[FFmpeg-devel] [PATCH] avformat/utils: Fix undefined NULL + 0

[James Almer]

On 2/14/2021 6:28 PM, Andreas Rheinhardt wrote:
> This is undefined behaviour in C, so use data = len ? data + len : data
> instead if data += len. GCC optimizes the branch away in this case;
> Clang unfortunately doesn't.
> 
> Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt at gmail.com>
> ---
> Checking for len != 0 instead of > 0 allows the compiler to optimize the
> branch away; maybe future versions of Clang (I used 11) will do so, too.
> 
>   libavformat/utils.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/libavformat/utils.c b/libavformat/utils.c
> index 3e955b85bc..cea6d4ca92 100644
> --- a/libavformat/utils.c
> +++ b/libavformat/utils.c
> @@ -1426,7 +1426,7 @@ static int parse_packet(AVFormatContext *s, AVPacket *pkt,
>           pkt->pts = pkt->dts = AV_NOPTS_VALUE;
>           pkt->pos = -1;
>           /* increment read pointer */
> -        data += len;
> +        data  = len ? data + len : data;

Isn't adding any offset to a NULL pointer UB? That was afaik the entire 
point behind the change in c40d36076a.

>           size -= len;
>   
>           got_output = !!out_pkt.size;
>