tools/target_dec_fuzzer: add a custom get_buffer2() implementation

Unlike avcodec_default_get_buffer2(), this version does not allocate more than
what the lavu image helper functions consider should be allocated for a given
frame.
Since the get_buffer2() documentation does not require any kind of buffer
padding for any of the planes, this should help detect bugs in our DR1 decoders
if they read 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>

tools/target_dec_fuzzer.c

@@ -104,6 +104,60 @@ 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 i, 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 (i = 0; i < 4 && frame->linesize[i]; i++)
+        linesize1[i] = frame->linesize[i] =
+            FFALIGN(frame->linesize[i], linesize_align[i]);
+    for (; i < 4; i++)
+        linesize1[i] = 0;
+
+    ret = av_image_fill_plane_sizes(size, ctx->pix_fmt, h, linesize1);
+    if (ret < 0)
+        return ret;
+
+    frame->extended_data = frame->data;
+    for (i = 0; i < 4 && size[i]; i++) {
+        frame->buf[i] = av_buffer_alloc(size[i]);
+        if (!frame->buf[i])
+            goto fail;
+        frame->data[i] = frame->buf[i]->data;
+    }
+    for (; i < AV_NUM_DATA_POINTERS; i++) {
+        frame->data[i] = NULL;
+        frame->linesize[i] = 0;
+    }
+
+    return 0;
+fail:
+    av_frame_unref(frame);
+    return AVERROR(ENOMEM);
+}
+
+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 +295,7 @@ 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;