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

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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




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