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

["Tomas Härdin" <git@haerdin.se>]

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

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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