Re: [PATCH v6 2/2] drm: Prevent release fb after cleanup mode config

[jeffy]

Hi Sean,

On 04/11/2017 04:31 AM, Sean Paul wrote:
> On Mon, Apr 10, 2017 at 06:00:45PM +0800, Jeffy Chen wrote:
> > After unbinding drm, the user space may still owns the drm dev fd,
> > and may trigger fb release after cleanup mode config.
> >
> > Add a sanity check to prevent that.
> >
> > Signed-off-by: Jeffy Chen <jeffy.chen@xxxxxxxxxxxxxx>
> > ---
> >
> > Changes in v6: None
> > Changes in v5: None
> > Changes in v2: None
> >
> >   drivers/gpu/drm/drm_framebuffer.c | 5 +++++
> >   1 file changed, 5 insertions(+)
> >
> > diff --git a/drivers/gpu/drm/drm_framebuffer.c b/drivers/gpu/drm/drm_framebuffer.c
> > index e8f9c13..03c1632 100644
> > --- a/drivers/gpu/drm/drm_framebuffer.c
> > +++ b/drivers/gpu/drm/drm_framebuffer.c
> > @@ -583,6 +583,11 @@ void drm_fb_release(struct drm_file *priv)
> >   {
> >   	struct drm_framebuffer *fb, *tfb;
> >   	struct drm_mode_rmfb_work arg;
> > +	struct drm_minor *minor = priv->minor;
> > +	struct drm_device *dev = minor->dev;
> > +
> > +	if (WARN_ON(!dev->mode_config.num_fb && !list_empty(&priv->fbs)))
>
> Have you actually seen this happen? num_fb should be tightly couple to
> priv->fbs, so it seems like this could only result from a driver bug (or I'm not
> reading the code correctly).
yes, 100% repro by:
1/ start display server
2/ unbind drm
3/ stop display server

the num_fb would be decreased(with a warning in 
drm_mode_config_cleanup's fb_list check) in 
drm_mode_config_cleanup->drm_framebuffer_free->rockchip_drm_fb_destroy->drm_framebuffer_cleanup

this flow would not modify the priv->fbs at the same time. so it would 
still remains the pointer of those freed fb.

> Sean
>
> > +		return;
> >
> >   	INIT_LIST_HEAD(&arg.fbs);
> >
> > --
> > 2.1.4