Re: [FFmpeg-devel] [PATCH 3/6] avformat/aviobuf: Don't use NULL as src for memcpy

[James Almer <jamrial@gmail.com>]

On 9/28/2022 3:58 PM, Andreas Rheinhardt wrote:
> This might happen in avio_write() if size == 0
> when the direct codepath is taken. It is undefined behaviour
> according to the spec although it happens to work in practice.
> Fixes the webm-webvtt-remux FATE-test under UBSan.
> 
> Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>
> ---
>   libavformat/aviobuf.c | 4 +++-
>   1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/libavformat/aviobuf.c b/libavformat/aviobuf.c
> index b20b1a611a..5b6a42d7f4 100644
> --- a/libavformat/aviobuf.c
> +++ b/libavformat/aviobuf.c
> @@ -231,6 +231,8 @@ void ffio_fill(AVIOContext *s, int b, int64_t count)
>   
>   void avio_write(AVIOContext *s, const unsigned char *buf, int size)
>   {
> +    if (size <= 0)
> +        return;
>       if (s->direct && !s->update_checksum) {
>           avio_flush(s);
>           writeout(s, buf, size);
> @@ -246,7 +248,7 @@ void avio_write(AVIOContext *s, const unsigned char *buf, int size)
>   
>           buf += len;
>           size -= len;
> -    }
> +    } while (size > 0);

Why are you adding this at the end of a similar while statement? Did you 
mean to replace the previous one with do()?

>   }
>   
>   void avio_flush(AVIOContext *s)
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