[FFmpeg-devel] [PATCH 2/2] avformat/assenc: fix strstr calls on non-string

[Tim Angus]

On 31/01/2023 11:13, Tim Angus wrote:
> In the write_header function of the ASS encoder, extradata is
> searched for a substring using the strstr function. strstr expects a
> null terminated C string as its first parameter, but extradata is
> (notionally) not one of these, meaning that the calls to strstr only
> happen to work because (presumably) they encounter a 0 byte somewhere
> downstream of extradata. I say notionally because often extradata will
> in fact be null terminated (which causes other problems, see parent
> commit).
>
> Signed-off-by: Tim Angus <tim at ngus.net>
> ---
>   libavformat/assenc.c | 8 ++++----
>   1 file changed, 4 insertions(+), 4 deletions(-)
>
> diff --git a/libavformat/assenc.c b/libavformat/assenc.c
> index 4c9ea6f982..f6f5293bb7 100644
> --- a/libavformat/assenc.c
> +++ b/libavformat/assenc.c
> @@ -83,12 +83,12 @@ static int write_header(AVFormatContext *s)
>           if (header_string[strlen(header_string) - 1] != '\n')
>               avio_printf(s->pb, "\r\n");
>   
> -        av_free(header_string);
> -
> -        ass->ssa_mode = !strstr(par->extradata, "\n[V4+ Styles]");
> -        if (!strstr(par->extradata, "\n[Events]"))
> +        ass->ssa_mode = !strstr(header_string, "\n[V4+ Styles]");
> +        if (!strstr(header_string, "\n[Events]"))
>               avio_printf(s->pb, "[Events]\r\nFormat: %s, Start, End, Style, Name, MarginL, MarginR, MarginV, Effect, Text\r\n",
>                           ass->ssa_mode ? "Marked" : "Layer");
> +
> +        av_free(header_string);
>       }
>   
>       return 0;

Apparently extradata is padded with zeroes, so this is probably 
unnecessary, strictly speaking. Having said that, reasoning about why 
the code works is less obscure with the patch applied.