Re: [FFmpeg-devel] [PATCH 1/4] avcodec/ac3_parser: use a padded buffer in av_ac3_parse_header()

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

James Almer:
> On 6/5/2025 9:32 PM, Andreas Rheinhardt wrote:
>> James Almer:
>>> On 6/5/2025 9:29 PM, Andreas Rheinhardt wrote:
>>>> James Almer:
>>>>> The GetBitContext API requires the buffer to be padded, and the
>>>>> documentation for
>>>>> av_ac3_parse_header() does not specify it, so use a temporary local
>>>>> buffer.
>>>>>
>>>>> Signed-off-by: James Almer <jamrial@gmail.com>
>>>>> ---
>>>>>    libavcodec/ac3_parser.c | 16 +++++++++++++---
>>>>>    1 file changed, 13 insertions(+), 3 deletions(-)
>>>>>
>>>>> diff --git a/libavcodec/ac3_parser.c b/libavcodec/ac3_parser.c
>>>>> index 69989690dd..9065d700e2 100644
>>>>> --- a/libavcodec/ac3_parser.c
>>>>> +++ b/libavcodec/ac3_parser.c
>>>>> @@ -202,14 +202,24 @@ int av_ac3_parse_header(const uint8_t *buf,
>>>>> size_t size,
>>>>>    {
>>>>>        GetBitContext gb;
>>>>>        AC3HeaderInfo hdr;
>>>>> +    uint8_t *tmp = av_malloc(size + AV_INPUT_BUFFER_PADDING_SIZE);
>>>>>        int err;
>>>>>    -    err = init_get_bits8(&gb, buf, size);
>>>>> -    if (err < 0)
>>>>> +    if (!tmp)
>>>>> +        return AVERROR(ENOMEM);
>>>>> +
>>>>> +    memcpy(tmp, buf, size);
>>>>> +    memset(tmp + size, 0, AV_INPUT_BUFFER_PADDING_SIZE);
>>>>> +    err = init_get_bits8(&gb, tmp, size);
>>>>> +    if (err < 0) {
>>>>> +        av_free(tmp);
>>>>>            return AVERROR_INVALIDDATA;
>>>>> +    }
>>>>>        err = ff_ac3_parse_header(&gb, &hdr);
>>>>> -    if (err < 0)
>>>>> +    if (err < 0) {
>>>>> +        av_free(tmp);
>>>>>            return AVERROR_INVALIDDATA;
>>>>> +    }
>>>>>          *bitstream_id = hdr.bitstream_id;
>>>>>        *frame_size   = hdr.frame_size;
>>>>
>>>> There is no need for an allocation here; (E)AC-3 frames have a bounded
>>>> size and the number of bytes read by ff_ac3_parse_header() is even
>>>> smaller.
>>>
>>> What's the max size? Is there a define for it?
>>
>> Not yet.
> 
> Ok, what size should the static buffer be?
> 

It should not be static. And I don't know. Just look at the code to get
the maximum.

- Andreas

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