[FFmpeg-devel] [PATCH] avformat/concat: finalize the AVBprint buffer immediately

[James Almer]

On 7/26/2021 6:49 PM, Marton Balint wrote:
> 
> 
> On Mon, 26 Jul 2021, James Almer wrote:
> 
>> Don't attempt to read its contents in place.
>> Fixes invalid reads when run under Valgrind.
> 
> As far as I remember AVBPrint buffer CAN be read in place by design, 
> zero terminator is always guaranteed, not only after finalizing. So this 
> should not be needed. What is causing the invalid reads exactly?
> 
> Thanks,
> Marton

It happens only when you use a URI string that doesn't end with a line 
break. I just noticed that after this patch some invalid reads still 
happen in any case.

I think i found out the culprit, and the following seems to fix it:

> diff --git a/libavformat/concat.c b/libavformat/concat.c
> index aec1f52d8e..94917840c6 100644
> --- a/libavformat/concat.c
> +++ b/libavformat/concat.c
> @@ -251,7 +251,7 @@ static av_cold int concatf_open(URLContext *h, const char *uri, int flags)
>              err = AVERROR(ENOMEM);
>              break;
>          }
> -        cursor++;
> +        if (*cursor) cursor++;
> 
>          if (++len == SIZE_MAX / sizeof(*nodes)) {
>              av_free(node_uri);

Basically, av_get_token() returns a pointer to the terminating 
character, so cursor++ for the next loop when said character was \0 was 
wrong.