Re: [PATCH v4 4/5] blk-iocost: fix divide by 0 error in calc_lcoefs()

[Tejun Heo]

On Tue, Jan 17, 2023 at 03:08:05PM +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>

Acked-by: Tejun Heo <tj@xxxxxxxxxx>

Thanks.

-- 
tejun