Re: [PATCH 6/7] v4l2-device: v4l2_device_release_subdev_node can't reference sd

[Laurent Pinchart <laurent.pinchart@ideasonboard.com>]

On Fri, Mar 01, 2019 at 12:46:52PM +0100, Hans Verkuil wrote:
> On 2/22/19 12:32 PM, Laurent Pinchart wrote:
> > Hi Hans,
> > 
> > Thank you for the patch.
> > 
> > On Thu, Feb 21, 2019 at 03:21:47PM +0100, Hans Verkuil wrote:
> >> When the v4l-subdev device node is released it calls the
> >> v4l2_device_release_subdev_node() function which sets sd->devnode
> >> to NULL.
> >>
> >> However, the v4l2_subdev struct may already be released causing this
> >> to write in freed memory.
> >>
> >> Instead just use the regular video_device_release release function
> >> (just calls kfree) and set sd->devnode to NULL right after the
> >> video_unregister_device() call.
> > 
> > This seems a bit of a workaround. The devnode can access the subdev in
> > multiple ways, it should really keep a reference to the subdev to ensure
> > it doesn't get freed early.
> 
> It's not the link from the devnode to the subdev (that's done through
> video_get_drvdata()), it's the link from the subdev to the devnode.

Right, my bad.

> As soon as the video device is unregistered sd->devnode should be set
> to NULL. It is in fact how sd->devnode is used: as a check if the devnode
> was registered.
> 
> The only other place where it is used is in v4l2_subdev_notify_event to
> send an event to the devnode, and after unregistering the video device
> you no longer want to do that, so setting sd->devnode to NULL after it
> is unregistered is the right thing to do.

Is there a risk the two function could race each other ?

> FYI, I'll post a v2 of this series soon.
> 
> >> Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
> >> ---
> >>  drivers/media/v4l2-core/v4l2-device.c | 10 ++--------
> >>  1 file changed, 2 insertions(+), 8 deletions(-)
> >>
> >> diff --git a/drivers/media/v4l2-core/v4l2-device.c b/drivers/media/v4l2-core/v4l2-device.c
> >> index e0ddb9a52bd1..57a7b220fa4d 100644
> >> --- a/drivers/media/v4l2-core/v4l2-device.c
> >> +++ b/drivers/media/v4l2-core/v4l2-device.c
> >> @@ -216,13 +216,6 @@ int v4l2_device_register_subdev(struct v4l2_device *v4l2_dev,
> >>  }
> >>  EXPORT_SYMBOL_GPL(v4l2_device_register_subdev);
> >>  
> >> -static void v4l2_device_release_subdev_node(struct video_device *vdev)
> >> -{
> >> -	struct v4l2_subdev *sd = video_get_drvdata(vdev);
> >> -	sd->devnode = NULL;
> >> -	kfree(vdev);
> >> -}
> >> -
> >>  int v4l2_device_register_subdev_nodes(struct v4l2_device *v4l2_dev)
> >>  {
> >>  	struct video_device *vdev;
> >> @@ -250,7 +243,7 @@ int v4l2_device_register_subdev_nodes(struct v4l2_device *v4l2_dev)
> >>  		vdev->dev_parent = sd->dev;
> >>  		vdev->v4l2_dev = v4l2_dev;
> >>  		vdev->fops = &v4l2_subdev_fops;
> >> -		vdev->release = v4l2_device_release_subdev_node;
> >> +		vdev->release = video_device_release;
> >>  		vdev->ctrl_handler = sd->ctrl_handler;
> >>  		err = __video_register_device(vdev, VFL_TYPE_SUBDEV, -1, 1,
> >>  					      sd->owner);
> >> @@ -319,6 +312,7 @@ void v4l2_device_unregister_subdev(struct v4l2_subdev *sd)
> >>  	}
> >>  #endif
> >>  	video_unregister_device(sd->devnode);
> >> +	sd->devnode = NULL;
> >>  	if (!sd->owner_v4l2_dev)
> >>  		module_put(sd->owner);
> >>  }

-- 
Regards,

Laurent Pinchart