On Thu, Aug 26, 2021 at 10:01:18AM +0800, Desmond Cheong Zhi Xi wrote:
> In a future patch, a read lock on drm_device.master_rwsem is
> held in the ioctl handler before the check for ioctl
> permissions. However, this inverts the lock hierarchy of
> drm_global_mutex --> master_rwsem.
>
> To avoid this, we do some prep work to grab the drm_global_mutex
> before checking for ioctl permissions.
>
> Signed-off-by: Desmond Cheong Zhi Xi <desmondcheongzx@xxxxxxxxx>
> ---
> drivers/gpu/drm/drm_ioctl.c | 18 +++++++++---------
> 1 file changed, 9 insertions(+), 9 deletions(-)
>
> diff --git a/drivers/gpu/drm/drm_ioctl.c b/drivers/gpu/drm/drm_ioctl.c
> index d25713b09b80..158629d88319 100644
> --- a/drivers/gpu/drm/drm_ioctl.c
> +++ b/drivers/gpu/drm/drm_ioctl.c
> @@ -772,19 +772,19 @@ long drm_ioctl_kernel(struct file *file, drm_ioctl_t *func, void *kdata,
> if (drm_dev_is_unplugged(dev))
> return -ENODEV;
>
> + /* Enforce sane locking for modern driver ioctls. */
> + if (unlikely(drm_core_check_feature(dev, DRIVER_LEGACY)) && !(flags & DRM_UNLOCKED))
Maybe have a local bool locked_ioctl for this so it's extremely clear it's
the same condition in both?
Either way: Reviewed-by: Daniel Vetter <daniel.vetter@xxxxxxxx>
> + mutex_lock(&drm_global_mutex);
> +
> retcode = drm_ioctl_permit(flags, file_priv);
> if (unlikely(retcode))
> - return retcode;
> + goto out;
>
> - /* Enforce sane locking for modern driver ioctls. */
> - if (likely(!drm_core_check_feature(dev, DRIVER_LEGACY)) ||
> - (flags & DRM_UNLOCKED))
> - retcode = func(dev, kdata, file_priv);
> - else {
> - mutex_lock(&drm_global_mutex);
> - retcode = func(dev, kdata, file_priv);
> + retcode = func(dev, kdata, file_priv);
> +
> +out:
> + if (unlikely(drm_core_check_feature(dev, DRIVER_LEGACY)) && !(flags & DRM_UNLOCKED))
> mutex_unlock(&drm_global_mutex);
> - }
> return retcode;
> }
> EXPORT_SYMBOL(drm_ioctl_kernel);
> --
> 2.25.1
>
--
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch
