[FFmpeg-devel] [PATCH 1/2] avcodec/adpcm_data: extend ff_adpcm_ima_cunning_index_table

[Michael Niedermayer]

On Fri, May 29, 2020 at 03:47:03AM +0000, Zane van Iperen wrote:
> On Thu, 28 May 2020 18:44:55 +0200
> "Michael Niedermayer" <michael at niedermayer.cc> wrote:
> 
> 
> > > The index table should only ever be indexed between [0,7], so this
> > > looks like a bug in adpcm_ima_cunning_expand_nibble() instead.  
> > 
> > maybe it should be only 0..7 but abs(-8 .. 7) needs a 8th entrty or a
> > check
> > 
> > 
> > >
> > > Where would one go to see the inputs for this? I'd like to
> > > investigate (and test against the original decoder).  
> > 
> > The file is from a fuzzer so is random trash and it needs
> > target_dec_fuzzer.c to be read. It wont be read as "Pro Pinball
> > Series Soundbank" so i think that file will not be useful to test the
> > behavior of the original decoder as i would assume it will not accept
> > the target_dec_fuzzer data but i surely can send it to you privatly
> > if you need it. just say if you want me to send you the file
> > 
> 
> No need, I've found an existing file that has the same problem. Out of
> all the 453 files I have, only one of them triggers this.
> 
> tl;dr: Change the 5 to -1.
> 
> 
> ff_adpcm_ima_cunning_index_table[abs(nibble)] is wrong in the case
> where nibble == -8.
> 
> If you take the unsigned nibble, and apply f():
>   f(x) = 16 - x if x > 8 else x & 0x7
> 
> you'll get the same value as abs() applied with the signed nibble,
> except for this one case (abs(-8) == 8, f(8) == 0).
> 
> 
> Instead of changing the abs(), a cleaner fix is to simply change the 5
> to -1. That should give the correct output.

will apply with -1

thanks

[...]
-- 
Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

I have never wished to cater to the crowd; for what I know they do not
approve, and what they approve I do not know. -- Epicurus
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