Re: [FFmpeg-devel] [PATCH] avcodec/jpeg2000dsp: Use unsigned to avoid overflow

[Andreas Rheinhardt <andreas.rheinhardt@outlook.com>]

Tomas Härdin:
> tis 2022-09-27 klockan 13:20 +0200 skrev Andreas Rheinhardt:
>> Tomas Härdin:
>>> tis 2022-09-27 klockan 03:47 +0200 skrev Andreas Rheinhardt:
>>>> Affected the jpeg2000dsp checkasm test.
>>>>
>>>> Signed-off-by: Andreas Rheinhardt
>>>> <andreas.rheinhardt@outlook.com>
>>>> ---
>>>>  libavcodec/jpeg2000dsp.c | 9 ++++-----
>>>>  1 file changed, 4 insertions(+), 5 deletions(-)
>>>>
>>>> diff --git a/libavcodec/jpeg2000dsp.c b/libavcodec/jpeg2000dsp.c
>>>> index b61be3b72f..b1bff6d5b1 100644
>>>> --- a/libavcodec/jpeg2000dsp.c
>>>> +++ b/libavcodec/jpeg2000dsp.c
>>>> @@ -76,14 +76,13 @@ static void ict_int(void *_src0, void *_src1,
>>>> void *_src2, int csize)
>>>>  
>>>>  static void rct_int(void *_src0, void *_src1, void *_src2, int
>>>> csize)
>>>>  {
>>>> -    int32_t *src0 = _src0, *src1 = _src1, *src2 = _src2;
>>>> -    int32_t i0, i1, i2;
>>>> +    uint32_t *src0 = _src0, *src1 = _src1, *src2 = _src2;
>>>>      int i;
>>>>  
>>>>      for (i = 0; i < csize; i++) {
>>>> -        i1 = *src0 - (*src2 + *src1 >> 2);
>>>> -        i0 = i1 + *src2;
>>>> -        i2 = i1 + *src1;
>>>> +        uint32_t i1 = *src0 - ((int32_t)(*src2 + *src1) >> 2);
>>>
>>> The addition could conceivably overflow. Also could just use / 4
>>> instead of >> 2.
>>
>> The addition uses unsigned types, so that overflow is defined.
> 
> Wups, I was looking at the original code. You're right of course. What
> about subtracting src0 (unsigned) from that result (signed)? Do they
> get promoted to int64_t?
> 

No. signed int + unsigned int uses unsigned.

> 
>> Furthermore, the shift is performed on signed types and the rounding
>> for
>> negative numbers divided by four is different than what >> 2 produces
>> (integer division is defined to use rounding towards zero, whereas
>> right
>> shifts of negative numbers are implementation defined and typically
>> use
>> rounding towards -inf (we require this behaviour)).
> 
> Tricky
> 
> /Tomas
> 
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