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

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Message-ID: <d3fe83e4-db71-6180-40e8-e0cfaf52be34@yandex-team.ru>
Date:   Thu, 16 Feb 2023 09:42:13 +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 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!

>>> * 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
>>>>