Re: [FFmpeg-devel] [PATCH 1/5] avcodec/lpc: copy levenson coeffs only when they have been computed

From: Michael Niedermayer <michael@niedermayer.cc>
To: FFmpeg development discussions and patches <ffmpeg-devel@ffmpeg.org>
Subject: Re: [FFmpeg-devel] [PATCH 1/5] avcodec/lpc: copy levenson coeffs only when they have been computed
Date: Sun, 12 May 2024 02:18:50 +0200
Message-ID: <20240512001850.GV6420@pb2> (raw)
In-Reply-To: <83bec738-e2dc-41a3-88af-a53ab6958628@lynne.ee>

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On Sun, May 12, 2024 at 02:13:06AM +0200, Lynne via ffmpeg-devel wrote:
> On 12/05/2024 02:03, Michael Niedermayer wrote:
> > Fixes: CID1473514 Uninitialized scalar variable
> > 
> > Sponsored-by: Sovereign Tech Fund
> > Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
> > ---
> >   libavcodec/lpc.c | 5 +++--
> >   1 file changed, 3 insertions(+), 2 deletions(-)
> > 
> > diff --git a/libavcodec/lpc.c b/libavcodec/lpc.c
> > index 8305cc0596a..981dacce8a5 100644
> > --- a/libavcodec/lpc.c
> > +++ b/libavcodec/lpc.c
> > @@ -282,8 +282,9 @@ int ff_lpc_calc_coefs(LPCContext *s,
> >           double av_uninit(weight);
> >           memset(var, 0, FFALIGN(MAX_LPC_ORDER+1,4)*sizeof(*var));
> > -        for(j=0; j<max_order; j++)
> > -            m[0].coeff[max_order-1][j] = -lpc[max_order-1][j];
> > +        if (lpc_passes > 1)
> > +            for(j=0; j<max_order; j++)
> > +                m[0].coeff[max_order-1][j] = -lpc[max_order-1][j];
> >           for(; pass<lpc_passes; pass++){
> >               avpriv_init_lls(&m[pass&1], max_order);
> 
> max_order is a function argument, I don't think that's the right place to
> fix this.

max_orders is fine

what the problem is, is that CHOLESKY with lpc_passes = 1
skips the first pass LEVINSON but this line copies the output
from LEVINSON so it copies Uninitialized data.
a few lines later thats cleared with avpriv_init_lls()
but that access to uninitialized data i think is undefined behavior

if my analysis is not wrong then i think my fix is correct

thx

[...]
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Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

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