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-ci...@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
