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

[Daniil Tatianin]

On 3/7/23 8:50 PM, Manish Chopra wrote:
> > -----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

Will do. 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