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

[Bartosz Golaszewski]

On Tue, Aug 8, 2023 at 8:11 PM Andy Shevchenko
<andriy.shevchenko@xxxxxxxxxxxxxxx> wrote:
>
> On Tue, Aug 08, 2023 at 04:56:05PM +0200, Bartosz Golaszewski wrote:
> > From: Bartosz Golaszewski <bartosz.golaszewski@xxxxxxxxxx>
> >
> > 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;
>

This is still find, though it doesn't really save us even a single line of code.

> But again, why not
>
>         timer_data->val ^= 1;
>

This is not ok in my book. If I need to think for more than a second
about what it does, then it's worse. I put clarity over brevity.

> ?
>
> > +     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?
>

No, because we also want to remove the trailing spaces and newlines.
But if you have a different suggestion with existing helpers, let me
know. I didn't find any.

> ...
>
> > +     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?
>

I added this to the documentation. Negative number means: lookup by
line name, positive or zero - lookup offset in chip.

> > +     if (offset > (U16_MAX - 1))
>
> And how does it related to this -1 if related at all?
>

GPIOLIB interprets U16_MAX as "lookup by line name". So we can allow
max (U16_MAX - 1). I will add a comment.

> > +             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.
>

Whooaah! In february this year I suggested basic C RAII during my talk
at fosdem and here we are? I missed this one. Yeah, I will use it!
Even better, I will abuse the cr*p out of it in gpio-sim as well!
Thanks for bringing this to my attention. This may be the best thing
that happened to kernel C code in years if people widely adopt it.
(This paragraph was written by a fan of GLib's autopointer paradigm.
:) )

Bartosz

> > +             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
>
>