drm: implement experimental render nodes

Render nodes provide an API for userspace to use non-privileged GPU
commands without any running DRM-Master. It is useful for offscreen
rendering, GPGPU clients, and normal render clients which do not perform
modesetting.

Compared to legacy clients, render clients no longer need any
authentication to perform client ioctls. Instead, user-space controls
render/client access to GPUs via filesystem access-modes on the
render-node. Once a render-node was opened, a client has full access to
the client/render operations on the GPU. However, no modesetting or ioctls
that affect global state are allowed on render nodes.

To prevent privilege-escalation, drivers must explicitly state that they
support render nodes. They must mark their render-only ioctls as
DRM_RENDER_ALLOW so render clients can use them. Furthermore, they must
support clients without any attached master.

If filesystem access-modes are not enough for fine-grained access control
to render nodes (very unlikely, considering the versaitlity of FS-ACLs),
you may still fall-back to fd-passing from server to client (which allows
arbitrary access-control). However, note that revoking access is
currently impossible and unlikely to get implemented.

Note: Render clients no longer have any associated DRM-Master as they are
supposed to be independent of any server state. DRM core highly depends on
file_priv->master to be non-NULL for modesetting/ctx/etc. commands.
Therefore, drivers must be very careful to not require DRM-Master if they
support DRIVER_RENDER.

So far render-nodes are protected by "drm_rnodes". As long as this
module-parameter is not set to 1, a driver will not create render nodes.
This allows us to experiment with the API a bit before we stabilize it.

v2: drop insecure GEM_FLINK to force use of dmabuf

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

1 files changed, 69 insertions, 0 deletions

diff --git a/Documentation/DocBook/drm.tmpl b/Documentation/DocBook/drm.tmpl
index 9fc8ed4ac0f4..ed1d6d289022 100644
--- a/Documentation/DocBook/drm.tmpl
+++ b/Documentation/DocBook/drm.tmpl
@@ -205,6 +205,12 @@
               Driver implements DRM PRIME buffer sharing.
             </para></listitem>
           </varlistentry>
+          <varlistentry>
+            <term>DRIVER_RENDER</term>
+            <listitem><para>
+              Driver supports dedicated render nodes.
+            </para></listitem>
+          </varlistentry>
         </variablelist>
       </sect3>
       <sect3>
@@ -2644,6 +2650,69 @@ int (*resume) (struct drm_device *);</synopsis>
       info, since man pages should cover the rest.
     </para>
 
+  <!-- External: render nodes -->
+
+    <sect1>
+      <title>Render nodes</title>
+      <para>
+        DRM core provides multiple character-devices for user-space to use.
+        Depending on which device is opened, user-space can perform a different
+        set of operations (mainly ioctls). The primary node is always created
+        and called <term>card&lt;num&gt;</term>. Additionally, a currently
+        unused control node, called <term>controlD&lt;num&gt;</term> is also
+        created. The primary node provides all legacy operations and
+        historically was the only interface used by userspace. With KMS, the
+        control node was introduced. However, the planned KMS control interface
+        has never been written and so the control node stays unused to date.
+      </para>
+      <para>
+        With the increased use of offscreen renderers and GPGPU applications,
+        clients no longer require running compositors or graphics servers to
+        make use of a GPU. But the DRM API required unprivileged clients to
+        authenticate to a DRM-Master prior to getting GPU access. To avoid this
+        step and to grant clients GPU access without authenticating, render
+        nodes were introduced. Render nodes solely serve render clients, that
+        is, no modesetting or privileged ioctls can be issued on render nodes.
+        Only non-global rendering commands are allowed. If a driver supports
+        render nodes, it must advertise it via the <term>DRIVER_RENDER</term>
+        DRM driver capability. If not supported, the primary node must be used
+        for render clients together with the legacy drmAuth authentication
+        procedure.
+      </para>
+      <para>
+        If a driver advertises render node support, DRM core will create a
+        separate render node called <term>renderD&lt;num&gt;</term>. There will
+        be one render node per device. No ioctls except  PRIME-related ioctls
+        will be allowed on this node. Especially <term>GEM_OPEN</term> will be
+        explicitly prohibited. Render nodes are designed to avoid the
+        buffer-leaks, which occur if clients guess the flink names or mmap
+        offsets on the legacy interface. Additionally to this basic interface,
+        drivers must mark their driver-dependent render-only ioctls as
+        <term>DRM_RENDER_ALLOW</term> so render clients can use them. Driver
+        authors must be careful not to allow any privileged ioctls on render
+        nodes.
+      </para>
+      <para>
+        With render nodes, user-space can now control access to the render node
+        via basic file-system access-modes. A running graphics server which
+        authenticates clients on the privileged primary/legacy node is no longer
+        required. Instead, a client can open the render node and is immediately
+        granted GPU access. Communication between clients (or servers) is done
+        via PRIME. FLINK from render node to legacy node is not supported. New
+        clients must not use the insecure FLINK interface.
+      </para>
+      <para>
+        Besides dropping all modeset/global ioctls, render nodes also drop the
+        DRM-Master concept. There is no reason to associate render clients with
+        a DRM-Master as they are independent of any graphics server. Besides,
+        they must work without any running master, anyway.
+        Drivers must be able to run without a master object if they support
+        render nodes. If, on the other hand, a driver requires shared state
+        between clients which is visible to user-space and accessible beyond
+        open-file boundaries, they cannot support render nodes.
+      </para>
+    </sect1>
+
   <!-- External: vblank handling -->
 
     <sect1>