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

[James Almer <jamrial@gmail.com>]

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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?

> Anyway: The buffer leaks on success.

I'm aware, i said as much in a reply :p

> 
> - Andreas
> 
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