Re: [PATCH v3] gpio: consumer: new virtual driver

[Bartosz Golaszewski]

On Thu, Aug 10, 2023 at 5:03 PM Andy Shevchenko
<andriy.shevchenko@xxxxxxxxxxxxxxx> wrote:
>
> On Wed, Aug 09, 2023 at 04:27:09PM +0200, Bartosz Golaszewski wrote:
> >
> > 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.
>
> ...
>
> > +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 ? 1 : 0;
>
> I guess it should be 0 : 1.
>

Eek!

> > +     gpiod_set_value_cansleep(timer_data->desc, timer_data->val);
> > +     mod_timer(&timer_data->timer, jiffies + msecs_to_jiffies(1000));
> > +}
>
> ...
>
> > +static ssize_t
> > +gpio_consumer_lookup_config_key_show(struct config_item *item, char *page)
> > +{
> > +     struct gpio_consumer_lookup *lookup = to_gpio_consumer_lookup(item);
> > +     struct gpio_consumer_device *dev = lookup->parent;
>
> > +     int ret;
>
> Why is it needed now? Seems you were too fast to send v3, look at my comments
> in v2 thread.
>
> > +     scoped_guard(mutex, &dev->lock)
> > +             ret = sprintf(page, "%s\n", lookup->key);
> > +
> > +     return ret;
> > +}
>
> ...
>
> > +static ssize_t
> > +gpio_consumer_lookup_config_key_store(struct config_item *item,
> > +                                   const char *page, size_t count)
> > +{
> > +     struct gpio_consumer_lookup *lookup = to_gpio_consumer_lookup(item);
> > +     struct gpio_consumer_device *dev = lookup->parent;
> > +     char *key __free(kfree) = NULL;
> > +     char *stripped;
> > +
> > +     key = kstrndup(page, count, GFP_KERNEL);
>
> skip_spaces() will allow you to get rid of memmove().
>
> > +     if (!key)
> > +             return -ENOMEM;
> > +
> > +     stripped = strstrip(key);
> > +     memmove(key, stripped, strlen(stripped) + 1);
>
> And this become something like
>
>         /* Get rid of trailing newline and spaces */
>         strim(key);
>

I did it this way for v3.

> > +     guard(mutex)(&dev->lock);
> > +
> > +     if (gpio_consumer_device_is_live_unlocked(dev))
> > +             return -EBUSY;
> > +
> > +     kfree(lookup->key);
> > +     lookup->key = no_free_ptr(key);
> > +
> > +     return count;
> > +}
>
> ...
>
> > +static enum gpio_lookup_flags
> > +gpio_consumer_lookup_get_flags(struct config_item *item)
> > +{
> > +     struct gpio_consumer_lookup *lookup = to_gpio_consumer_lookup(item);
> > +     struct gpio_consumer_device *dev = lookup->parent;
> > +     enum gpio_lookup_flags flags;
> > +
> > +     scoped_guard(mutex, &dev->lock)
>
> > +             flags = lookup->flags;
> > +
> > +     return flags;
>
>         guard()
>         return lookup->flags;
>
> ?
>
> > +}
>
> ...
>
> > +static ssize_t
> > +gpio_consumer_device_config_live_store(struct config_item *item,
> > +                                    const char *page, size_t count)
> > +{
> > +     struct gpio_consumer_device *dev = to_gpio_consumer_device(item);
> > +     bool live;
> > +     int ret;
> > +
> > +     ret = kstrtobool(page, &live);
> > +     if (ret)
> > +             return ret;
> > +
> > +     guard(mutex)(&dev->lock);
> > +
> > +     if (live == gpio_consumer_device_is_live_unlocked(dev))
> > +             ret = -EPERM;
>
>                 return ... ?
>
> > +     else if (live)
>
>         if () ?
>
> > +             ret = gpio_consumer_device_activate_unlocked(dev);
>
> > +     else
>
> drop it ?

We need to return count, not ret so this version is clearer.

Bart

>
> > +             gpio_consumer_device_deactivate_unlocked(dev);
> > +
> > +     return ret ?: count;
> > +}
>
> --
> With Best Regards,
> Andy Shevchenko
>
>