Re: [FFmpeg-devel] [PATCH] avformat/cafdec: Implement FLAC-in-CAF parsing

["Jan Ekström" <jeebjp@gmail.com>]

On Fri, Jun 3, 2022 at 8:11 PM Andreas Rheinhardt
<andreas.rheinhardt@outlook.com> wrote:
>
> Martijn van Beurden:
> > The afconvert utility shipped with MacOS supports muxing of FLAC
> > in CAF, see afconvert help output on a recent Mac here:
> > https://hydrogenaud.io/index.php?topic=122509.0 A file created
> > with afconvert free of copyright (licensed CC0) can be found here:
> > http://www.audiograaf.nl/misc_stuff/afconvert-FLAC-in-CAF.caf
> >
> > This patch implements parsing of such a file
> > ---
> >  libavformat/caf.c    |  1 +
> >  libavformat/cafdec.c | 41 +++++++++++++++++++++++++++++++++++++++++
> >  2 files changed, 42 insertions(+)
> >
> > diff --git a/libavformat/caf.c b/libavformat/caf.c
> > index a700e4055b..a61c39fae5 100644
> > --- a/libavformat/caf.c
> > +++ b/libavformat/caf.c
> > @@ -46,6 +46,7 @@ const AVCodecTag ff_codec_caf_tags[] = {
> >      { AV_CODEC_ID_GSM,             MKTAG('a','g','s','m') },
> >      { AV_CODEC_ID_GSM_MS,          MKTAG('m','s', 0, '1') },
> >      { AV_CODEC_ID_ILBC,            MKTAG('i','l','b','c') },
> > +    { AV_CODEC_ID_FLAC,            MKTAG('f','l','a','c') },
> >      { AV_CODEC_ID_MACE3,           MKTAG('M','A','C','3') },
> >      { AV_CODEC_ID_MACE6,           MKTAG('M','A','C','6') },
> >      { AV_CODEC_ID_MP1,             MKTAG('.','m','p','1') },
> > diff --git a/libavformat/cafdec.c b/libavformat/cafdec.c
> > index 168f69f20b..72a74ab297 100644
> > --- a/libavformat/cafdec.c
> > +++ b/libavformat/cafdec.c
> > @@ -31,6 +31,7 @@
> >  #include "demux.h"
> >  #include "internal.h"
> >  #include "isom.h"
> > +#include "libavcodec/flac.h"
> >  #include "mov_chan.h"
> >  #include "libavutil/intreadwrite.h"
> >  #include "libavutil/intfloat.h"
> > @@ -170,6 +171,46 @@ static int read_kuki_chunk(AVFormatContext *s, int64_t size)
> >              }
> >              avio_skip(pb, size - ALAC_NEW_KUKI);
> >          }
> > +    } else if (st->codecpar->codec_id == AV_CODEC_ID_FLAC) {
> > +        int last, type, flac_metadata_size;
> > +        uint8_t buf[4];
> > +        /* The magic cookie format for FLAC consists mostly of an mp4 dfLa atom. */
> > +        if (size < (16 + FLAC_STREAMINFO_SIZE)) {
> > +            av_log(s, AV_LOG_ERROR, "invalid FLAC magic cookie\n");
> > +            return AVERROR_INVALIDDATA;
> > +        }
> > +        /* Check cookie version. */
> > +        if (avio_r8(pb) != 0) {
> > +            av_log(s, AV_LOG_ERROR, "unknown FLAC magic cookie\n");
> > +            return AVERROR_INVALIDDATA;
> > +        }
> > +        avio_rb24(pb); /* Flags */
> > +        /* read dfLa fourcc */
> > +        if (avio_read(pb, buf, 4) != 4) {
> > +            av_log(s, AV_LOG_ERROR, "failed to read FLAC magic cookie\n");
> > +            return AVERROR_INVALIDDATA;
> > +        }
> > +        if (memcmp(buf,"dfLa",4)) {
> > +            av_log(s, AV_LOG_ERROR, "invalid FLAC magic cookie\n");
> > +            return AVERROR_INVALIDDATA;
> > +        }
> > +        /* Check dfLa version. */
> > +        if (avio_r8(pb) != 0) {
> > +            av_log(s, AV_LOG_ERROR, "unknown dfLa version\n");
> > +            return AVERROR_INVALIDDATA;
> > +        }
> > +        avio_rb24(pb); /* Flags */
> > +        avio_read(pb, buf, sizeof(buf));
>
> You are not checking this; this could lead to uninitialized values being
> used in flac_parse_block_header() below.
>
> > +        flac_parse_block_header(buf, &last, &type, &flac_metadata_size);
> > +        if (type != FLAC_METADATA_TYPE_STREAMINFO || flac_metadata_size != FLAC_STREAMINFO_SIZE) {
> > +            av_log(s, AV_LOG_ERROR, "STREAMINFO must be first FLACMetadataBlock\n");
> > +            return AVERROR_INVALIDDATA;
> > +        }
> > +        ret = ff_get_extradata(s, st->codecpar, pb, FLAC_STREAMINFO_SIZE);
> > +        if (ret < 0)
> > +            return ret;
> > +        if (!last)
> > +            av_log(s, AV_LOG_WARNING, "non-STREAMINFO FLACMetadataBlock(s) ignored\n");
> >      } else if (st->codecpar->codec_id == AV_CODEC_ID_OPUS) {
> >          // The data layout for Opus is currently unknown, so we do not export
> >          // extradata at all. Multichannel streams are not supported.
>
> Is there an official spec for this?

CAF mostly follows ISOBMFF in these things, so
https://github.com/xiph/flac/blob/master/doc/isoflac.txt is probably
the source of truth for dfLa FLACSpecificBox and such.

Base CAF specification is at
https://developer.apple.com/library/archive/documentation/MusicAudio/Reference/CAFSpec/CAF_spec/CAF_spec.html
, but it stops at AAC, and ignores both Opus and FLAC.

Jan
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