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

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

On 26-Apr-19 1:03 PM, Burakov, Anatoly wrote:
> 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...
> 

I think the real source of the problem is not that, and i believe 
there's something wrong with Coverity's analysis here.

For some reason Coverity thinks that multiplying two floating point 
values (100f and miss_diff converted to float) will result in /integer/ 
overflow (lolwut?), *and* it assumes that miss_diff is negative at that 
point when it *can't* be, because if miss_diff was negative, we would've 
done an early exit on line 77.

My guess is, this is the culprit:

"overflow: Multiply operation overflows on operands (float)miss_diff and 
100f. Example values for operands: *100f = 268435456*, (float)miss_diff 
= -2147483648."

The "100f = 268435456" part makes me suspect that Coverity somehow 
thinks that "100f" is a variable name?

>>
>> 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