blk_iocost: fix more out of bound shifts

Recently running UBSAN caught few out of bound shifts in the
ioc_forgive_debts() function:

UBSAN: shift-out-of-bounds in block/blk-iocost.c:2142:38
shift exponent 80 is too large for 64-bit type 'u64' (aka 'unsigned long
long')
...
UBSAN: shift-out-of-bounds in block/blk-iocost.c:2144:30
shift exponent 80 is too large for 64-bit type 'u64' (aka 'unsigned long
long')
...
Call Trace:
<IRQ>
dump_stack_lvl+0xca/0x130
__ubsan_handle_shift_out_of_bounds+0x22c/0x280
? __lock_acquire+0x6441/0x7c10
ioc_timer_fn+0x6cec/0x7750
? blk_iocost_init+0x720/0x720
? call_timer_fn+0x5d/0x470
call_timer_fn+0xfa/0x470
? blk_iocost_init+0x720/0x720
__run_timer_base+0x519/0x700
...

Actual impact of this issue was not identified but I propose to fix the
undefined behaviour.
The proposed fix to prevent those out of bound shifts consist of
precalculating exponent before using it the shift operations by taking
min value from the actual exponent and maximum possible number of bits.

Reported-by: Breno Leitao <[email protected]>
Signed-off-by: Konstantin Ovsepian <[email protected]>
Acked-by: Tejun Heo <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Jens Axboe <[email protected]>

Author: Konstantin Ovsepian

block/blk-iocost.c

@@ -2076,7 +2076,7 @@ static void ioc_forgive_debts(struct ioc *ioc, u64 usage_us_sum, int nr_debtors,
 			      struct ioc_now *now)
 {
 	struct ioc_gq *iocg;
-	u64 dur, usage_pct, nr_cycles;
+	u64 dur, usage_pct, nr_cycles, nr_cycles_shift;
 
 	/* if no debtor, reset the cycle */
 	if (!nr_debtors) {
@@ -2138,10 +2138,12 @@ static void ioc_forgive_debts(struct ioc *ioc, u64 usage_us_sum, int nr_debtors,
 		old_debt = iocg->abs_vdebt;
 		old_delay = iocg->delay;
 
+		nr_cycles_shift = min_t(u64, nr_cycles, BITS_PER_LONG - 1);
 		if (iocg->abs_vdebt)
-			iocg->abs_vdebt = iocg->abs_vdebt >> nr_cycles ?: 1;
+			iocg->abs_vdebt = iocg->abs_vdebt >> nr_cycles_shift ?: 1;
+
 		if (iocg->delay)
-			iocg->delay = iocg->delay >> nr_cycles ?: 1;
+			iocg->delay = iocg->delay >> nr_cycles_shift ?: 1;
 
 		iocg_kick_waitq(iocg, true, now);