Re: [PATCH v3 4/5] blk-iocost: fix divide by 0 error in calc_lcoefs()

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Hi,

在 2023/01/05 5:54, Tejun Heo 写道:
On Mon, Dec 26, 2022 at 04:58:58PM +0800, Yu Kuai wrote:
From: Li Nan <linan122@xxxxxxxxxx>

echo max of u64 to cost.model can cause divide by 0 error.

   # echo 8:0 rbps=18446744073709551615 > /sys/fs/cgroup/io.cost.model

   divide error: 0000 [#1] PREEMPT SMP
   RIP: 0010:calc_lcoefs+0x4c/0xc0
   Call Trace:
    <TASK>
    ioc_refresh_params+0x2b3/0x4f0
    ioc_cost_model_write+0x3cb/0x4c0
    ? _copy_from_iter+0x6d/0x6c0
    ? kernfs_fop_write_iter+0xfc/0x270
    cgroup_file_write+0xa0/0x200
    kernfs_fop_write_iter+0x17d/0x270
    vfs_write+0x414/0x620
    ksys_write+0x73/0x160
    __x64_sys_write+0x1e/0x30
    do_syscall_64+0x35/0x80
    entry_SYSCALL_64_after_hwframe+0x63/0xcd

calc_lcoefs() uses the input value of cost.model in DIV_ROUND_UP_ULL,
overflow would happen if bps plus IOC_PAGE_SIZE is greater than
ULLONG_MAX, it can cause divide by 0 error.

Fix the problem by setting basecost

Signed-off-by: Li Nan <linan122@xxxxxxxxxx>
Signed-off-by: Yu Kuai <yukuai3@xxxxxxxxxx>
---
  block/blk-iocost.c | 10 +++++++---
  1 file changed, 7 insertions(+), 3 deletions(-)

diff --git a/block/blk-iocost.c b/block/blk-iocost.c
index f8726e20da20..c6b39024117b 100644
--- a/block/blk-iocost.c
+++ b/block/blk-iocost.c
@@ -866,9 +866,13 @@ static void calc_lcoefs(u64 bps, u64 seqiops, u64 randiops,
*page = *seqio = *randio = 0; - if (bps)
-		*page = DIV64_U64_ROUND_UP(VTIME_PER_SEC,
-					   DIV_ROUND_UP_ULL(bps, IOC_PAGE_SIZE));
+	if (bps) {
+		if (bps >= U64_MAX - IOC_PAGE_SIZE)
+			*page = 1;
+		else
+			*page = DIV64_U64_ROUND_UP(VTIME_PER_SEC,
+					DIV_ROUND_UP_ULL(bps, IOC_PAGE_SIZE));
+	}

This is a nitpick but wouldn't something like the following be easier to
understand?

         if (bps) {
                 u64 bps_pages = DIV_ROUND_UP_ULL(bps, IOC_PAGE_SIZE);

                 if (bps_pages)
                         *pages = DIV64_U64_ROUND_UP(VTIME_PER_SEC, bps_pages);
                 else
                         *pages = 1;
         }

Yes, I agree that this is better to understand. I'll send a new version.

Thanks,
Kuai




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