[FFmpeg-devel] MpegEncContext->class is never initialized

[Andreas Rheinhardt]

John Dorian:
> Yes, I have a custom log callback. I have multiple files open and I'd like
> to associate logged errors with the correct file, which I am doing through
> the objects that have an "opaque" member (not all, but most do). I don't
> think this is any abuse of the API as it does state that the first member
> of a struct passed through av_log should be AVClass*.
> 

I meant: We (as in the libavcodec library/the FFmpeg project) don't
abide by our own APIs when we pass objects with a NULL AVClass* to av_log().

> The solution for now is to check for a null pointer e.g. in the log
> callback, "if ((AVClass**)ptr[0])==NULL)" The helper log functions such as
> av_default_item_name() do not have this check, perhaps that is the real
> problem here.
> 

Here is how the default log callback handles this:
    AVClass* avc = avcl ? *(AVClass **) avcl : NULL;
Further down the case of no object and an object with AVClass* set to
NULL are therefore handled the same.

> I'm not so sure it is required to be non-null given that the features of
> AVClass may not be useful to every object, e.g. options and such may not be
> appropriate for private objects.
> 

If no AVClass* is set, then this object should not be used with the
av_log API.

> 
> On Tue, Mar 4, 2025 at 4:36 PM Andreas Rheinhardt <
> andreas.rheinhardt at outlook.com> wrote:
> 
>> John Dorian:
>>> I discovered a crash here if log handler function tries to get the class
>>> name from "s"
>>>
>>> mpeg12dec.c:
>>> if (get_bits_left(&s->gb) < 0) {
>>>          av_log(s, AV_LOG_ERROR, "overread %d\n",
>> -get_bits_left(&s->gb));
>>>          return AVERROR_INVALIDDATA;
>>>      }
>>>
>>> And it seems to be because MpegEncContext->class is never initialized (or
>>> rather it is initialized to 0) due to av_malloc_z() I assume.
>>>
>>> So the question is, is it valid for any av object to have its av_class
>> set
>>> to 0.
>>
>> Here is the patch:
>> https://ffmpeg.org/pipermail/ffmpeg-devel/2025-March/340615.html
>> My reading of log.h is that it is illegal for the pointer to an AVClass
>> to be NULL. The default log callback handles this gracefully, so I
>> presume you are using a custom log callback.
>>
>> - Andreas
>>