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

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Date:	Mon, 13 Jul 2015 09:23:55 +0200
From:	Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Jörg Otte 
	<jrg.otte@...il.com>, David Airlie <airlied@...ux.ie>,
	DRI <dri-devel@...ts.freedesktop.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [4.2.0-rc1-00201-g59c3cb5] Regression: kernel NULL pointer dereference

Op 13-07-15 om 08:22 schreef Daniel Vetter:
> On Sun, Jul 12, 2015 at 09:52:51AM -0700, Linus Torvalds wrote:
>> On Sun, Jul 12, 2015 at 1:03 AM, Jörg Otte <jrg.otte@...il.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?
> Indeed embarrassing. I've missed that we still have 1 caller left that's
> using the transitional helpers, and those don't fill out
> plane_state->state backpointers to the global atomic update since there is
> no global atomic update for transitional helpers. Below diff should fix
> this - we need to preferentially check crts_state->active and if that's
> not set intel_crtc->active should yield the right result for the one
> remaining caller (it's in the crtc_disable paths).
>
> For cheap excuses why i915 is so crap in 4.2: Thanks to a hipshot decision
> to transition to a different QA team ("we'll do this in 1 week without
> upfront planing") I essentially don't have proper QA support for 1-2
> months by now. The other trouble in this area specifically is that this
> code is already completely changed in -next again, so any testing done on
> integration trees (like -next or drm-intel-nightly) won't test any patches
> for 4.2.
> -Daniel
>
> Oh and Signed-off-by: Daniel Vetter <daniel.vetter@...el.com> in case you
> decide to apply this right away.
>
Well your version has the benefit of compiling without errors. :-)

Reviewed-by: Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>
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