On Thu, 2021-07-15 at 13:02 +0100, Colin Ian King wrote:
> Hi
>
> Static analysis with cppcheck picked up an interesting issue with the
> following inline helper function in arch/s390/net/bpf_jit_comp.c :
>
> static inline void reg_set_seen(struct bpf_jit *jit, u32 b1)
> {
> u32 r1 = reg2hex[b1];
>
> if (!jit->seen_reg[r1] && r1 >= 6 && r1 <= 15)
> jit->seen_reg[r1] = 1;
> }
>
> Although I believe r1 is always within range, the range check on r1
> is
> being performed before the more cache/memory expensive lookup on
> jit->seen_reg[r1]. I can't see why the range change is being
> performed
> after the access of jit->seen_reg[r1]. The following seems more
> correct:
>
> if (r1 >= 6 && r1 <= 15 && !jit->seen_reg[r1])
> jit->seen_reg[r1] = 1;
>
> ..since the check on r1 are less expensive than !jit->seen_reg[r1]
> and
> also the range check ensures the array access is not out of bounds. I
> was just wondering if I'm missing something deeper to why the order
> is
> the way it is.
>
> Colin
Hi,
I think your analysis is correct, thanks for spotting this!
Even though I don't think the performance difference would be
measurable here, not confusing future readers is a good reason
to make a change that you suggest.
Do you plan to send a patch?
Best regards,
Ilya
On 15/07/2021 13:09, Ilya Leoshkevich wrote:
> On Thu, 2021-07-15 at 13:02 +0100, Colin Ian King wrote:
>> Hi
>>
>> Static analysis with cppcheck picked up an interesting issue with the
>> following inline helper function in arch/s390/net/bpf_jit_comp.c :
>>
>> static inline void reg_set_seen(struct bpf_jit *jit, u32 b1)
>> {
>> u32 r1 = reg2hex[b1];
>>
>> if (!jit->seen_reg[r1] && r1 >= 6 && r1 <= 15)
>> jit->seen_reg[r1] = 1;
>> }
>>
>> Although I believe r1 is always within range, the range check on r1
>> is
>> being performed before the more cache/memory expensive lookup on
>> jit->seen_reg[r1]. I can't see why the range change is being
>> performed
>> after the access of jit->seen_reg[r1]. The following seems more
>> correct:
>>
>> if (r1 >= 6 && r1 <= 15 && !jit->seen_reg[r1])
>> jit->seen_reg[r1] = 1;
>>
>> ..since the check on r1 are less expensive than !jit->seen_reg[r1]
>> and
>> also the range check ensures the array access is not out of bounds. I
>> was just wondering if I'm missing something deeper to why the order
>> is
>> the way it is.
>>
>> Colin
>
> Hi,
>
> I think your analysis is correct, thanks for spotting this!
> Even though I don't think the performance difference would be
> measurable here, not confusing future readers is a good reason
> to make a change that you suggest.
> Do you plan to send a patch?
I'll send a patch later today. Colin
>
> Best regards,
> Ilya
>