Re: [PATCH v2] gpio: consumer: new virtual driver

[Andy Shevchenko <andriy.shevchenko@linux.intel.com>]

On Tue, Aug 08, 2023 at 04:56:05PM +0200, Bartosz Golaszewski wrote:
> From: Bartosz Golaszewski <bartosz.golaszewski@linaro.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.

I'll read documentation later. Some code comments below.

...

> +static void gpio_consumer_on_timer(struct timer_list *timer)
> +{
> +	struct gpio_consumer_timer_data *timer_data = to_timer_data(timer);

> +	timer_data->val = timer_data->val == 0 ? 1 : 0;

Can be

	timer_data->val = timer_data->val ? 0 : 1;

But again, why not

	timer_data->val ^= 1;

?

> +	gpiod_set_value_cansleep(timer_data->desc, timer_data->val);
> +	mod_timer(&timer_data->timer, jiffies + msecs_to_jiffies(1000));
> +}

...

> +	key = kstrndup(page, count, GFP_KERNEL);
> +	if (!key)
> +		return -ENOMEM;

> +	stripped = strstrip(key);
> +	memmove(key, stripped, strlen(stripped) + 1);

This can be avoided by

	key = kstrndup(skip_spaces(page), count, GFP_KERNEL);

no?

...

> +	ret = kstrtoint(page, 0, &offset);
> +	if (ret)
> +		return ret;
> +
> +	/* Use -1 to indicate lookup by name. */

This comment is unclear as offset can be -1 given by the user.
What does above mean in that context?

> +	if (offset > (U16_MAX - 1))

And how does it related to this -1 if related at all?

> +		return -EINVAL;

...

> +static struct config_group *
> +gpio_consumer_config_make_device_group(struct config_group *group,
> +				       const char *name)
> +{
> +	struct gpio_consumer_device *dev;
> +
> +	dev = kzalloc(sizeof(*dev), GFP_KERNEL);
> +	if (!dev)
> +		return ERR_PTR(-ENOMEM);
> +
> +	dev->id = ida_alloc(&gpio_consumer_ida, GFP_KERNEL);
> +	if (dev->id < 0) {
> +		kfree(dev);

Wondering if you can utilize cleanup.h.

> +		return ERR_PTR(dev->id);
> +	}
> +
> +	config_group_init_type_name(&dev->group, name,
> +				    &gpio_consumer_device_config_group_type);
> +	mutex_init(&dev->lock);
> +	INIT_LIST_HEAD(&dev->lookup_list);
> +	dev->bus_notifier.notifier_call = gpio_consumer_bus_notifier_call;
> +	dev->function = GPIO_CONSUMER_FUNCTION_ACTIVE;
> +	init_completion(&dev->probe_completion);
> +
> +	return &dev->group;
> +}

-- 
With Best Regards,
Andy Shevchenko