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

[Daniil Tatianin]

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!

> > > * 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