drm: remove redundant drm_file->uid

Each DRM file-context caches the EUID of the process that opened the file.
It is used exclusively for debugging purposes in /proc/dri/ and friends.

Note, however, that we can already fetch the EUID from
priv->pid->task->creds. The pointer-chasing will not hurt us, since it is
only about debugging, anyway.

Since we already are in an rcu-read-side, we can use __task_cred() rather
than task_cred_xxx().
Signed-off-by: David Herrmann <dh.herrmann@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20160901124837.680-2-dh.herrmann@gmail.com

drivers/gpu/drm/drm_fops.c

@@ -199,7 +199,6 @@ static int drm_open_helper(struct file *filp, struct drm_minor *minor)
 
 	filp->private_data = priv;
 	priv->filp = filp;
-	priv->uid = current_euid();
 	priv->pid = get_pid(task_pid(current));
 	priv->minor = minor;
 

drivers/gpu/drm/drm_info.c

@@ -80,6 +80,7 @@ int drm_clients_info(struct seq_file *m, void *data)
 	struct drm_info_node *node = (struct drm_info_node *) m->private;
 	struct drm_device *dev = node->minor->dev;
 	struct drm_file *priv;
+	kuid_t uid;
 
 	seq_printf(m,
 		   "%20s %5s %3s master a %5s %10s\n",
@@ -98,13 +99,14 @@ int drm_clients_info(struct seq_file *m, void *data)
 
 		rcu_read_lock(); /* locks pid_task()->comm */
 		task = pid_task(priv->pid, PIDTYPE_PID);
+		uid = task ? __task_cred(task)->euid : GLOBAL_ROOT_UID;
 		seq_printf(m, "%20s %5d %3d   %c    %c %5d %10u\n",
 			   task ? task->comm : "<unknown>",
 			   pid_vnr(priv->pid),
 			   priv->minor->index,
 			   drm_is_current_master(priv) ? 'y' : 'n',
 			   priv->authenticated ? 'y' : 'n',
-			   from_kuid_munged(seq_user_ns(m), priv->uid),
+			   from_kuid_munged(seq_user_ns(m), uid),
 			   priv->magic);
 		rcu_read_unlock();
 	}

include/drm/drmP.h

@@ -379,7 +379,6 @@ struct drm_file {
 	unsigned is_master:1;
 
 	struct pid *pid;
-	kuid_t uid;
 	drm_magic_t magic;
 	struct list_head lhead;
 	struct drm_minor *minor;