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

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Message-ID: <CAL_JsqLLTL-_H3XgsZRZaxE6qO6y8CDpoHfDkOgwekb1ysNQFw@mail.gmail.com>
Date:   Mon, 13 Dec 2021 08:50:07 -0600
From:   Rob Herring <robh+dt@...nel.org>
To:     Javier Martinez Canillas <javier@...hile0.org>
Cc:     Hector Martin <marcan@...can.st>,
        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>,
        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 5:30 AM Javier Martinez Canillas
<javier@...hile0.org> wrote:
>
> 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.

All/most DT functions work with a NULL node ptr, so why should this
one be different?

Rob