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

[quietvoid <tcchlisop0@gmail.com>]

On 26/10/2023 17.44, Michael Niedermayer wrote:
> 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;
>      }

Hi Michael. I like the idea and it's a cool property.

However the then printed CRC on mismatch is not a useful value, so I'm
unsure if it's better to simplify here.
I like having the expected CRC logged here.

Thanks.

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