linux-kernel - Re: [PATCH] gpiolib: fix the size of the ngpios variable

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Message-ID: <CAMRc=Md011QRB1z0h=ktYwxKJbH1EQhvZWLnNzU1bhpXuo9sqA@mail.gmail.com>
Date:   Sun, 6 Mar 2022 20:22:16 +0100
From:   Bartosz Golaszewski <brgl@...ev.pl>
To:     Andy Shevchenko <andy.shevchenko@...il.com>
Cc:     Linus Walleij <linus.walleij@...aro.org>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        "open list:GPIO SUBSYSTEM" <linux-gpio@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] gpiolib: fix the size of the ngpios variable

On Sat, Mar 5, 2022 at 10:27 PM Andy Shevchenko
<andy.shevchenko@...il.com> wrote:
>
> On Sat, Mar 5, 2022 at 10:14 PM Bartosz Golaszewski <brgl@...ev.pl> wrote:
> >
> > The ngpio fields both in struct gpio_device as well as gpio_chip are
> > 16 bit unsigned integers. Let's not risk an overflow and use the correct
> > size when reading the device property.
>
> ...
>
> > -               ret = device_property_read_u32(&gdev->dev, "ngpios", &ngpios);
> > +               ret = device_property_read_u16(&gdev->dev, "ngpios", &ngpios);
>
> AFAIU DT format you may not do this. The property is ABI-like and it's
> 32-bit (BE!). Reading as u16 will bring you 2 MSB (bytes) out of 4
> from the 32-bit value.
>

Ah right. It's not very obvious from the docs but the DT syntax must
be different for 16 bits. In that case our best shot is to read a
32-bit unsigned integer and check if its value doesn't exceed
UINT16_MAX.

Bart