Re: [PATCH -next v2 5/9] blk-iocost: fix divide by 0 error in calc_lcoefs()

Tejun Heo
2022-11-30T20:22:43Z

On Wed, Nov 30, 2022 at 09:21:52PM +0800, Li Nan wrote:
> 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.I_LCOEF_MAX is introduced to
> prevent it.
>
> Signed-off-by: Li Nan <linan122@huawei.com>
> ---
> block/blk-iocost.c | 5 +++++
> 1 file changed, 5 insertions(+)
>
> diff --git a/block/blk-iocost.c b/block/blk-iocost.c
> index 3a96cd557c47..f4a754b9d10f 100644
> --- a/block/blk-iocost.c
> +++ b/block/blk-iocost.c
> @@ -306,6 +306,9 @@ enum {
> IOC_PAGE_SIZE = 1 << IOC_PAGE_SHIFT,
> IOC_SECT_TO_PAGE_SHIFT = IOC_PAGE_SHIFT - SECTOR_SHIFT,
>
> + /* avoid overflow */
> + I_LCOEF_MAX = ULLONG_MAX - IOC_PAGE_SIZE,
> +
> /* if apart further than 16M, consider randio for linear model */
> LCOEF_RANDIO_PAGES = 4096,
> };
> @@ -3431,6 +3434,8 @@ static ssize_t ioc_cost_model_write(struct kernfs_open_file *of, char *input,
> goto out_unlock;
> }
>
> + if (v > I_LCOEF_MAX)
> + goto out_unlock;

This is kinda round-about. Can you just make calc_lcoefs to not divide by
zero (e.g. just make it 1 if zero)?

Thanks.

--
tejun