linux-kernel - Re: [PATCH v5] gpio: consumer: new virtual driver

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Date:   Thu, 17 Aug 2023 13:03:22 +0300
From:   Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
To:     Bartosz Golaszewski <brgl@...ev.pl>
Cc:     Linus Walleij <linus.walleij@...aro.org>,
        Kent Gibson <warthog618@...il.com>,
        Jonathan Corbet <corbet@....net>, linux-gpio@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-doc@...r.kernel.org,
        Bartosz Golaszewski <bartosz.golaszewski@...aro.org>
Subject: Re: [PATCH v5] gpio: consumer: new virtual driver

On Tue, Aug 15, 2023 at 08:56:50PM +0200, Bartosz Golaszewski wrote:
> From: Bartosz Golaszewski <bartosz.golaszewski@...aro.org>
> 
> The GPIO subsystem has a serious problem with undefined behavior and
> use-after-free bugs on hot-unplug of GPIO chips. This can be considered a
> corner-case by some as most GPIO controllers are enabled early in the
> boot process and live until the system goes down but most GPIO drivers
> do allow unbind over sysfs, many are loadable modules that can be (force)
> unloaded and there are also GPIO devices that can be dynamically detached,
> for instance CP2112 which is a USB GPIO expender.
> 
> Bugs can be triggered both from user-space as well as by in-kernel users.
> We have the means of testing it from user-space via the character device
> but the issues manifest themselves differently in the kernel.
> 
> This is a proposition of adding a new virtual driver - a configurable
> GPIO consumer that can be configured over configfs (similarly to
> gpio-sim).
> 
> The configfs interface allows users to create dynamic GPIO lookup tables
> that are registered with the GPIO subsystem. Every config group
> represents a consumer device. Every sub-group represents a single GPIO
> lookup. The device can work in three modes: just keeping the line
> active, toggling it every second or requesting its interrupt and
> reporting edges. Every lookup allows to specify the key, offset and
> flags as per the lookup struct defined in linux/gpio/machine.h.
> 
> The module together with gpio-sim allows to easily trigger kernel
> hot-unplug errors. A simple use-case is to create a simulated chip,
> setup the consumer to lookup one of its lines in 'monitor' mode, unbind
> the simulator, unbind the consumer and observe the fireworks in dmesg.
> 
> This driver is aimed as a helper in tackling the hot-unplug problem in
> GPIO as well as basis for future regression testing once the fixes are
> upstream.

...

> +	struct gpio_consumer_device *dev = lookup->parent;
> +
> +	guard(mutex)(&dev->lock);
> +
> +	return sprintf(page, "%s\n", lookup->key);

...

> +static ssize_t
> +gpio_consumer_lookup_config_offset_show(struct config_item *item, char *page)
> +{
> +	struct gpio_consumer_lookup *lookup = to_gpio_consumer_lookup(item);
> +	struct gpio_consumer_device *dev = lookup->parent;
> +	unsigned int offset;
> +
> +	scoped_guard(mutex, &dev->lock)
> +		offset = lookup->offset;
> +
> +	return sprintf(page, "%d\n", offset);

Consistently it can be simplified same way

	guard(mutex)(&dev->lock);

	return sprintf(page, "%d\n", lookup->offset);

BUT. Thinking about this more. With guard() we put sprintf() inside the lock,
which is suboptimal from runtime point of view. So, I think now that all these
should actually use scoped_guard() rather than guard().

> +}

...

> +	guard(mutex)(&dev->lock);
> +
> +	return lookup->flags;

...

> +static ssize_t
> +gpio_consumer_lookup_config_transitory_show(struct config_item *item,
> +					    char *page)
> +{

> +	enum gpio_lookup_flags flags;
> +
> +	flags = gpio_consumer_lookup_get_flags(item);

This is perfectly one line < 80 characters.

> +	return sprintf(page, "%s\n", flags & GPIO_TRANSITORY ? "1" : "0");
> +}

-- 
With Best Regards,
Andy Shevchenko