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

[Daniil Tatianin <d-tatianin@yandex-team.ru>]

On 3/7/23 8:50 PM, Manish Chopra wrote:
>> -----Original Message-----
>> From: Daniil Tatianin <d-tatianin@yandex-team.ru>
>> Sent: Friday, March 3, 2023 5:48 PM
>> To: Manish Chopra <manishc@marvell.com>; Simon Horman
>> <simon.horman@corigine.com>
>> Cc: Ariel Elior <aelior@marvell.com>; David S. Miller
>> <davem@davemloft.net>; Eric Dumazet <edumazet@google.com>; Jakub
>> Kicinski <kuba@kernel.org>; Paolo Abeni <pabeni@redhat.com>; Yuval Mintz
>> <Yuval.Mintz@qlogic.com>; netdev@vger.kernel.org; linux-
>> kernel@vger.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@yandex-team.ru>
>>>>> Sent: Tuesday, February 14, 2023 12:53 PM
>>>>> To: Simon Horman <simon.horman@corigine.com>
>>>>> Cc: Ariel Elior <aelior@marvell.com>; Manish Chopra
>>>>> <manishc@marvell.com>; David S. Miller <davem@davemloft.net>; Eric
>>>>> Dumazet <edumazet@google.com>; Jakub Kicinski <kuba@kernel.org>;
>>>>> Paolo Abeni <pabeni@redhat.com>; Yuval Mintz
>>>>> <Yuval.Mintz@qlogic.com>; netdev@vger.kernel.org;
>>>>> linux-kernel@vger.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@yandex-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
>>
> 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
>>>>>>>