Re: [FFmpeg-devel] [PATCH v2] avcodec/jpeg2000htdec: check if block decoding will exceed internal precision

[Pierre-Anthony Lemieux <pal@sandflow.com>]

On Mon, Aug 14, 2023 at 1:14 AM Tomas Härdin <git@haerdin.se> wrote:
>
> lör 2023-08-12 klockan 13:31 -0700 skrev pal@sandflow.com:
> > From: Pierre-Anthony Lemieux <pal@palemieux.com>
> >
> > Intended to replace
> > https://patchwork.ffmpeg.org/project/ffmpeg/patch/20230802000135.26482-3-michael@niedermayer.cc/
> > with a more accurate block decoding magnitude bound.
> >
> > ---
> >  libavcodec/jpeg2000htdec.c | 34 +++++++++++++++++++++++++++++++++-
> >  1 file changed, 33 insertions(+), 1 deletion(-)
> >
> > diff --git a/libavcodec/jpeg2000htdec.c b/libavcodec/jpeg2000htdec.c
> > index 2c4cea5dd9..a7329206de 100644
> > --- a/libavcodec/jpeg2000htdec.c
> > +++ b/libavcodec/jpeg2000htdec.c
> > @@ -572,6 +572,14 @@ static int
> > jpeg2000_decode_ht_cleanup_segment(const Jpeg2000DecoderContext *s,
> >
> >      const uint8_t *vlc_buf = Dcup + Pcup;
> >
> > +    /*
> > +     * Bound on the recision needed to process the codeblock. The
> > number of
>
> precision
>
> > +     * decoded bit planes is equal to at most cblk->zbp + 2 since
> > S_blk = P if
> > +     * there are no placeholder passes or HT Sets and P = cblk->zbp.
> > See Rec.
> > +     * ITU-T T.814, 7.6.
> > +     */
> > +    int maxbp = cblk->zbp + 2;
> > +
> >      /* convert to raster-scan */
> >      const uint16_t is_border_x = width % 2;
> >      const uint16_t is_border_y = height % 2;
> > @@ -590,6 +598,12 @@ static int
> > jpeg2000_decode_ht_cleanup_segment(const Jpeg2000DecoderContext *s,
> >          goto free;
> >      }
> >
> > +    /* do we have enough precision, assuming a 32-bit decoding path
> > */
> > +    if (maxbp >= 32) {
> > +        return AVERROR_INVALIDDATA;
> > +        goto free;
> > +    }
>
> Why not move this check to just after maxbp is declared? That way a
> bunch of allocations are avoided

I had it that way but then the compiler complained about mixing
declarations and code.

>
> The rest of the patch looks fine, though I haven't studied the HT spec
> deeply. The +2 on maxbp is familiar.
>
> /Tomas
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