drm: fix returning -EINVAL on setmaster if another master is active

We link every DRM "file_priv" to a "drm_master" structure. Currently, the
drmSetMaster() call returns 0 when there is _any_ active master associated
with the "drm_master" structure of the calling "file_priv". This means,
that after drmSetMaster() we are not guaranteed to be DRM-Master and might
not be able to perform mode-setting.

A way to reproduce this is by starting weston with the DRM backend from
within an X-console (eg., xterm). Because the xserver's "drm_master" is
currently active, weston is assigned to the same master but is inactive
because its VT is inactive and the xserver is still active. But when
"fake-activating" weston, it calls drmSetMaster(). With current behavior
this returns "0/success" and weston thinks that it is DRM-Master, even
though it is not (as the xserver is still DRM-Master).
Expected behavior would be drmSetMaster() to return -EINVAL, because the
xserver is still DRM-Master. This patch changes exactly that.

The only way this bogus behavior would be useful is for clients to check
whether their associated "drm_master" is currently the active DRM-Master.
But this logic fails if no DRM-Master is currently active at all. Because
then the client itself would become DRM-Master (if it is root) and this
makes this whole thing useles.

Also note that the second "if-condition":
  file_priv->minor->master != file_priv->master
is always true and can be skipped.

Signed-off-by: David Herrmann <dh.herrmann@googlemail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>

diff --git a/drivers/gpu/drm/drm_stub.c b/drivers/gpu/drm/drm_stub.c
index c236fd27eba634878fd4455756d76761602a38fd..581e61d04057003049d3bdeecef8dca428f0a0a5 100644 (file)
--- a/drivers/gpu/drm/drm_stub.c
+++ b/drivers/gpu/drm/drm_stub.c
@@ -221,20 +221,20 @@ int drm_setmaster_ioctl(struct drm_device *dev, void *data,
        if (!file_priv->master)
                return -EINVAL;
 
-       if (!file_priv->minor->master &&
-           file_priv->minor->master != file_priv->master) {
-               mutex_lock(&dev->struct_mutex);
-               file_priv->minor->master = drm_master_get(file_priv->master);
-               file_priv->is_master = 1;
-               if (dev->driver->master_set) {
-                       ret = dev->driver->master_set(dev, file_priv, false);
-                       if (unlikely(ret != 0)) {
-                               file_priv->is_master = 0;
-                               drm_master_put(&file_priv->minor->master);
-                       }
+       if (file_priv->minor->master)
+               return -EINVAL;
+
+       mutex_lock(&dev->struct_mutex);
+       file_priv->minor->master = drm_master_get(file_priv->master);
+       file_priv->is_master = 1;
+       if (dev->driver->master_set) {
+               ret = dev->driver->master_set(dev, file_priv, false);
+               if (unlikely(ret != 0)) {
+                       file_priv->is_master = 0;
+                       drm_master_put(&file_priv->minor->master);
                }
-               mutex_unlock(&dev->struct_mutex);
        }
+       mutex_unlock(&dev->struct_mutex);
 
        return 0;
 }