Re: [4.2.0-rc1-00201-g59c3cb5] Regression: kernel NULL pointer dereference

From: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
To: "Linus Torvalds" <torvalds@linux-foundation.org>,
	"Jörg Otte" <jrg.otte@gmail.com>,
	"Daniel Vetter" <daniel.vetter@ffwll.ch>
Cc: David Airlie <airlied@linux.ie>,
	DRI <dri-devel@lists.freedesktop.org>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: [4.2.0-rc1-00201-g59c3cb5] Regression: kernel NULL pointer dereference
Date: Mon, 13 Jul 2015 07:56:10 +0200	[thread overview]
Message-ID: <55A352FA.1000300@linux.intel.com> (raw)
In-Reply-To: <CA+55aFxc=Xi1sjM+VFGw9ZR5-awgOy3VkE7goSuEdTJbYWfMGA@mail.gmail.com>

Op 12-07-15 om 18:52 schreef Linus Torvalds:
> On Sun, Jul 12, 2015 at 1:03 AM, Jörg Otte <jrg.otte@gmail.com> wrote:
>> BUG: unable to handle kernel NULL pointer dereference at 0000000000000009
>> IP: [<ffffffffbd3447bb>] 0xffffffffbd3447bb
> Ugh. Please enable KALLSYMS to get sane symbols.
>
> But yes, "crtc_state->base.active" is at offset 9 from "crtc_state",
> so it's pretty clearly just that change frm
>
> -       if (intel_crtc->active) {
> +       if (crtc_state->base.active) {
>
> and "crtc_state" is NULL.
>
> And the code very much knows that crtc_state can be NULL, since it's
> initialized with
>
>         crtc_state = state->base.state ?
>                 intel_atomic_get_crtc_state(state->base.state,
> intel_crtc) : NULL;
>
> Tssk. Daniel? Should I just revert that commit dec4f799d0a4
> ("drm/i915: Use crtc_state->active in primary check_plane func") for
> now, or is there a better fix? Like just checking crtc_state for NULL?
>
>                     Linus
More symbols would be nice.

With the transitional helpers when crtc_state == NULL you don't want to update the scalers or funny things happen.
Fix is probably something like this:

diff --git a/drivers/gpu/drm/i915/intel_display.c b/drivers/gpu/drm/i915/intel_display.c
index ba9321998a41..830e07b23a15 100644
--- a/drivers/gpu/drm/i915/intel_display.c
+++ b/drivers/gpu/drm/i915/intel_display.c
@@ -13276,7 +13276,7 @@ intel_check_primary_plane(struct drm_plane *plane,
 	if (ret)
 		return ret;
 
-	if (crtc_state->base.active) {
+	if (crtc_state ? crtc_state->base.active || crtc->state->active) {
 		struct intel_plane_state *old_state =
 			to_intel_plane_state(plane->state);
 

