linux-kernel - Re: [PATCH] drm/radeon: don't add files at control minor debugfs directory

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Date:   Mon, 5 Dec 2016 09:04:48 +0100
From:   Daniel Vetter <daniel@...ll.ch>
To:     Christian König <christian.koenig@....com>
Cc:     Nicolai Stange <nicstange@...il.com>,
        Alex Deucher <alexander.deucher@....com>,
        Daniel Vetter <daniel.vetter@...el.com>,
        linux-kernel@...r.kernel.org, dri-devel@...ts.freedesktop.org
Subject: Re: [PATCH] drm/radeon: don't add files at control minor debugfs
 directory

On Mon, Dec 05, 2016 at 08:42:49AM +0100, Christian König wrote:
> Am 05.12.2016 um 08:27 schrieb Daniel Vetter:
> > On Sat, Dec 03, 2016 at 03:47:00PM +0100, Nicolai Stange wrote:
> > > Since commit 8a357d10043c ("drm: Nerf DRM_CONTROL nodes"), a
> > > struct drm_device's ->control member is always NULL.
> > > 
> > > In the case of CONFIG_DEBUG_FS=y, radeon_debugfs_add_files() accesses
> > > ->control->debugfs_root though. This results in the following Oops:
> > > 
> > >    BUG: unable to handle kernel NULL pointer dereference at 0000000000000018
> > >    IP: radeon_debugfs_add_files+0x90/0x100 [radeon]
> > >    PGD 0
> > >    Oops: 0000 [#1] SMP
> > >    [...]
> > >    Call Trace:
> > >     ? work_on_cpu+0xb0/0xb0
> > >     radeon_fence_driver_init+0x120/0x150 [radeon]
> > >     si_init+0x122/0xd50 [radeon]
> > >     ? _raw_spin_unlock_irq+0x2c/0x40
> > >     ? device_pm_check_callbacks+0xb3/0xc0
> > >     radeon_device_init+0x958/0xda0 [radeon]
> > >     radeon_driver_load_kms+0x9a/0x210 [radeon]
> > >     drm_dev_register+0xa9/0xd0 [drm]
> > >     drm_get_pci_dev+0x9c/0x1e0 [drm]
> > >     radeon_pci_probe+0xb8/0xe0 [radeon]
> > >    [...]
> > > 
> > > Fix this by omitting the drm_debugfs_create_files() call for the
> > > control minor debugfs directory which is now non-existent anyway.
> > > 
> > > Fixes: 8a357d10043c ("drm: Nerf DRM_CONTROL nodes")
> > > Signed-off-by: Nicolai Stange <nicstange@...il.com>
> > Applied to drm-misc with Dave's irc ack, thanks for your patch.
> 
> If it's still worth it the patch is Reviewed-by: Christian König
> <christian.koenig@....com>.
> 
> On the other hand when ->control is always NULL, why do we still have
> ->control anyway?

I'm chicken and didn't want to mass-delete code from the start. But yeah
after a few kernel releases to make sure no one noticed the removal of
control nodes we can garbage collected. I guess I've been bitten a few too
many times when trying to remove old stuff ;-)
 
> And BTW: Please double check the other drivers as well.

Yeah, I've done a full audit, only qxl turned up to have similar code.
Already merged with Dave's irc ack.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch