diff mbox series

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

Message ID 20221226085859.2701195-5-yukuai1@huaweicloud.com
State New
Headers
Series blk-iocost: random bugfix |

Commit Message

Yu Kuai Dec. 26, 2022, 8:58 a.m. UTC 
  From: Li Nan <linan122@huawei.com>

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

Comments

Tejun Heo Jan. 4, 2023, 9:54 p.m. UTC | #1 
On Mon, Dec 26, 2022 at 04:58:58PM +0800, Yu Kuai wrote:
> From: Li Nan <linan122@huawei.com>
> 
> 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@huawei.com>
> Signed-off-by: Yu Kuai <yukuai3@huawei.com>
> ---
>  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;
        }

Out of scope but this seems more like a bug in the DIV macros. The fact that
it returns 0 is an implementation artifact more than anything and a
surprising one at that as it ends up returning 0 for an input that a regular
division would handle just fine and the rounded up result fits well within
the result type.

Thanks.
Yu Kuai Jan. 5, 2023, 1:53 a.m. UTC | #2 
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@huawei.com>
>>
>> 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@huawei.com>
>> Signed-off-by: Yu Kuai <yukuai3@huawei.com>
>> ---
>>   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
diff mbox series

Patch

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));
+	}
 
 	if (seqiops) {
 		v = DIV64_U64_ROUND_UP(VTIME_PER_SEC, seqiops);