Re: [PATCH v0] qed/qed_dev: guard against a possible division by zero

[Daniil Tatianin]

On 2/9/23 2:13 PM, Simon Horman wrote:
> On Thu, Feb 09, 2023 at 01:38:13PM +0300, Daniil Tatianin wrote:
> > Previously we would divide total_left_rate by zero if num_vports
> > happened to be 1 because non_requested_count is calculated as
> > num_vports - req_count. Guard against this by explicitly checking for
> > zero when doing the division.
> >
> > Found by Linux Verification Center (linuxtesting.org) with the SVACE
> > static analysis tool.
> >
> > Fixes: bcd197c81f63 ("qed: Add vport WFQ configuration APIs")
> > Signed-off-by: Daniil Tatianin <d-tatianin@xxxxxxxxxxxxxx>
> > ---
> >   drivers/net/ethernet/qlogic/qed/qed_dev.c | 2 +-
> >   1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/drivers/net/ethernet/qlogic/qed/qed_dev.c b/drivers/net/ethernet/qlogic/qed/qed_dev.c
> > index d61cd32ec3b6..90927f68c459 100644
> > --- a/drivers/net/ethernet/qlogic/qed/qed_dev.c
> > +++ b/drivers/net/ethernet/qlogic/qed/qed_dev.c
> > @@ -5123,7 +5123,7 @@ static int qed_init_wfq_param(struct qed_hwfn *p_hwfn,
> >   
> >   	total_left_rate	= min_pf_rate - total_req_min_rate;
> >   
> > -	left_rate_per_vp = total_left_rate / non_requested_count;
> > +	left_rate_per_vp = total_left_rate / (non_requested_count ?: 1);
>
> I don't know if num_vports can be 1.
> But if it is then I agree that the above will be a divide by zero.
>
> I do, however, wonder if it would be better to either:
>
> * Treat this case as invalid and return with -EINVAL if num_vports is 1; or
I think that's a good idea considering num_vports == 1 is indeed an 
invalid value.
I'd like to hear a maintainer's opinion on this.
> * Skip both the calculation immediately above and the code
>    in the if condition below, which is the only place where
>    the calculated value is used, if num_vports is 1.
>    I don't think the if clause makes much sense if num_vports is one.left_rate_per_vp is also used below the if clause, it is assigned to 
.min_speed in a for loop. Looking at that code division by 1 seems to 
make sense to me in this case.
> >   	if (left_rate_per_vp <  min_pf_rate / QED_WFQ_UNIT) {
> >   		DP_VERBOSE(p_hwfn, NETIF_MSG_LINK,
> >   			   "Non WFQ configured vports rate [%d Mbps] is less than one percent of configured PF min rate[%d Mbps]\n",
> > --
> > 2.25.1