drm: hollow-out GET_CLIENT ioctl

We not only have debugfs files to do pretty much the equivalent of
lsof, we also have an ioctl. Not that compared to lsof this dumps a
wee bit more information, but we can still get at that from debugfs
easily.

I've dug around in mesa, libdrm and ddx histories and the only users
seem to be drm/tests/dristat.c and drm/tests/getclients.c. The later
is a testcase for the ioctl itself since up to

commit b018fcdaa5e8b4eabb8cffda687d00004a3c4785
Author: Eric Anholt <eric@anholt.net>
Date:   Thu Nov 22 18:46:54 2007 +1000

    drm: Make DRM_IOCTL_GET_CLIENT return EINVAL when it can't find client #idx

there was actually no way at all for userspace to enumerate all
clients since the kernel just wouldn't tell it when to stop. Which
completely broke it's only user, dristat -c.

So obviously that ioctl wasn't much use for debugging. Hence I don't
see any point in keeping support for a tool which was pretty obviously
never really used, and while we have good replacements in the form of
equivalent debugfs files.

Still, to keep dristat -c from looping forever again stop it early by
returning an unconditional -EINVAL. Also add a comment in the code
about why.

v2: Slightly less hollowed-out implementation. libva uses GET_CLIENTS
to figure out whether the fd it has is already authenticated or not.
So we need to keep that part of things working. Simplest way is to
just return one entry to keep va_drm_is_authenticated in
libva/va/drm/va_drm_auth.c working.

This is exercised by igt/drm_get_client_auth which contains a
copypasta of the libva auth check code.

Cc: Gwenole Beauchesne <gwenole.beauchesne@intel.com>
Cc: David Herrmann <dh.herrmann@gmail.com>
Reviewed-by: David Herrmann <dh.herrmann@gmail.com>
Reviewed-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Dave Airlie <airlied@redhat.com>

drivers/gpu/drm/drm_ioctl.c

@@ -217,29 +217,30 @@ int drm_getclient(struct drm_device *dev, void *data,
 		  struct drm_file *file_priv)
 {
 	struct drm_client *client = data;
-	struct drm_file *pt;
-	int idx;
-	int i;
-
-	idx = client->idx;
-	i = 0;
 
-	mutex_lock(&dev->struct_mutex);
-	list_for_each_entry(pt, &dev->filelist, lhead) {
-		if (i++ >= idx) {
-			client->auth = pt->authenticated;
-			client->pid = pid_vnr(pt->pid);
-			client->uid = from_kuid_munged(current_user_ns(), pt->uid);
-			client->magic = pt->magic;
-			client->iocs = pt->ioctl_count;
-			mutex_unlock(&dev->struct_mutex);
-
-			return 0;
-		}
+	/*
+	 * Hollowed-out getclient ioctl to keep some dead old drm tests/tools
+	 * not breaking completely. Userspace tools stop enumerating one they
+	 * get -EINVAL, hence this is the return value we need to hand back for
+	 * no clients tracked.
+	 *
+	 * Unfortunately some clients (*cough* libva *cough*) use this in a fun
+	 * attempt to figure out whether they're authenticated or not. Since
+	 * that's the only thing they care about, give it to the directly
+	 * instead of walking one giant list.
+	 */
+	if (client->idx == 0) {
+		client->auth = file_priv->authenticated;
+		client->pid = pid_vnr(file_priv->pid);
+		client->uid = from_kuid_munged(current_user_ns(),
+					       file_priv->uid);
+		client->magic = 0;
+		client->iocs = 0;
+
+		return 0;
+	} else {
+		return -EINVAL;
 	}
-	mutex_unlock(&dev->struct_mutex);
-
-	return -EINVAL;
 }
 
 /**