Re: [PATCH v2] blk-iocost: fix shift-out-of-bounds in iocg_hick_delay()

[Yu Kuai]

Hi,

在 2022/11/29 10:49, Li Jinlin 写道:
> On 2022/11/29 9:14, Yu Kuai wrote:
> > Hi,
> >
> > 在 2022/11/29 3:58, Tejun Heo 写道:
> > > On Mon, Nov 28, 2022 at 11:04:13AM +0800, Li Jinlin wrote:
> > > >        /* calculate the current delay in effect - 1/2 every second */
> > > >        tdelta = now->now - iocg->delay_at;
> > > >        if (iocg->delay)
> > > > -        delay = iocg->delay >> div64_u64(tdelta, USEC_PER_SEC);
> > > > +        delay = iocg->delay >>
> > > > +            min_t(u64, div64_u64(tdelta, USEC_PER_SEC), 63);
> > >
> > > I replied earlier but the right thing to do here is setting delay to 0 if
> > > the shift is >= 64.
> >
> > Perhaps following change will make more sense?
> >
> > @@ -1322,18 +1323,19 @@ static bool iocg_kick_delay(struct ioc_gq *iocg, struct ioc_now *now)
> >   {
> >          struct ioc *ioc = iocg->ioc;
> >          struct blkcg_gq *blkg = iocg_to_blkg(iocg);
> > -       u64 tdelta, delay, new_delay;
> > +       u64 delay = 0;
> > +       u64 new_delay;
> >          s64 vover, vover_pct;
> >          u32 hwa;
> >
> >          lockdep_assert_held(&iocg->waitq.lock);
> >
> >          /* calculate the current delay in effect - 1/2 every second */
> > -       tdelta = now->now - iocg->delay_at;
> > -       if (iocg->delay)
> > +       if (iocg->delay && now->now > iocg->delay_at) {
> > +               u64 tdelta = now->now - iocg->delay_at;
> > +
> >                  delay = iocg->delay >> div64_u64(tdelta, USEC_PER_SEC);
> > -       else
> > -               delay = 0;
> > +       }
> I think "now->now > iocg->delay_at" is unnecessary, it is almost inevitable.

From what I see, following can only happen if now->now < iocg->delay_at:

"shift exponent 18446744073709"

Or something else triggers it?

Thanks,
Kuai