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: 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
> 
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> 
> 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. 

> > * 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
> >>