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

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Message-ID: <63334964-d63d-7625-e46f-a6e6ec19e908@marcan.st>
Date:   Mon, 13 Dec 2021 19:45:56 +0900
From:   Hector Martin <marcan@...can.st>
To:     Javier Martinez Canillas <javier@...hile0.org>
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 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 
was suggested verbatim by Rob in v2 without the NULL check and digging 
through I found that the NULL codepath is safe.

of_platform_device_create calls of_platform_device_create_pdata 
directly, and:

static struct platform_device *of_platform_device_create_pdata(
                                         struct device_node *np,
                                         const char *bus_id,
                                         void *platform_data,
                                         struct device *parent)
{
         struct platform_device *dev;

         if (!of_device_is_available(np) ||
             of_node_test_and_set_flag(np, OF_POPULATED))
                 return NULL;

of_device_is_available takes a global spinlock and then calls 
__of_device_is_available, and that does:

static bool __of_device_is_available(const struct device_node *device)
{
         const char *status;
         int statlen;

         if (!device)
                 return false;

... so I don't see how this can do anything but immediately return false 
if node is NULL.

-- 
Hector Martin (marcan@...can.st)
Public Key: https://mrcn.st/pub