[FFmpeg-devel] [PATCH 2/2] avcodec/fitsdec: Prevent division by 0 with huge data_max

[Reimar Döffinger]

On 16.07.2019, at 00:50, Michael Niedermayer <michael at niedermayer.cc> wrote:

> Fixes: division by 0
> Fixes: 15657/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_FITS_fuzzer-5738154838982656
> 
> Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
> Signed-off-by: Michael Niedermayer <michael at niedermayer.cc>
> ---
> libavcodec/fitsdec.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/libavcodec/fitsdec.c b/libavcodec/fitsdec.c
> index 4f452422ef..fe941f199d 100644
> --- a/libavcodec/fitsdec.c
> +++ b/libavcodec/fitsdec.c
> @@ -174,7 +174,7 @@ static int fits_read_header(AVCodecContext *avctx, const uint8_t **ptr, FITSHead
>             return AVERROR_INVALIDDATA;
>         }
>         av_log(avctx, AV_LOG_WARNING, "data min/max indicates a blank image\n");
> -        header->data_max ++;
> +        header->data_max += fabs(header->data_max) / 10000000 + 1;

This is really non-obvious, both by itself, in where/how it causes the division by 0 and that the error here isn't worse than the division by 0 for example, and why this constant was chosen.
Also why a division and not a multiply by the inverse?
Why not * (1.0f / (1 << 24)) for example, which for single-precision IEEE I think should result in exactly 1 ULP (well, possibly 2 with rounding) increments?
Why is this even using floating-point? And why not double-precision at least?