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

[Manish Chopra]

> -----Original Message-----
> From: Daniil Tatianin <d-tatianin@xxxxxxxxxxxxxx>
> Sent: Friday, March 3, 2023 5:48 PM
> To: Manish Chopra <manishc@xxxxxxxxxxx>; Simon Horman
> <simon.horman@xxxxxxxxxxxx>
> Cc: Ariel Elior <aelior@xxxxxxxxxxx>; David S. Miller
> <davem@xxxxxxxxxxxxx>; Eric Dumazet <edumazet@xxxxxxxxxx>; Jakub
> Kicinski <kuba@xxxxxxxxxx>; Paolo Abeni <pabeni@xxxxxxxxxx>; Yuval Mintz
> <Yuval.Mintz@xxxxxxxxxx>; netdev@xxxxxxxxxxxxxxx; linux-
> kernel@xxxxxxxxxxxxxxx
> Subject: Re: [EXT] Re: [PATCH v0] qed/qed_dev: guard against a possible
> division by zero
> 
> On 2/16/23 9:42 AM, Daniil Tatianin wrote:
> > On 2/16/23 12:20 AM, Manish Chopra wrote:
> >>> -----Original Message-----
> >>> From: Daniil Tatianin <d-tatianin@xxxxxxxxxxxxxx>
> >>> Sent: Tuesday, February 14, 2023 12:53 PM
> >>> To: Simon Horman <simon.horman@xxxxxxxxxxxx>
> >>> Cc: Ariel Elior <aelior@xxxxxxxxxxx>; Manish Chopra
> >>> <manishc@xxxxxxxxxxx>; David S. Miller <davem@xxxxxxxxxxxxx>; Eric
> >>> Dumazet <edumazet@xxxxxxxxxx>; Jakub Kicinski <kuba@xxxxxxxxxx>;
> >>> Paolo Abeni <pabeni@xxxxxxxxxx>; Yuval Mintz
> >>> <Yuval.Mintz@xxxxxxxxxx>; netdev@xxxxxxxxxxxxxxx;
> >>> linux-kernel@xxxxxxxxxxxxxxx
> >>> Subject: [EXT] Re: [PATCH v0] qed/qed_dev: guard against a possible
> >>> division by zero
> >>>
> >>> External Email
> >>>
> >>> --------------------------------------------------------------------
> >>> --
> >>>
> >>>
> >>> 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.
> >> Practically, this flow will only hit with presence of SR-IOV VFs. In
> >> that case it's always expected to have num_vports > 1.
> >
> > In that case, should we add a check and return with -EINVAL otherwise?
> > Thank you!
> >
> 
> Ping
>
It should be fine, please add some log indicating "Unexpected num_vports" before returning error.

Thanks,
Manish
 
> >>>> * 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
> >>>>>