[PATCH v2] drm/meson: fix potential NULL pointer exception in meson_drv_unbind()

Fix a potential NULL pointer exception when meson_drv_unbind()
attempts to operate on the driver_data priv which may be NULL.
Add a null pointer check on the priv struct to avoid the NULL
pointer dereference after calling dev_get_drvdata(), just like
the null pointer checks done on the struct priv in the function
meson_drv_shutdown(), meson_drv_pm_suspend() and meson_drv_pm_resume().

Signed-off-by: Jiajun Cao <redacted>
Signed-off-by: Xin Tan <redacted>
---
 drivers/gpu/drm/meson/meson_drv.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/meson/meson_drv.c b/drivers/gpu/drm/meson/meson_drv.c
index 07fcd12dca16..f544fba8c44e 100644
--- a/drivers/gpu/drm/meson/meson_drv.c
+++ b/drivers/gpu/drm/meson/meson_drv.c

@@ -380,7 +380,10 @@ static int meson_drv_bind(struct device *dev)
 static void meson_drv_unbind(struct device *dev)
 {
 	struct meson_drm *priv = dev_get_drvdata(dev);
-	struct drm_device *drm = priv->drm;
+	struct drm_device *drm;
+
+	if (!priv)
+		return;
 
 	if (priv->canvas) {
 		meson_canvas_free(priv->canvas, priv->canvas_id_osd1);

@@ -389,6 +392,7 @@ static void meson_drv_unbind(struct device *dev)
 		meson_canvas_free(priv->canvas, priv->canvas_id_vd1_2);
 	}
 
+	drm = priv->drm;
 	drm_dev_unregister(drm);
 	drm_kms_helper_poll_fini(drm);
 	drm_atomic_helper_shutdown(drm);

-- 
2.17.1


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Hello,

first of all: thanks for your patch and sorry for being late with my
review question.

On Fri, Jun 18, 2021 at 7:28 AM Jiajun Cao [off-list ref] wrote:

Fix a potential NULL pointer exception when meson_drv_unbind()
attempts to operate on the driver_data priv which may be NULL.
Add a null pointer check on the priv struct to avoid the NULL
pointer dereference after calling dev_get_drvdata(), just like
the null pointer checks done on the struct priv in the function
meson_drv_shutdown(), meson_drv_pm_suspend() and meson_drv_pm_resume().

I am trying to review Amlogic Meson related patches in the DRM
subsystem so I can help Neil with this.
However, I am still new to this so please help me educate on this topic.

[...]

 static void meson_drv_unbind(struct device *dev)
 {
        struct meson_drm *priv = dev_get_drvdata(dev);
-       struct drm_device *drm = priv->drm;
+       struct drm_device *drm;
+
+       if (!priv)
+               return;

My understanding of the component framework is that meson_drv_unbind()
is only called if previously meson_drv_bind() was called (and did not
return any error).
This is different from meson_drv_shutdown() (for example) because that
can be called if meson_drv_probe() returns 0 (success) in case the
"count" variable was 0 (then the probe function does nothing).

As I mentioned before: I am still learning about the DRM subsystem in
the Linux kernel.
So it would be great if you could help me understand for which
scenarios this newly added if-condition is needed.


Thank you!
Best regards,
Martin

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