[FFmpeg-devel] [PATCH 1/3] avcodec/avcodec: Add codec_tags array to AVCodec

[James Almer]

On 1/21/2020 4:30 PM, Michael Niedermayer wrote:
> On Tue, Jan 21, 2020 at 07:48:38PM +0100, Anton Khirnov wrote:
>> Quoting Michael Niedermayer (2019-12-30 00:38:17)
>>> This allows the fuzzer to target meaningfull codec tags instead
>>> of hunting the 4gb space, which it seems to have problems with.
>>>
>>> Suggested-by: James
>>> Signed-off-by: Michael Niedermayer <michael at niedermayer.cc>
>>> ---
>>>  libavcodec/avcodec.h | 6 ++++++
>>>  1 file changed, 6 insertions(+)
>>>
>>> diff --git a/libavcodec/avcodec.h b/libavcodec/avcodec.h
>>> index 119b32dc1f..b0c6a8f2e3 100644
>>> --- a/libavcodec/avcodec.h
>>> +++ b/libavcodec/avcodec.h
>>> @@ -3634,6 +3634,12 @@ typedef struct AVCodec {
>>>       * The user can only access this field via avcodec_get_hw_config().
>>>       */
>>>      const struct AVCodecHWConfigInternal **hw_configs;
>>> +
>>> +    /**
>>> +     * List of supported codec_tags, terminated by CODEC_TAGS_END.
>>> +     */
>>> +    const uint32_t *codec_tags;
>>> +#define CODEC_TAGS_END -1
>>
>> Is this supposed to be public or for fuzzer use only?
>> If the latter, then CODEC_TAGS_END doesn't need to live in a public
>> header.
> 
> the codec_tag field is public. So eventually the list of valid codec tags
> should become too.

It's at the end of AVCodec, which is where the fields that must not be
used or accessed from outside of lavc reside. They are documented as
"can be removed at any time without warning and without bumping
version". So basically, it's not public.

Since you are accessing them directly from the fuzzer anyway, might as
well just assume the -1 termination and not bother adding a public
define for it. If we ever want this public then it can be added, perhaps
after a better design is found.

> In this initial patch codec_tags was not really intended to be public
> and i can certainly move CODEC_TAGS_END to a internal header if preferred.
> but this is a change that we will likely undo in the future ...
> 
> Thanks
> 
> [...]
> 
> 
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