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 9:29 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.

> 
>> Anyway: The buffer leaks on success.
> 
> I'm aware, i said as much in a reply :p
> 

Should have read all your mails.

- Andreas

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