linux-kernel - Re: [PATCH 1/1] drm/sun4i: hdmi: Check for null pointer before cleanup

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Message-ID: <20191217114906.lrjup35tngrjhwuj@gilmour.lan>
Date:   Tue, 17 Dec 2019 12:49:06 +0100
From:   Maxime Ripard <mripard@...nel.org>
To:     Stefan Mavrodiev <stefan@...mex.com>
Cc:     Chen-Yu Tsai <wens@...e.org>, David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel@...ll.ch>,
        "open list:DRM DRIVERS FOR ALLWINNER A10" 
        <dri-devel@...ts.freedesktop.org>,
        "moderated list:ARM/Allwinner sunXi SoC support" 
        <linux-arm-kernel@...ts.infradead.org>,
        open list <linux-kernel@...r.kernel.org>,
        linux-sunxi@...glegroups.com
Subject: Re: [PATCH 1/1] drm/sun4i: hdmi: Check for null pointer before
 cleanup

On Tue, Dec 17, 2019 at 12:46:03PM +0100, Maxime Ripard wrote:
> On Tue, Dec 17, 2019 at 08:45:07AM +0200, Stefan Mavrodiev wrote:
> > Hi,
> >
> > On 12/16/19 6:12 PM, Maxime Ripard wrote:
> > > Hi,
> > >
> > > On Mon, Dec 16, 2019 at 04:43:48PM +0200, Stefan Mavrodiev wrote:
> > > > It's possible hdmi->connector and hdmi->encoder divices to be NULL.
> > > > This can happen when building as kernel module and you try to remove
> > > > the module.
> > > >
> > > > This patch make simple null check, before calling the cleanup functions.
> > > >
> > > > Signed-off-by: Stefan Mavrodiev <stefan@...mex.com>
> > > > ---
> > > >   drivers/gpu/drm/sun4i/sun4i_hdmi_enc.c | 6 ++++--
> > > >   1 file changed, 4 insertions(+), 2 deletions(-)
> > > >
> > > > diff --git a/drivers/gpu/drm/sun4i/sun4i_hdmi_enc.c b/drivers/gpu/drm/sun4i/sun4i_hdmi_enc.c
> > > > index a7c4654445c7..b61e00f2ecb8 100644
> > > > --- a/drivers/gpu/drm/sun4i/sun4i_hdmi_enc.c
> > > > +++ b/drivers/gpu/drm/sun4i/sun4i_hdmi_enc.c
> > > > @@ -685,8 +685,10 @@ static void sun4i_hdmi_unbind(struct device *dev, struct device *master,
> > > >   	struct sun4i_hdmi *hdmi = dev_get_drvdata(dev);
> > > >
> > > >   	cec_unregister_adapter(hdmi->cec_adap);
> > > > -	drm_connector_cleanup(&hdmi->connector);
> > > > -	drm_encoder_cleanup(&hdmi->encoder);
> > > > +	if (hdmi->connector.dev)
> > > > +		drm_connector_cleanup(&hdmi->connector);
> > > > +	if (hdmi->encoder.dev)
> > > > +		drm_encoder_cleanup(&hdmi->encoder);
> > > Hmmm, this doesn't look right. Do you have more information on how you
> > > can reproduce it?
> >
> > Just build sun4i_drm_hdmi as module (CONFIG_DRM_SUN4I_HDMI=m). Then try to
> > unload the module:
> >
> > # rmmod sun4i_drm_hdmi
> >
> > And you get this:
> >
> > Unable to handle kernel NULL pointer dereference at virtual address 00000000
> > pgd = 6b032436
> > [00000000] *pgd=00000000
> > Internal error: Oops: 5 [#1] SMP ARM
> > Modules linked in: sun4i_drm_hdmi(-)
> > CPU: 0 PID: 1081 Comm: rmmod Not tainted 5.5.0-rc1-00030-g6ec417030d93 #33
> > Hardware name: Allwinner sun7i (A20) Family
> > PC is at drm_connector_cleanup+0x40/0x208
> > LR is at sun4i_hdmi_unbind+0x10/0x54 [sun4i_drm_hdmi]
> > ...
> >
> >
> > I've tested that with sunxi/for-next branch on A20-OLinuXino board.
>
> Yeah, you detailed the symptoms nicely in the commit log, but my point
> was that we shouldn't end up in that situation in the first place.
>
> Your patch works around it, but it doesn't fix the underlying
> issue. Is drm_connector_cleanup (or the encoder variant) called twice?

Answering myself, yes it is. It's both the destroy call back and
called in unbind. We should just remove the one in the unbind then.

Maxime

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