linux-kernel - Re: [PATCH v2 1/9] gpio: sysfs: add a parallel class device for each GPIO chip using device IDs

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Message-ID: <CAMRc=Mci_q8PsJT2A33KtsPfSoO1BiDhB854M9__0KSv9YcB9w@mail.gmail.com>
Date: Mon, 30 Jun 2025 10:34:52 +0200
From: Bartosz Golaszewski <brgl@...ev.pl>
To: Andy Shevchenko <andriy.shevchenko@...el.com>
Cc: Ahmad Fatoum <a.fatoum@...gutronix.de>, Kent Gibson <warthog618@...il.com>, 
	Jan Lübbe <jlu@...gutronix.de>, Marek Vasut <marex@...x.de>, 
	Geert Uytterhoeven <geert+renesas@...der.be>, Linus Walleij <linus.walleij@...aro.org>, 
	linux-gpio@...r.kernel.org, linux-kernel@...r.kernel.org, 
	Bartosz Golaszewski <bartosz.golaszewski@...aro.org>
Subject: Re: [PATCH v2 1/9] gpio: sysfs: add a parallel class device for each
 GPIO chip using device IDs

On Fri, Jun 27, 2025 at 5:21 PM Andy Shevchenko
<andriy.shevchenko@...el.com> wrote:
>
> On Mon, Jun 23, 2025 at 10:59:49AM +0200, Bartosz Golaszewski wrote:
> >
> > In order to enable moving away from the global GPIO numberspace-based
> > exporting of lines over sysfs: add a parallel, per-chip entry under
> > /sys/class/gpio/ for every registered GPIO chip, denoted by device ID
> > in the file name and not its base GPIO number.
> >
> > Compared to the existing chip group: it does not contain the "base"
> > attribute as the goal of this change is to not refer to GPIOs by their
> > global number from user-space anymore. It also contains its own,
> > per-chip export/unexport attribute pair which allow to export lines by
> > their hardware offset within the chip.
> >
> > Caveat #1: the new device cannot be a link to (or be linked to by) the
> > existing "gpiochip<BASE>" entry as we cannot create links in
> > /sys/class/xyz/.
> >
> > Caveat #2: the new entry cannot be named "gpiochipX" as it could
> > conflict with devices whose base is statically defined to a low number.
> > Let's go with "chipX" instead.
> >
> > While at it: the chip label is unique so update the untrue statement
> > when extending the docs.
>
> ...
>
> >  struct gpiodev_data {
> >       struct gpio_device *gdev;
> >       struct device *cdev_base; /* Class device by GPIO base */
> > +     struct device *cdev_id; /* Class device by GPIO device ID */
>
> I would add it in the middle in a way of the possible drop or conditional
> compiling of the legacy access in the future.
>

I'm not sure what difference it makes?

> >  };
>
> ...
>
> > +static int export_gpio_desc(struct gpio_desc *desc)
> > +{
> > +     int offset, ret;
>
> Why offset is signed?
>

Because gpio_chip_hwgpio() returns a signed int.

> > +     CLASS(gpio_chip_guard, guard)(desc);
> > +     if (!guard.gc)
> > +             return -ENODEV;
> > +
> > +     offset = gpio_chip_hwgpio(desc);
> > +     if (!gpiochip_line_is_valid(guard.gc, offset)) {
> > +             pr_debug_ratelimited("%s: GPIO %d masked\n", __func__,
> > +                                  gpio_chip_hwgpio(desc));
>
> Can we use gdev here? (IIRC we can't due to some legacy corner cases)
>

Yeah, I think there was some revert here? In any case: it's material
for a different series, I'm just moving the code here.

> > +             return -EINVAL;
> > +     }
> > +
> > +     /*
> > +      * No extra locking here; FLAG_SYSFS just signifies that the
> > +      * request and export were done by on behalf of userspace, so
> > +      * they may be undone on its behalf too.
> > +      */
> > +
> > +     ret = gpiod_request_user(desc, "sysfs");
> > +     if (ret)
> > +             return ret;
> > +
> > +     ret = gpiod_set_transitory(desc, false);
> > +     if (ret) {
> > +             gpiod_free(desc);
> > +             return ret;
> > +     }
> > +
> > +     ret = gpiod_export(desc, true);
> > +     if (ret < 0) {
> > +             gpiod_free(desc);
> > +     } else {
> > +             set_bit(FLAG_SYSFS, &desc->flags);
> > +             gpiod_line_state_notify(desc, GPIO_V2_LINE_CHANGED_REQUESTED);
> > +     }
> > +
> > +     return ret;
> > +}
>
> ...
>
> > +static struct device_attribute dev_attr_export = __ATTR(export, 0200, NULL,
> > +                                                     chip_export_store);
>
> __ATTR_WO()
>

No can do, the attribute would have to be called "chip_export". A
function called export_store() already exists in this file.

> ...
>
> > +static struct device_attribute dev_attr_unexport = __ATTR(unexport, 0200,
> > +                                                       NULL,
> > +                                                       chip_unexport_store);
>
> Ditto.
>
> ...
>
> > +static struct attribute *gpiochip_ext_attrs[] = {
> > +     &dev_attr_label.attr,
> > +     &dev_attr_ngpio.attr,
> > +     &dev_attr_export.attr,
> > +     &dev_attr_unexport.attr,
> > +     NULL,
>
> No comma for the terminator, please.
>

Ok.

> > +};
>
> ...
>
> > +     data->cdev_id = device_create_with_groups(&gpio_class, parent,
> > +                                               MKDEV(0, 0), data,
> > +                                               gpiochip_ext_groups,
> > +                                               "chip%d", gdev->id);
> > +     if (IS_ERR(data->cdev_id)) {
> > +             device_unregister(data->cdev_base);
> > +             kfree(data);
>
> UAF
>

Ok.

> > +             return PTR_ERR(data->cdev_id);
> > +     }
>
> --
> With Best Regards,
> Andy Shevchenko
>
>

Thanks,
Bart