There are cases (in mesa and applications) where one would open the
primary node without properly authenticating the client.

Sometimes we don't check if the authentication succeeds, but there's
also cases we simply forget to do it.

The former was a case for Mesa where it did not not check the return
value of drmGetMagic() [1]. That was fixed recently although, there's
the question of older drivers or other apps that exbibit this behaviour.

While omitting the call results in issues as seen in [2] and [3].

In the libva case, libva itself doesn't authenticate the DRM client and
the vaGetDisplayDRM documentation doesn't mention if the app should
either.

As of today, the official vainfo utility doesn't authenticate.

To workaround issues like these, some users resort to running their apps
under sudo. Which admittedly isn't always a good idea.

Since any DRIVER_RENDER driver has sufficient isolation between clients,
we can use that, for unauthenticated [primary node] ioctls that require
DRM_AUTH. But only if the respective ioctl is tagged as DRM_RENDER_ALLOW.

v2:
- Rework/simplify if check (Daniel V)
- Add examples to commit messages, elaborate. (Daniel V)

v3:
- Use single unlikely (Daniel V)

[1] https://gitlab.freedesktop.org/mesa/mesa/blob/2bc1f5c2e70fe3b4d41f060af9859bc2a94c5b62/src/egl/drivers/dri2/platform_wayland.c#L1136
[2] https://lists.freedesktop.org/archives/libva/2016-July/004185.html
[3] https://gitlab.freedesktop.org/mesa/kmscube/issues/1
Testcase: igt/core_unauth_vs_render
Cc: intel-gfx@lists.freedesktop.org
Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20190114085408.15933-2-emil.l.velikov@gmail.com