linux-kernel - Re: [PATCH v3 1/3] of: Move simple-framebuffer device handling from simplefb to of

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Date:   Mon, 13 Dec 2021 12:30:33 +0100
From:   Javier Martinez Canillas <javier@...hile0.org>
To:     Hector Martin <marcan@...can.st>
Cc:     Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        Maxime Ripard <mripard@...nel.org>,
        Thomas Zimmermann <tzimmermann@...e.de>,
        David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel@...ll.ch>,
        Rob Herring <robh+dt@...nel.org>,
        Hans de Goede <hdegoede@...hat.com>,
        Alyssa Rosenzweig <alyssa@...enzweig.io>,
        Pekka Paalanen <ppaalanen@...il.com>,
        devicetree@...r.kernel.org,
        dri-devel <dri-devel@...ts.freedesktop.org>,
        Linux Kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3 1/3] of: Move simple-framebuffer device handling from
 simplefb to of

On Mon, Dec 13, 2021 at 11:46 AM Hector Martin <marcan@...can.st> wrote:
>
> On 13/12/2021 17.44, Javier Martinez Canillas wrote:
> > Hello Hector,
> >
> > On Sun, Dec 12, 2021 at 7:24 AM Hector Martin <marcan@...can.st> wrote:
> >>
> >> This code is required for both simplefb and simpledrm, so let's move it
> >> into the OF core instead of having it as an ad-hoc initcall in the
> >> drivers.
> >>
> >> Acked-by: Thomas Zimmermann <tzimmermann@...e.de>
> >> Signed-off-by: Hector Martin <marcan@...can.st>
> >> ---
> >>   drivers/of/platform.c          |  4 ++++
> >>   drivers/video/fbdev/simplefb.c | 21 +--------------------
> >>   2 files changed, 5 insertions(+), 20 deletions(-)
> >>
> >
> > This is indeed a much better approach than what I suggested. I just
> > have one comment.
> >
> >> diff --git a/drivers/of/platform.c b/drivers/of/platform.c
> >> index b3faf89744aa..793350028906 100644
> >> --- a/drivers/of/platform.c
> >> +++ b/drivers/of/platform.c
> >> @@ -540,6 +540,10 @@ static int __init of_platform_default_populate_init(void)
> >>                  of_node_put(node);
> >>          }
> >>
> >> +       node = of_get_compatible_child(of_chosen, "simple-framebuffer");
> >
> > You have to check if the node variable is NULL here.
> >
> >> +       of_platform_device_create(node, NULL, NULL);
> >
> > Otherwise this could lead to a NULL pointer dereference if debug
> > output is enabled (the node->full_name is printed).
>
> Where is it printed? I thought I might need a NULL check, but this code

Sorry, I misread of_amba_device_create() as
of_platform_device_create(), which uses the "%pOF" printk format
specifier [0] to print the node's full name as a debug output [1].

[0]: https://elixir.bootlin.com/linux/v5.16-rc5/source/Documentation/core-api/printk-formats.rst#L462
[1]: https://elixir.bootlin.com/linux/v5.16-rc5/source/drivers/of/platform.c#L233

> was suggested verbatim by Rob in v2 without the NULL check and digging
> through I found that the NULL codepath is safe.
>

You are right that passing NULL is a safe code path for now due the
of_device_is_available(node) check, but that seems fragile to me since
just adding a similar debug output to of_platform_device_create()
could trigger the NULL pointer dereference.

Best regards,
Javier