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References: <20210618052800.89007-1-jjcao20@fudan.edu.cn>
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From:   Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Date:   Thu, 1 Jul 2021 23:34:37 +0200
Message-ID: <CAFBinCCGpbCNOcbgCZcLa1O8mJd0wgLtt4_sAMvwQW4xOxkuWg@mail.gmail.com>
Subject: Re: [PATCH v2] drm/meson: fix potential NULL pointer exception in meson_drv_unbind()
To:     Jiajun Cao <jjcao20@fudan.edu.cn>
Cc:     yuanxzhang@fudan.edu.cn, Xin Tan <tanxin.ctf@gmail.com>,
        Neil Armstrong <narmstrong@baylibre.com>,
        David Airlie <airlied@linux.ie>,
        Daniel Vetter <daniel@ffwll.ch>,
        Kevin Hilman <khilman@baylibre.com>,
        Jerome Brunet <jbrunet@baylibre.com>,
        dri-devel@lists.freedesktop.org, linux-amlogic@lists.infradead.org,
        linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org
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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 <jjcao20@fudan.edu.cn> 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