Re: [PATCH v6] gpio: consumer: new virtual driver

[Bartosz Golaszewski]

On Fri, Aug 18, 2023 at 11:16 AM Andy Shevchenko
<andriy.shevchenko@xxxxxxxxxxxxxxx> wrote:
>
> On Thu, Aug 17, 2023 at 08:43:56PM +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.
>
> ...
>
> > @@ -46,6 +46,7 @@ obj-$(CONFIG_GPIO_BT8XX)            += gpio-bt8xx.o
> >  obj-$(CONFIG_GPIO_CADENCE)           += gpio-cadence.o
> >  obj-$(CONFIG_GPIO_CLPS711X)          += gpio-clps711x.o
> >  obj-$(CONFIG_GPIO_SNPS_CREG)         += gpio-creg-snps.o
> > +obj-$(CONFIG_GPIO_CONSUMER)          += gpio-consumer.o
>
> Order?
>
> >  obj-$(CONFIG_GPIO_CRYSTAL_COVE)              += gpio-crystalcove.o
> >  obj-$(CONFIG_GPIO_CS5535)            += gpio-cs5535.o
> >  obj-$(CONFIG_GPIO_DA9052)            += gpio-da9052.o
>
> ...
>
> > +             return dev_err_probe(dev, ret,
> > +                                  "Failed to read GPIO line names\n");
>
> With one line it takes 83 characters (and note, that long before checkpatch
> went for 100, the string literals at the end of a long line were accepted)...
>
> ...
>
> > +                             return dev_err_probe(dev, ret,
> > +                                             "Failed to request GPIO line interrupt\n");
>
> And here with broken indentation you got 91.
> Can you be consistent?
>
> (I prefer as you know less LoCs)
>
> ...
>
> > +static ssize_t
> > +gpio_consumer_lookup_config_drive_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;
> > +
> > +     guard(mutex)(&dev->lock);
> > +
> > +     if (gpio_consumer_device_is_live_unlocked(dev))
> > +             return -EBUSY;
> > +
> > +     if (sysfs_streq(page, "push-pull")) {
> > +             lookup->flags &= ~(GPIO_OPEN_DRAIN | GPIO_OPEN_SOURCE);
> > +     } else if (sysfs_streq(page, "open-drain")) {
> > +             lookup->flags &= ~GPIO_OPEN_SOURCE;
> > +             lookup->flags |= GPIO_OPEN_DRAIN;
> > +     } else if (sysfs_streq(page, "open-source")) {
> > +             lookup->flags &= ~GPIO_OPEN_DRAIN;
> > +             lookup->flags |= GPIO_OPEN_SOURCE;
> > +     } else {
>
> > +             count = -EINVAL;
>
> Strictly speaking this is incorrect.
> You need
>
>         ssize_t ret;
>         ...
>         ret = count;
>         if (...)
>                 ret = -EINVAL;
>
> > +     }
> > +
> > +     return count;
> > +}
>
> > +static ssize_t
> > +gpio_consumer_lookup_config_pull_store(struct config_item *item,
> > +                                    const char *page, size_t count)
> > +{
>
> As per above.
>
> > +}
>
> ...
>
> > +             curr->chip_hwnum = lookup->offset < 0 ?
> > +                                     U16_MAX : lookup->offset;
>
> I found this way better
>
>                 curr->chip_hwnum =
>                         lookup->offset < 0 ? U16_MAX : lookup->offset;
>
>
> ...
>
> > +     return ret ?: count;
>
> Also possible way in the above mentioned cases.
>
> ...
>
>
> I'm not going to bikeshed, I believe you can fix above accordingly,
> either way
>
> Reviewed-by: Andy Shevchenko <andriy.shevchenko@xxxxxxxxxxxxxxx>
>

Actually when experimenting more, I started thinking we could improve
the driver even more. Expose a sysfs interface that would allow to
control the GPIOs from the kernel on user-space's behalf. For
instance: change value, change direction, toggle active-low watch for
interrupts (and log them to a sysfs buffer) etc.

I won't be queueing it for v6.6, I want to spend some more time on it.

Thanks for the reviews.

Bart

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