Re: [FFmpeg-devel] [PATCH v2] tools/target_dec_fuzzer: add a custom get_buffer2() implementation

[James Almer <jamrial@gmail.com>]

On 5/31/2022 5:42 PM, James Almer wrote:
> Unlike avcodec_default_get_buffer2(), this version does not allocate more than
> what the normal image helper functions consider should be allocated for a given
> frame.
> Since the get_buffer2() documentation does not require any kind of buffer
> overallocation for any of the planes, this should help detect bugs in our DR1
> decoders if they overread beyond the end of the buffer, simulating what some
> library users might experience when they use their own custom get_buffer2()
> implementations.
> 
> Signed-off-by: James Almer <jamrial@gmail.com>
> ---
> Now making sure to not allocate more plane buffers than needed.
> 
>   tools/target_dec_fuzzer.c | 52 +++++++++++++++++++++++++++++++++++++++
>   1 file changed, 52 insertions(+)
> 
> diff --git a/tools/target_dec_fuzzer.c b/tools/target_dec_fuzzer.c
> index 288aa63313..2e43ed3d88 100644
> --- a/tools/target_dec_fuzzer.c
> +++ b/tools/target_dec_fuzzer.c
> @@ -104,6 +104,57 @@ const uint32_t maxiteration = 8096;
>   
>   static const uint64_t FUZZ_TAG = 0x4741542D5A5A5546ULL;
>   
> +static int fuzz_video_get_buffer(AVCodecContext *ctx, AVFrame *frame)
> +{
> +    ptrdiff_t linesize1[4];
> +    size_t size[4];
> +    int linesize_align[AV_NUM_DATA_POINTERS];
> +    int ret, w = frame->width, h = frame->height;
> +
> +    avcodec_align_dimensions2(ctx, &w, &h, linesize_align);
> +    ret = av_image_fill_linesizes(frame->linesize, ctx->pix_fmt, w);
> +    if (ret < 0)
> +        return ret;
> +
> +    for (int i = 0; i < 4; i++)
> +        linesize1[i] = frame->linesize[i] =
> +            FFALIGN(frame->linesize[i], linesize_align[i]);
> +
> +    ret = av_image_fill_plane_sizes(size, ctx->pix_fmt, h, linesize1);
> +    if (ret < 0)
> +        goto fail;
> +
> +    for (int i = 0; i < 4; i++) {
> +        if (!size[i])
> +            break;
> +        frame->buf[i] = av_buffer_alloc(size[i]);
> +        if (!frame->buf[i]) {
> +            ret = AVERROR(ENOMEM);
> +            goto fail;
> +        }
> +        frame->data[i] = frame->buf[i]->data;
> +    }
> +
> +fail:
> +    if (ret < 0)
> +        av_frame_unref(frame);
> +    return ret;
> +}
> +
> +static int fuzz_get_buffer2(AVCodecContext *ctx, AVFrame *frame, int flags)
> +{
> +    switch (ctx->codec_type) {
> +    case AVMEDIA_TYPE_VIDEO:
> +        return (ctx->codec->capabilities & AV_CODEC_CAP_DR1)
> +               ? fuzz_video_get_buffer(ctx, frame)
> +               : avcodec_default_get_buffer2(ctx, frame, flags);
> +    case AVMEDIA_TYPE_AUDIO:
> +        return avcodec_default_get_buffer2(ctx, frame, flags);
> +    default:
> +        return AVERROR(EINVAL);
> +    }
> +}
> +
>   int LLVMFuzzerTestOneInput(const uint8_t *data, size_t size) {
>       uint64_t maxpixels_per_frame = 4096 * 4096;
>       uint64_t maxpixels;
> @@ -241,6 +292,7 @@ int LLVMFuzzerTestOneInput(const uint8_t *data, size_t size) {
>           ctx->max_pixels = maxpixels_per_frame; //To reduce false positive OOM and hangs
>   
>       ctx->max_samples = maxsamples_per_frame;
> +    ctx->get_buffer2 = fuzz_get_buffer2;
>   
>       if (size > 1024) {
>           GetByteContext gbc;

Will push soon if nobody objects.
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