Re: [FFmpeg-devel] [PATCH] avformat/mov: (v2) fix get_eia608_packet

[Pavel Koshevoy <pkoshevoy@gmail.com>]

I've submitted a v3 patch that doesn't do as much over-allocation, similar
to https://lists.ffmpeg.org/pipermail/ffmpeg-devel/2023-June/310456.html

On Thu, Feb 13, 2025 at 10:55 AM Pavel Koshevoy <pkoshevoy@gmail.com> wrote:

> The problem is reproducible with "Test for Quicktime 608 CC file.mov"
> from https://samples.ffmpeg.org/MPEG2/subcc/
>
> ffmpeg_g -i "Test for Quicktime 608 CC file.mov" -map 0 -c copy -y
> remuxed.mov
>
> Prior to the fix QuickTime Player playback of remuxed.mov would
> render cdt2 atom type tags as part of "English CC" subtitles.
> ---
>  libavformat/mov.c | 54 +++++++++++++++++++++++++++++++++++++++--------
>  1 file changed, 45 insertions(+), 9 deletions(-)
>
> diff --git a/libavformat/mov.c b/libavformat/mov.c
> index 85aef33b19..65b4f3dd5c 100644
> --- a/libavformat/mov.c
> +++ b/libavformat/mov.c
> @@ -10790,22 +10790,58 @@ static int mov_change_extradata(AVStream *st,
> AVPacket *pkt)
>
>  static int get_eia608_packet(AVIOContext *pb, AVPacket *pkt, int size)
>  {
> -    int new_size, ret;
> +    /* We can't make assumptions about the structure of the payload,
> +       because it may include multiple cdat and cdt2 samples. */
> +    int ret, out_size_max, out_size = 0;
>
> -    if (size <= 8)
> +    /* a valide payload must have size, 4cc, and at least 1 byte pair: */
> +    if (size <= 10 || size % 2 != 0)
>          return AVERROR_INVALIDDATA;
> -    new_size = ((size - 8) / 2) * 3;
> -    ret = av_new_packet(pkt, new_size);
> +
> +    /* avoid an int overflow: */
> +    if ((size - 8) / 2 >= (INT_MAX - size) / 3)
> +        return AVERROR_INVALIDDATA;
> +
> +    /* Allocate the buffer larger enough to store the output cc_data_pkts
> +       and the original c608 payload, safely. */
> +    out_size_max = ((size - 8) / 2) * 3;
> +    ret = av_new_packet(pkt, out_size_max + size);
>      if (ret < 0)
>          return ret;
>
> -    avio_skip(pb, 8);
> -    for (int j = 0; j < new_size; j += 3) {
> -        pkt->data[j] = 0xFC;
> -        pkt->data[j+1] = avio_r8(pb);
> -        pkt->data[j+2] = avio_r8(pb);
> +    /* Load the original c608 payload into the last 3rd of the buffer. */
> +    if (avio_read(pb, pkt->data + out_size_max, size) != size)
> +        return AVERROR_EOF;
> +
> +    /* parse and re-format the c608 payload in one pass. */
> +    for (uint8_t *out = pkt->data, *src = out + out_size_max, *end = src
> + size; src + 8 <= end; ) {
> +        uint32_t atom_size = AV_RB32(src);
> +        uint8_t cc_field =
> +            memcmp(src + 4, "cdat", 4) == 0 ? 1 :
> +            memcmp(src + 4, "cdt2", 4) == 0 ? 2 :
> +            0;
> +
> +        /* make sure the atom size stays within the buffer boundaries. */
> +        if (src + atom_size > end)
> +            return AVERROR_INVALIDDATA;
> +
> +        /* make sure the payload size is consistent with N byte pairs. */
> +        if ((atom_size - 8) % 2 != 0)
> +            return AVERROR_INVALIDDATA;
> +
> +        if (cc_field != 0) {
> +            for (uint8_t *cc = src + 8, *cc_end = src + atom_size; cc <
> cc_end; cc += 2) {
> +                out[0] = (0x1F << 3) | (1 << 2) | (cc_field - 1);
> +                out[1] = cc[0];
> +                out[2] = cc[1];
> +                out_size += 3;
> +                out += 3;
> +            }
> +        }
> +        src += atom_size;
>      }
>
> +    av_shrink_packet(pkt, out_size);
>      return 0;
>  }
>
> --
> 2.43.0
>
>
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