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

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Message-ID: <20191217115757.ptoscpwuyv6tmwtc@gilmour.lan>
Date:   Tue, 17 Dec 2019 12:57:57 +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 01:54:56PM +0200, Stefan Mavrodiev wrote:
> Hi,
>
> On 12/17/19 1:49 PM, Maxime Ripard wrote:
> > 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.
>
> Should I do this or leave it to you?

You started that discussion, so it's only fair that you do the patch :)

Maxime

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