Re: [FFmpeg-devel] [PATCH] lavc/dovi_rpu: Fix UB for possible left shift of negative values

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

Thilo Borgmann:
> Am 16.06.22 um 12:13 schrieb Andreas Rheinhardt:
>> Thilo Borgmann:
>>> diff --git a/libavcodec/dovi_rpu.c b/libavcodec/dovi_rpu.c
>>> index a87562c8a3..833ce9e705 100644
>>> --- a/libavcodec/dovi_rpu.c
>>> +++ b/libavcodec/dovi_rpu.c
>>> @@ -153,7 +153,7 @@ static inline uint64_t get_ue_coef(GetBitContext
>>> *gb, const AVDOVIRpuDataHeader
>>>       case RPU_COEFF_FIXED:
>>>           ipart = get_ue_golomb_long(gb);
>>>           fpart.u32 = get_bits_long(gb, hdr->coef_log2_denom);
>>> -        return (ipart << hdr->coef_log2_denom) + fpart.u32;
>>> +        return ipart * (1 << hdr->coef_log2_denom) + fpart.u32;
>>>         case RPU_COEFF_FLOAT:
>>>           fpart.u32 = get_bits_long(gb, 32);
>>> @@ -172,7 +172,7 @@ static inline int64_t get_se_coef(GetBitContext
>>> *gb, const AVDOVIRpuDataHeader *
>>>       case RPU_COEFF_FIXED:
>>>           ipart = get_se_golomb_long(gb);
>>>           fpart.u32 = get_bits_long(gb, hdr->coef_log2_denom);
>>> -        return (ipart << hdr->coef_log2_denom) + fpart.u32;
>>> +        return ipart * (1 << hdr->coef_log2_denom) + fpart.u32;
>>>         case RPU_COEFF_FLOAT:
>>>           fpart.u32 = get_bits_long(gb, 32);
>>
>> coef_log2_denom can be in the range 13..32. This means that 1 <<
>> hdr->coef_log2_denom can be UB (namely if coef_log2_denom is 31 or 32
>> for ordinary 32 bit ints); this time it is not UB that happens to work
>> as expected, because 1 << 32 will be 0 or 1 (depending upon the system)
>> and not 2^32. In case of get_ue_coef() this actually adds UB to
>> otherwise fine code.
> 
> So 1LL it needs to be, not? Am I still missing something?
> 

This version should not add new UB.
I btw don't get why you are changing get_ue_coef() at all: It's fine
as-is; consistency should only trump when the choice is between several
equally good alternatives which is IMO not true here. (The reason that C
makes left shifts of negative values UB is because of non-two's
complement systems, so it is unfortunate for a project like FFmpeg that
requires two's complement that it has to workaround this restriction by
using a * (1 << b).)

- Andreas
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