[FFmpeg-devel] [PATCH 4/4] avcodec/cbs_h265: add missing support for reserved_payload_extension_data SEI bits

[Andreas Rheinhardt]

James Almer:
> On 4/22/2020 9:17 PM, Andreas Rheinhardt wrote:
>> James Almer:
>>> Fixes ticket #8622
>>>
>>> Signed-off-by: James Almer <jamrial at gmail.com>
>>> ---
>>> It fixes it assuming the Picture Timing in that sample is not being misparsed
>>> by cbs_h265.
>>> We're compliant with the latest revision of the spec, so any
>>> reserved_payload_extension_data found in a sample created today is almost
>>> guaranteed to be bogus. But the spec states that they should be skipped if
>>> found, for forward compatibility reasons.
>>> Would be worth checking if the nvenc encoder is at fault, writing faulty SEI
>>> messages.
>>>
>>> This patch could for that matter make many potential cases of undiscovered
>>> cbs_h265 SEI misparsing be handled as if the remaining bits were these reserved
>>> bits.
>>
>> That's unfortunately true. I don't see a way to avoid that.
>>
>>>
>>>  libavcodec/cbs_h265.h                 |  1 +
>>>  libavcodec/cbs_h265_syntax_template.c | 44 +++++++++++++++++++++++++--
>>>  2 files changed, 43 insertions(+), 2 deletions(-)
>>>
>>> diff --git a/libavcodec/cbs_h265.h b/libavcodec/cbs_h265.h
>>> index 2c1e153ad9..73897f77a4 100644
>>> --- a/libavcodec/cbs_h265.h
>>> +++ b/libavcodec/cbs_h265.h
>>> @@ -715,6 +715,7 @@ typedef struct H265RawSEIPayload {
>>>              AVBufferRef *data_ref;
>>>          } other;
>>>      } payload;
>>> +    H265RawExtensionData extension_data;
>>>  } H265RawSEIPayload;
>>>  
>>>  typedef struct H265RawSEI {
>>> diff --git a/libavcodec/cbs_h265_syntax_template.c b/libavcodec/cbs_h265_syntax_template.c
>>> index f978e16549..ef3254d27f 100644
>>> --- a/libavcodec/cbs_h265_syntax_template.c
>>> +++ b/libavcodec/cbs_h265_syntax_template.c
>>> @@ -2054,11 +2054,43 @@ static int FUNC(sei_alpha_channel_info)(CodedBitstreamContext *ctx,
>>>      return 0;
>>>  }
>>>  
>>> +static int FUNC(payload_extension)(CodedBitstreamContext *ctx, RWContext *rw,
>>> +                                   H265RawExtensionData *current, uint32_t payload_size,
>>> +                                   int cur_pos)
>>> +{
>>> +    int err, i;
>>> +
>>> +#ifdef READ
>>> +    if (cbs_h265_payload_extension_present(rw, payload_size, cur_pos)) {
>>> +        GetBitContext tmp = *rw;
>>> +        int payload_zero_bits, bits_left = 8 * payload_size - cur_pos;
>>> +        if (bits_left > 8)
>>> +            skip_bits_long(&tmp, bits_left - 8);
>>> +        payload_zero_bits = ff_ctz(get_bits(&tmp, FFMIN(bits_left, 8)));
>>> +        if (payload_zero_bits >= bits_left - 1)
>>> +            return AVERROR_INVALIDDATA;
>>> +        current->bit_length = bits_left - payload_zero_bits - 1;
>>> +        allocate(current->data, (current->bit_length + 7) / 8);
>>> +        for (i = 0; i < current->bit_length; i++) {
>>> +            uint8_t bit;
>>> +            xu(1, reserved_payload_extension_data, bit, 0, 1, 0);
>>> +            current->data[i / 8] |= bit << (7 - i % 8);
>>> +        }
>>> +    }
>>> +#else
>>> +    for (i = 0; i < current->bit_length; i++)
>>> +        xu(1, reserved_payload_extension_data,
>>> +           current->data[i / 8] >> (7 - i % 8) & 1, 0, 1, 0);
>>
>> Do we really need to write one bit at a time? Why not simply one byte at
>> a time and then output the rest in one go? The specs treat it as a
>> bitfield and not as individual bits, so I think we are free to group it
>> as we please.
> 
> Mmh, ok.
> 
>>
>> The same goes for the other extension_data() function, of course. And
>> for reading.
> 
> Doing it for the other function will affect the existing trace output,
> so I'm inclined to leave it as is.
> 
>>
>> And shouldn't i be better size_t?
> 
> The correct type would be ptrdiff_t, but we never bothered with it and
> always used int.
> 

The bit_length field of the extension data struct is size_t. If you
presume that extension data will always only be passed through, then you
are on the safe side; if not, you are not.

>>
>>> +#endif
>>> +
>>> +    return 0;
>>> +}
>>> +
>>>  static int FUNC(sei_payload)(CodedBitstreamContext *ctx, RWContext *rw,
>>>                               H265RawSEIPayload *current, int prefix)
>>>  {
>>>      int err, i;
>>> -    int start_position, end_position;
>>> +    int start_position, current_position, end_position;
>>>  
>>>  #ifdef READ
>>>      start_position = get_bits_count(rw);
>>> @@ -2122,7 +2154,15 @@ static int FUNC(sei_payload)(CodedBitstreamContext *ctx, RWContext *rw,
>>>          }
>>>      }
>>>  
>>> -    if (byte_alignment(rw)) {
>>> +#ifdef READ
>>> +    current_position = get_bits_count(rw) - start_position;
>>> +    if (current_position != 8 * current->payload_size) {
>>> +#else
>>> +    current_position = put_bits_count(rw) - start_position;
>>> +    if (byte_alignment(rw) || current->extension_data.bit_length) {
>>> +#endif
>>> +        CHECK(FUNC(payload_extension)(ctx, rw, &current->extension_data,
>>> +                                      current->payload_size, current_position));
>>>          fixed(1, bit_equal_to_one, 1);
>>>          while (byte_alignment(rw))
>>>              fixed(1, bit_equal_to_zero, 0);
>>>
>>