[FFmpeg-devel] [PATCH] avutil/frame: ensure the frame is writable in av_frame_copy()

[James Almer]

On 3/14/2021 7:47 AM, Anton Khirnov wrote:
> Quoting James Almer (2021-03-11 15:18:38)
>> On 3/11/2021 10:56 AM, Andreas Rheinhardt wrote:
>>> James Almer:
>>>> Signed-off-by: James Almer <jamrial at gmail.com>
>>>> ---
>>>>    libavutil/frame.c | 2 ++
>>>>    1 file changed, 2 insertions(+)
>>>>
>>>> diff --git a/libavutil/frame.c b/libavutil/frame.c
>>>> index eab51b6a32..ec79d053e1 100644
>>>> --- a/libavutil/frame.c
>>>> +++ b/libavutil/frame.c
>>>> @@ -800,6 +800,8 @@ int av_frame_copy(AVFrame *dst, const AVFrame *src)
>>>>    {
>>>>        if (dst->format != src->format || dst->format < 0)
>>>>            return AVERROR(EINVAL);
>>>> +    if (!av_frame_is_writable(dst))
>>>> +        return AVERROR(EINVAL);
>>>>    
>>>>        if (dst->width > 0 && dst->height > 0)
>>>>            return frame_copy_video(dst, src);
>>>>
>>> This will break scenarios where one owns all the references to a frame's
>>> data without the frame being writable?
>>
>> It would, but that's by design. We define a frame as writable when its
>> buffers are both reference counted and there's at most a single
>> reference to them. Who owns any of the references does not play a part
>> in that.
> 
> I disagree that this is a hard definition. It's merely a convention. If
> all the references are under control of a single bit of code, it can
> have its own convention. E.g. frame threading in libavcodec does exactly
> that.

Shouldn't this function then warn about it? A simple warning that 
ownership *and* writability of all buffer references within dst should 
be ensured before calling it would maybe suffice.
And i put emphasis on both characteristics because the user, despite 
having the only reference/s, may have not created the frame and have no 
knowledge of how it was created. A decoder could have handcrafted the 
frame by explicitly doing something like frame->buf[0] = 
av_buffer_create(..., AV_BUFFER_FLAG_READONLY), because they point at 
something that's effectively not writable (Internal buffers from an 
external library used by the lavc decoder).