Re: [PATCH] video: fbdev: arkfb: Cast ics5342_init() allocation type

[Kees Cook <kees@kernel.org>]

On April 29, 2025 1:17:26 PM PDT, Helge Deller <deller@gmx.de> wrote:
>On 4/28/25 08:36, Geert Uytterhoeven wrote:
>> Hi Kees,
>> 
>> On Sat, 26 Apr 2025 at 13:33, Helge Deller <deller@gmx.de> wrote:
>>> On 4/26/25 08:23, Kees Cook wrote:
>>>> In preparation for making the kmalloc family of allocators type aware,
>>>> we need to make sure that the returned type from the allocation matches
>>>> the type of the variable being assigned. (Before, the allocator would
>>>> always return "void *", which can be implicitly cast to any pointer type.)
>>>> 
>>>> The assigned type is "struct dac_info *" but the returned type will be
>>>> "struct ics5342_info *", which has a larger allocation size. This is
>>>> by design, as struct ics5342_info contains struct dac_info as its first
>>>> member. Cast the allocation type to match the assignment.
>>>> 
>>>> Signed-off-by: Kees Cook <kees@kernel.org>
>> 
>> Thanks for your patch, which is now commit 8d2f0f5bbac87b9d ("fbdev:
>> arkfb: Cast ics5342_init() allocation type") in fbdev/for-next.
>> 
>>> I applied your patch, but wouldn't this untested patch be cleaner and fulfill the
>>> same purpose to match a kzalloc return type?
>>> 
>>> diff --git a/drivers/video/fbdev/arkfb.c b/drivers/video/fbdev/arkfb.c
>>> index 7d131e3d159a..a57c8a992e11 100644
>>> --- a/drivers/video/fbdev/arkfb.c
>>> +++ b/drivers/video/fbdev/arkfb.c
>>> @@ -431,7 +431,8 @@ static struct dac_ops ics5342_ops = {
>>> 
>>>    static struct dac_info * ics5342_init(dac_read_regs_t drr, dac_write_regs_t dwr, void *data)
>>>    {
>>> -       struct dac_info *info = (struct dac_info *)kzalloc(sizeof(struct ics5342_info), GFP_KERNEL);
>>> +       struct ics5342_info *ics_info = kzalloc(sizeof(struct ics5342_info), GFP_KERNEL);
>> 
>> sizeof(*ics_info)?
>> 
>>> +       struct dac_info *info = &ics_info->dac;
>> 
>> Exactly my thought when I noticed this commit.  Adding casts makes
>> it harder to notice any future discrepancies.
>
>I've changed it accordingly.

Thanks! Yeah, that's a much nicer solution.

-- 
Kees Cook