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

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Date:   Fri, 3 Mar 2023 15:18:21 +0300
From:   Daniil Tatianin <d-tatianin@...dex-team.ru>
To:     Manish Chopra <manishc@...vell.com>,
        Simon Horman <simon.horman@...igine.com>
Cc:     Ariel Elior <aelior@...vell.com>,
        "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        Yuval Mintz <Yuval.Mintz@...gic.com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
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@...dex-team.ru>
>>> Sent: Tuesday, February 14, 2023 12:53 PM
>>> To: Simon Horman <simon.horman@...igine.com>
>>> Cc: Ariel Elior <aelior@...vell.com>; Manish Chopra
>>> <manishc@...vell.com>; David S. Miller <davem@...emloft.net>; Eric
>>> Dumazet <edumazet@...gle.com>; Jakub Kicinski <kuba@...nel.org>; Paolo
>>> Abeni <pabeni@...hat.com>; Yuval Mintz <Yuval.Mintz@...gic.com>;
>>> netdev@...r.kernel.org; linux-kernel@...r.kernel.org
>>> 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@...dex-team.ru>
>>>>> ---
>>>>>    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

>>>> * 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
>>>>>