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

[Yu Kuai]

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