Re: cdev/devm_* issues (was Re: [TECH TOPIC] Rust for Linux)

[Dan Carpenter <dan.carpenter@oracle.com>]

The other thing which is quite tricky to get right is kobj releases.  I
have a bunch of these static checker warning but they're annoying
because they're not something which really affects real life so I can't
be bothered to fix or report them.

Also some of them are just unfixable, for example
__video_register_device().

drivers/media/v4l2-core/v4l2-dev.c
  1031          /* Part 4: register the device with sysfs */
  1032          vdev->dev.class = &video_class;
  1033          vdev->dev.devt = MKDEV(VIDEO_MAJOR, vdev->minor);
  1034          vdev->dev.parent = vdev->dev_parent;
  1035          dev_set_name(&vdev->dev, "%s%d", name_base, vdev->num);
  1036          ret = device_register(&vdev->dev);

If device_register() fails then it's illegal to manually clean up.

  1037          if (ret < 0) {
  1038                  pr_err("%s: device_register failed\n", __func__);
  1039                  goto cleanup;
                        ^^^^^^^^^^^^^
But this does, but it turns out it's fine.

  1040          }
  1041          /* Register the release callback that will be called when the last
  1042             reference to the device goes away. */
  1043          vdev->dev.release = v4l2_device_release;

The ->release() function is not set until here so it's just going to
trigger the debug message in kobject_cleanup() about a missing
->release() function.  So no problems.

  1044  
  1045          if (nr != -1 && nr != vdev->num && warn_if_nr_in_use)
  1046                  pr_warn("%s: requested %s%d, got %s\n", __func__,
  1047                          name_base, nr, video_device_node_name(vdev));
  1048  
  1049          /* Increase v4l2_device refcount */
  1050          v4l2_device_get(vdev->v4l2_dev);
  1051  
  1052          /* Part 5: Register the entity. */
  1053          ret = video_register_media_controller(vdev);
                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
But then there is no way to clean up from this is
video_register_media_controller() fails.  If we call put_device() it
would lead to use after frees in the callers.  We just have to ignore
the error.

It's a minor thing, but it's so frustrating.

  1054  
  1055          /* Part 6: Activate this minor. The char device can now be used. */
  1056          set_bit(V4L2_FL_REGISTERED, &vdev->flags);
  1057  
  1058          return 0;

regards,
dan carpenter