Re: [FFmpeg-devel] [PATCH] avformat: add CRI USM demuxer

[Andreas Rheinhardt <andreas.rheinhardt@outlook.com>]

Paul B Mahol:
> On Wed, Sep 6, 2023 at 1:30 PM Andreas Rheinhardt <
> andreas.rheinhardt@outlook.com> wrote:
> 
>> Paul B Mahol:
>>> On Wed, Sep 6, 2023 at 11:26 AM Andreas Rheinhardt <
>>> andreas.rheinhardt@outlook.com> wrote:
>>>
>>>> Paul B Mahol:
>>>>>
>>>>> +    chunk_type = avio_rb32(pb);
>>>>> +    chunk_size = avio_rb32(pb);
>>>>
>>>> You are not checking whether the chunk here exceeds its containing
>> chunk.
>>>>
>>>>>
>>>>> +    av_fast_malloc(&usm->header, &usm->header_size,
>>>>> +                   chunk_size + AV_INPUT_BUFFER_PADDING_SIZE);
>>>>> +    if (!usm->header)
>>>>> +        return AVERROR(ENOMEM);
>>>>
>>>> The bytestream2 API does not rely on the buffer being padded at all.
>>>>
>>>>>
>>>>> +    bytestream2_skip(&sgb, string_offset);
>>>>
>>>> This is unnecessary, because you seek with an absolute offset lateron
>>>> anyway before using sgb.
>>>>
>>>>>
>>>>> +        bytestream2_seek(&sgb, string_offset + offset, SEEK_SET);
>>>>> +        while (bytestream2_get_bytes_left(&sgb) > 0) {
>>>>> +            key[n] = bytestream2_get_byte(&sgb);
>>>>> +            if (!key[n])
>>>>> +                break;
>>>>> +            if (n >= sizeof(key) - 1)
>>>>> +                break;
>>>>> +            n++;
>>>>> +        }
>>>>> +        key[n] = '\0';
>>>>
>>>> IMO this would be easier with strnlen(), avoiding sgb altogether.
>>>> You would of course need to explicitly check that you are not
>>>> overreading, but that is good practice anyway.
>>>>
>>>>>
>>>>> +    chunk_start = avio_tell(pb);
>>>>> +    avio_skip(pb, 1);
>>>>> +    payload_offset = avio_r8(pb);
>>>>> +    padding_size = avio_rb16(pb);
>>>>> +    stream_index = avio_r8(pb);
>>>>> +    avio_skip(pb, 2);
>>>>> +    payload_type = avio_r8(pb);
>>>>> +    frame_time = avio_rb32(pb);
>>>>> +    frame_rate = avio_rb32(pb);
>>>>> +    avio_skip(pb, 8);
>>>>
>>>> payload_offset and frame_time are set-but-unused; this might lead to
>>>> compiler warnings.
>>>>
>>>>> +        if (usm->ch[is_audio][stream_index].used == 1) {
>>>>> +            uint32_t pkt_size = chunk_size - (avio_tell(pb) -
>>>> chunk_start);
>>>>> +
>>>>
>>>> This is unnecessary: Unless we already had a read error, pkt_size is
>>>> chunk_size - (1 + 1 + 2 + 1 + 2 + 1 + 4 + 4 + 8).
>>>>
>>>> (Notice that in case padding_size is > 0, it will be part of the packet
>>>> with the current code; not sure if that is an issue.)
>>>>
>>>>>
>>>>> +
>>>>> +    avio_skip(pb, padding_size);
>>>>> +    avio_skip(pb, chunk_size - (avio_tell(pb) - chunk_start));
>>>>> +
>>>>
>>>> Simpler to just use avio_seek(pb, chunk_start + chunk_size, SEEK_SET);
>>>>
>>>
>>> But input might not be seekable.
>>>
>>
>> And? You know that avio_skip(pb, offset) is just avio_seek(pb, offset,
>> SEEK_CUR)?
>>
> 
> And? Do you know that SEEK_SET is different from SEEK_CUR with positive
> argument.
> 

You are using SEEK_CUR with -avio_tell(pb), which effectively makes it
an absolute seek.

- Andreas

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