Re: [FFmpeg-devel] [PATCH] avcodec/avcodec: don't uninitialize ch_layout in avcodec_close()

[James Almer <jamrial@gmail.com>]

On 3/19/2022 4:50 AM, Hendrik Leppkes wrote:
> On Sat, Mar 19, 2022 at 4:04 AM James Almer <jamrial@gmail.com> wrote:
>>
>> The function is not meant to clear codec parameters, and the lavf demux code
>> relies on this behavior.
>> Regression since 327efa66331ebdc0087c6b656059a8df2f404019.
>>
>> Signed-off-by: James Almer <jamrial@gmail.com>
>> ---
>>   libavcodec/avcodec.c | 6 ++++++
>>   1 file changed, 6 insertions(+)
>>
>> diff --git a/libavcodec/avcodec.c b/libavcodec/avcodec.c
>> index 38bdaad4fa..253c9f56cc 100644
>> --- a/libavcodec/avcodec.c
>> +++ b/libavcodec/avcodec.c
>> @@ -469,6 +469,7 @@ void avsubtitle_free(AVSubtitle *sub)
>>
>>   av_cold int avcodec_close(AVCodecContext *avctx)
>>   {
>> +    AVChannelLayout ch_layout;
>>       int i;
>>
>>       if (!avctx)
>> @@ -524,7 +525,12 @@ av_cold int avcodec_close(AVCodecContext *avctx)
>>
>>       if (avctx->priv_data && avctx->codec && avctx->codec->priv_class)
>>           av_opt_free(avctx->priv_data);
>> +    /* av_opt_free() will uninitialize avctx->ch_layout, but we want to keep it.
>> +       It will be uninitialized in avcodec_free_context() */
>> +    ch_layout = avctx->ch_layout;
>> +    memset(&avctx->ch_layout, 0, sizeof(avctx->ch_layout));
>>       av_opt_free(avctx);
>> +    avctx->ch_layout = ch_layout;
>>       av_freep(&avctx->priv_data);
>>       if (av_codec_is_encoder(avctx->codec)) {
>>           av_freep(&avctx->extradata);
> 
> This feels pretty ugly and still a bit risky that any call to
> av_opt_free could invalidate data its not supposed to. Maybe we should
> have a flag for AVOptions instead where av_opt_free won't touch an
> entry, because its only there to set/get it, not manage its memory?

Where would that flag be set? av_opt_free() takes none. And that 
function exists purely to free strings, dictionaries, and now 
uninitialize AVChannelLayout elements in a struct. If you don't want to 
free what av_opt_set() allocated, you shouldn't call av_opt_free() at all.

> 
> - Hendrik
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