Re: [dpdk-dev] [PATCH v1] examples/vm_power_manager: fix overflowed return value

["Burakov, Anatoly" <anatoly.burakov@intel.com>]

On 26-Apr-19 12:14 PM, Hunt, David wrote:
> Hi Anatoly,
> 
> On 26/4/2019 11:29 AM, Burakov, Anatoly wrote:
>> On 26-Apr-19 9:44 AM, David Hunt wrote:
>>> Coverity complains about the return of a value that may
>>> possibly overflow because of a multiply. Limit the value
>>> so it cannot overflow.
>>>
>>> Coverity issue: 337677
>>> Fixes: 4b1a631b8a ("examples/vm_power: add oob monitoring functions")
>>> CC: stable@dpdk.org
>>> Signed-off-by: David Hunt <david.hunt@intel.com>
>>> ---
>>>   examples/vm_power_manager/oob_monitor_x86.c | 5 ++++-
>>>   1 file changed, 4 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/examples/vm_power_manager/oob_monitor_x86.c 
>>> b/examples/vm_power_manager/oob_monitor_x86.c
>>> index ebd96b205..2074eec1e 100644
>>> --- a/examples/vm_power_manager/oob_monitor_x86.c
>>> +++ b/examples/vm_power_manager/oob_monitor_x86.c
>>> @@ -99,7 +99,10 @@ apply_policy(int core)
>>>           return -1.0;
>>>       }
>>>   -    ratio = (float)miss_diff * (float)100 / (float)hits_diff;
>>> +    ratio = (float)miss_diff / (float)hits_diff;
>>> +    if (ratio > 1.0)
>>> +        ratio = 1.0;
>>> +    ratio *= 100.0f;
>>
>> It should probably be the other way around - multiply first, then 
>> clamp. Also, please use RTE_MIN.
>>
> I tried that, but coverity still sees an overflow condition. I need to 
> clamp first, then multiply. Then coverity is happy.

That's weird. This may be a bug in Coverity then. Please correct me if 
i'm wrong, but floating point formats aren't precise, so by doing 
multiplication on a value that doesn't exceed 1.0, you may very well end 
up with a value that does exceed 100 by a tiny bit on account of 
floating point approximations, rounding errors etc.

The question is, do we want correct code, or do we want to keep Coverity 
happy? :) I'll have a look at the coverity issue itself, maybe i'm 
missing something here...

> 
> Also, do you really want me to change to use RTE_MIN? I honestly prefer 
> the code as it is.

No strong opinion here.

> 
> 
> 
>>>         if (ratio < ci->branch_ratio_threshold)
>>>           power_manager_scale_core_min(core);
>>>
>>
>>
> 


-- 
Thanks,
Anatoly