[FFmpeg-devel] [PATCH] avformat/format: Fix registering a format more than once and related races

[wm4]

On Thu,  7 Apr 2016 17:51:26 +0200
Michael Niedermayer <michael at niedermayer.cc> wrote:

> Signed-off-by: Michael Niedermayer <michael at niedermayer.cc>
> ---
>  libavformat/format.c |   16 ++++++++++------
>  1 file changed, 10 insertions(+), 6 deletions(-)
> 
> diff --git a/libavformat/format.c b/libavformat/format.c
> index 15fe167..f0abb5d 100644
> --- a/libavformat/format.c
> +++ b/libavformat/format.c
> @@ -62,20 +62,24 @@ void av_register_input_format(AVInputFormat *format)
>  {
>      AVInputFormat **p = last_iformat;
>  
> -    format->next = NULL;
> -    while(*p || avpriv_atomic_ptr_cas((void * volatile *)p, NULL, format))
> +    // Note, format could be added after the first 2 checks but that implies that *p is no longer NULL
> +    while(p != &format->next && !format->next && avpriv_atomic_ptr_cas((void * volatile *)p, NULL, format))
>          p = &(*p)->next;
> -    last_iformat = &format->next;
> +
> +    if (!format->next)
> +        last_iformat = &format->next;
>  }
>  
>  void av_register_output_format(AVOutputFormat *format)
>  {
>      AVOutputFormat **p = last_oformat;
>  
> -    format->next = NULL;
> -    while(*p || avpriv_atomic_ptr_cas((void * volatile *)p, NULL, format))
> +    // Note, format could be added after the first 2 checks but that implies that *p is no longer NULL
> +    while(p != &format->next && !format->next && avpriv_atomic_ptr_cas((void * volatile *)p, NULL, format))
>          p = &(*p)->next;
> -    last_oformat = &format->next;
> +
> +    if (!format->next)
> +        last_oformat = &format->next;
>  }
>  
>  int av_match_ext(const char *filename, const char *extensions)

This is a giant heap of crap, and it's probably not easy to do much
better. Have you considered just using a mutex instead? We have the
equivalent of pthread_init_once on all platforms now.