Re: [FFmpeg-devel] [PATCH v3] avcodec/dovi_rpu: verify RPU data CRC32

[Michael Niedermayer <michael@niedermayer.cc>]

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On Wed, Aug 09, 2023 at 01:46:57PM -0400, quietvoid wrote:
> The Dolby Vision RPU contains a CRC32 to validate the payload against.
> The implementation is CRC32/MPEG-2.
> 
> The CRC is only verified with the AV_EF_CRCCHECK flag.
> 
> Signed-off-by: quietvoid <tcChlisop0@gmail.com>
> ---
>  libavcodec/dovi_rpu.c | 46 ++++++++++++++++++++++++++++++++++++++++---
>  libavcodec/dovi_rpu.h |  3 ++-
>  libavcodec/hevcdec.c  |  3 ++-
>  3 files changed, 47 insertions(+), 5 deletions(-)
> 
> diff --git a/libavcodec/dovi_rpu.c b/libavcodec/dovi_rpu.c
> index dd38936552..1dfeee7564 100644
> --- a/libavcodec/dovi_rpu.c
> +++ b/libavcodec/dovi_rpu.c
> @@ -22,6 +22,7 @@
>   */
>  
>  #include "libavutil/buffer.h"
> +#include "libavutil/crc.h"
>  
>  #include "dovi_rpu.h"
>  #include "golomb.h"
> @@ -191,13 +192,17 @@ static inline int64_t get_se_coef(GetBitContext *gb, const AVDOVIRpuDataHeader *
>          }                                                                       \
>      } while (0)
>  
> -int ff_dovi_rpu_parse(DOVIContext *s, const uint8_t *rpu, size_t rpu_size)
> +int ff_dovi_rpu_parse(DOVIContext *s, const uint8_t *rpu, size_t rpu_size,
> +                      int err_recognition)
>  {
>      AVDOVIRpuDataHeader *hdr = &s->header;
>      GetBitContext *gb = &(GetBitContext){0};
>      DOVIVdrRef *vdr;
>      int ret;
>  
> +    size_t actual_rpu_size;
> +    uint8_t trailing_zeroes = 0;
> +
>      uint8_t nal_prefix;
>      uint8_t rpu_type;
>      uint8_t vdr_seq_info_present;
> @@ -205,7 +210,22 @@ int ff_dovi_rpu_parse(DOVIContext *s, const uint8_t *rpu, size_t rpu_size)
>      uint8_t use_prev_vdr_rpu;
>      uint8_t use_nlq;
>      uint8_t profile;
> -    if ((ret = init_get_bits8(gb, rpu, rpu_size)) < 0)
> +
> +    uint32_t rpu_data_crc32;
> +    uint32_t computed_crc32;
> +
> +    for (int i = rpu_size - 1; i > 0; i--) {
> +        if (!rpu[i]) {
> +            trailing_zeroes++;
> +        } else {
> +            break;
> +        }
> +    }
> +
> +    actual_rpu_size = rpu_size - trailing_zeroes;
> +
> +    /* Exclude trailing byte (0x80) from reader */
> +    if ((ret = init_get_bits8(gb, rpu, actual_rpu_size - 1)) < 0)
>          return ret;
>  
>      /* RPU header, common values */
> @@ -440,7 +460,27 @@ int ff_dovi_rpu_parse(DOVIContext *s, const uint8_t *rpu, size_t rpu_size)
>          color->source_diagonal = get_bits(gb, 10);
>      }
>  
> -    /* FIXME: verify CRC32, requires implementation of AV_CRC_32_MPEG_2 */
> +    if (!(err_recognition & AV_EF_CRCCHECK))
> +        return 0;
> +
> +    /* Skip unsupported until CRC32 */
> +    skip_bits_long(gb, get_bits_left(gb) - 32);
> +
> +    rpu_data_crc32 = get_bits_long(gb, 32);
> +
> +    /* Verify CRC32, buffer excludes the prefix, CRC32 and trailing byte */
> +    computed_crc32 = av_bswap32(av_crc(av_crc_get_table(AV_CRC_32_IEEE),
> +                                       -1, rpu + 1, actual_rpu_size - 6));
> +
> +    if (rpu_data_crc32 != computed_crc32) {
> +        av_log(s->logctx, AV_LOG_ERROR,
> +               "RPU CRC mismatch! Expected %"PRIu32", received %"PRIu32"\n",
> +               rpu_data_crc32, computed_crc32);
> +
> +        if (err_recognition & AV_EF_EXPLODE)
> +            goto fail;
> +    }

(correctly designed) CRCs have the beautifull symmetry that you can merge
the crc32 value into the crc computation and then a 0 means no CRC missmatch
(there are many other cool properties but this one allows to simplify the code)

This works too: (and is simpler)

    /* Skip unsupported until CRC32 */
    skip_bits_long(gb, get_bits_left(gb));

    /* Verify CRC32, buffer excludes the prefix, CRC32 and trailing byte */
    computed_crc32 = av_bswap32(av_crc(av_crc_get_table(AV_CRC_32_IEEE),
                                       -1, rpu + 1, actual_rpu_size - 2));

    if (computed_crc32) {
        av_log(s->logctx, AV_LOG_ERROR, "RPU CRC mismatch! %"PRIX32"\n",
               computed_crc32);

        if (err_recognition & AV_EF_EXPLODE)
            goto fail;
    }


[...]
-- 
Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

Into a blind darkness they enter who follow after the Ignorance,
they as if into a greater darkness enter who devote themselves
to the Knowledge alone. -- Isha Upanishad

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