[PATCH 2/4] drm/fb_helper: Fix references to dev->mode_config.num_connector

[Lyude]

commit 255f0e7c418ad95a4baeda017ae6182ba9b3c423 upstream.

During boot, MST hotplugs are generally expected (even if no physical
hotplugging occurs) and result in DRM's connector topology changing.
This means that using num_connector from the current mode configuration
can lead to the number of connectors changing under us. This can lead to
some nasty scenarios in fbcon:

- We allocate an array to the size of dev->mode_config.num_connectors.
- MST hotplug occurs, dev->mode_config.num_connectors gets incremented.
- We try to loop through each element in the array using the new value
  of dev->mode_config.num_connectors, and end up going out of bounds
  since dev->mode_config.num_connectors is now larger then the array we
  allocated.

fb_helper->connector_count however, will always remain consistent while
we do a modeset in fb_helper.

This fix is only required for 4.6 and below. David Airlie's patchseries
for 4.7 to add connector reference counting provides a more proper fix
for this.

Changes since v1:
 - Remove leftover *dev variable from drm_pick_crtcs since it's not used
   now

Upstream fix: 0552f7651bc2 ("drm/i915/mst: use reference counted
connectors. (v3)")
Reviewed-by: Daniel Vetter <daniel.vetter@xxxxxxxx>
Signed-off-by: Lyude <cpaul@xxxxxxxxxx>
---
 drivers/gpu/drm/drm_fb_helper.c | 5 ++---
 1 file changed, 2 insertions(+), 3 deletions(-)

diff --git a/drivers/gpu/drm/drm_fb_helper.c b/drivers/gpu/drm/drm_fb_helper.c
index 855108e..fe4df97 100644
--- a/drivers/gpu/drm/drm_fb_helper.c
+++ b/drivers/gpu/drm/drm_fb_helper.c
@@ -1895,7 +1895,6 @@ static int drm_pick_crtcs(struct drm_fb_helper *fb_helper,
 			  int n, int width, int height)
 {
 	int c, o;
-	struct drm_device *dev = fb_helper->dev;
 	struct drm_connector *connector;
 	const struct drm_connector_helper_funcs *connector_funcs;
 	struct drm_encoder *encoder;
@@ -1914,7 +1913,7 @@ static int drm_pick_crtcs(struct drm_fb_helper *fb_helper,
 	if (modes[n] == NULL)
 		return best_score;
 
-	crtcs = kzalloc(dev->mode_config.num_connector *
+	crtcs = kzalloc(fb_helper->connector_count *
 			sizeof(struct drm_fb_helper_crtc *), GFP_KERNEL);
 	if (!crtcs)
 		return best_score;
@@ -1960,7 +1959,7 @@ static int drm_pick_crtcs(struct drm_fb_helper *fb_helper,
 		if (score > best_score) {
 			best_score = score;
 			memcpy(best_crtcs, crtcs,
-			       dev->mode_config.num_connector *
+			       fb_helper->connector_count *
 			       sizeof(struct drm_fb_helper_crtc *));
 		}
 	}
-- 
2.5.5