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>

1 files changed, 12 insertions, 12 deletions

diff --git a/drivers/gpu/drm/drm_stub.c b/drivers/gpu/drm/drm_stub.c
index c236fd27eba6..581e61d04057 100644
--- 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;
 }