[FFmpeg-devel] [PATCH] avformat/assenc: avoid incorrect copy of null terminator

[Marton Balint]

On Wed, 29 Mar 2023, Marton Balint wrote:

> When writing a subtitle SSA/ASS subtitle file, the AVCodecParameters::extradata
> buffer is written directly to the output.  In the case where the buffer is
> filled from a matroska source file produced by some older versions of
> Handbrake, this buffer ends with a null terminating character, which is then
> erroneously copied into the middle of the output file. The change here avoids
> this problem by treating it as a string rather than a raw buffer. This way it
> is agnostic as to whether the source buffer was null terminated or not.
>
> Fixes ticket #10203.

Will apply.

Regards,
Marton

>
> Reported-by: Tim Angus <tim at ngus.net>
> Signed-off-by: Marton Balint <cus at passwd.hu>
> ---
> libavformat/assenc.c | 3 ++-
> 1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/libavformat/assenc.c b/libavformat/assenc.c
> index 85a1e53371..6ecfb04517 100644
> --- a/libavformat/assenc.c
> +++ b/libavformat/assenc.c
> @@ -70,8 +70,9 @@ static int write_header(AVFormatContext *s)
>                 ass->trailer = trailer;
>         }
>
> +        header_size = av_strnlen(par->extradata, header_size);
>         avio_write(s->pb, par->extradata, header_size);
> -        if (par->extradata[header_size - 1] != '\n')
> +        if (header_size && par->extradata[header_size - 1] != '\n')
>             avio_write(s->pb, "\r\n", 2);
>         ass->ssa_mode = !strstr(par->extradata, "\n[V4+ Styles]");
>         if (!strstr(par->extradata, "\n[Events]"))
> -- 
> 2.35.3
>
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