >                      "Use larger granularity");
> > --
> > 2.7.4
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> 
> Not sure that this is possible, as it's automatically available here:
> https://lists.gnu.org/archive/html/qemu-devel/2019-10/msg07336.html
> 
> 
> --
> Best regards,
> Vladimir

Thanks a lot for reviewing it