[FFmpeg-devel] [PATCH 1/3] aacsbr_fixed: avoid division by zero in sbr_gain_calc

[Andreas Cadhalpun]

On 11.11.2015 23:19, Michael Niedermayer wrote:
> On Wed, Nov 11, 2015 at 09:31:18PM +0100, Andreas Cadhalpun wrote:
>> On 11.11.2015 14:55, Michael Niedermayer wrote:
>>> On Sun, Nov 08, 2015 at 10:02:56PM +0100, Andreas Cadhalpun wrote:
>>>> This fixes a SIGFPE crash in the aac_fixed decoder.
>>>>
>>>> Signed-off-by: Andreas Cadhalpun <Andreas.Cadhalpun at googlemail.com>
>>>> ---
>>>>  libavcodec/aacsbr_fixed.c | 4 ++--
>>>>  1 file changed, 2 insertions(+), 2 deletions(-)
>>>>
>>>> diff --git a/libavcodec/aacsbr_fixed.c b/libavcodec/aacsbr_fixed.c
>>>> index f942d74..efe40f2 100644
>>>> --- a/libavcodec/aacsbr_fixed.c
>>>> +++ b/libavcodec/aacsbr_fixed.c
>>>> @@ -434,8 +434,8 @@ static void sbr_gain_calc(AACContext *ac, SpectralBandReplication *sbr,
>>>>                gain_max = FLOAT_100000;
>>>>              for (m = sbr->f_tablelim[k] - sbr->kx[1]; m < sbr->f_tablelim[k + 1] - sbr->kx[1]; m++) {
>>>>                  SoftFloat q_m_max = av_div_sf(
>>>> -                                        av_mul_sf(sbr->q_m[e][m], gain_max),
>>>> -                                        sbr->gain[e][m]);
>>>> +                                        av_add_sf(FLOAT_EPSILON, av_mul_sf(sbr->q_m[e][m], gain_max)),
>>>> +                                        av_add_sf(FLOAT_EPSILON, sbr->gain[e][m]));
>>>
>>> is gain == 0 here ?
>>
>> Yes: sbr->gain[e][m] == {mant = 0, exp = -126}
>>
>>> isnt that invalid ?
>>
>> I'm not sure, but it can happen, when q_mapped is very small, which can be
>> caused by noise_facs becoming tiny in sbr_dequant.
>> That's kind of the opposite problem of 'envelope scalefactors overflowing'.
> 
> sbr_dequant() does not look like it can set noise_facs to 0
> except by underflow of the exponent range

That's exactly what happens.

> that "has" to be invalid as this depends on the implementation 

So should sbr_dequant error out instead?
If so, what's the minimal value it should accept?

Best regards,
Andreas