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

[Michael Niedermayer]

On Thu, Apr 07, 2016 at 08:08:41PM +0200, wm4 wrote:
> 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.

it could be done with a mutex but it would be alot more code and
it seems noone works on implementing that

so unles there are objections ill push this, leaving the race is worst
of all options

[...]
-- 
Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

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