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

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Message-ID: <CAMRc=MdUWXZVnjkPqH2BZvDY0v-OOysQ=NMjwQEi1rt+16NEQQ@mail.gmail.com>
Date:   Thu, 17 Aug 2023 14:14:04 +0200
From:   Bartosz Golaszewski <brgl@...ev.pl>
To:     Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
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 Thu, Aug 17, 2023 at 12:03 PM Andy Shevchenko
<andriy.shevchenko@...ux.intel.com> wrote:
>
> 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().
>

Precisely why I used a scoped guard here. Same elsewhere.

> > +}
>
> ...
>
> > +     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");
> > +}
>

There's nothing wrong with setting the variable on another line though.

Bart

> --
> With Best Regards,
> Andy Shevchenko
>
>