Re: [RFC PATCH 07/14] iio: core: add simple centralized mechanism for ioctl() handlers

Jonathan Cameron
2020-05-24T16:45:45Z

On Fri, 8 May 2020 16:53:41 +0300
Alexandru Ardelean <alexandru.ardelean@analog.com> wrote:

> The aim of this is to reduce the organization violation of ioctl() calls in
> IIO core. Currently, since the chardev is split across files, event ioctl()
> calls need to be called in buffer ioctl() calls.
>
> The 'industrialio-core.c' file will provide a 'iio_device_ioctl()' which
> will iterate over a list of ioctls registered with the IIO device. These
> can be event ioctl() or buffer ioctl() calls, or something else.
> This is needed, since there is currently one chardev per IIO device and
> that is used for both event handling and reading from the buffer.
>
> Each ioctl() will have to return a IIO_IOCTL_UNHANDLED code (which is
> positive 1), if the ioctl() did not handle the call in any. This eliminates
> any potential ambiguities; if we were to have used error codes it would
> have been uncertain whether they were actual errors, or whether
> the registered ioctl() doesn't service the command.
>
> If any ioctl() returns 0, it was considered that it was serviced
> successfully and the loop will exit.
>
> One assumption for all registered ioctl() handlers is that they are
> statically allocated, so the iio_device_unregister() which just remove all
> of them from the device's ioctl() handler list.
>
> Also, something that is a bit hard to do [at this point] and may not be
> worth the effort of doing, is to check whether registered ioctl()
> calls/commands overlap. This should be unlikely to happen, and should get
> caught at review time. Though, new ioctl() calls would likely not be added
> too often.
>
> Signed-off-by: Alexandru Ardelean <alexandru.ardelean@analog.com>

We seem to have dropped the locking in here. What am I missing that
stops us racing a remove with the ioctl? If there is a reason that
can't race, please add comments there so I don't wonders sometime in
the future.

The check on iio_dev->info means we won't start the ioctl if the
remove has been called, but if we switch immediately after that,
anything can happen before we start calling the ioctls.

J

> ---
> drivers/iio/iio_core.h | 14 ++++++++++++++
> drivers/iio/industrialio-core.c | 33 +++++++++++++++++++++++++++++++++
> include/linux/iio/iio.h | 2 ++
> 3 files changed, 49 insertions(+)
>
> diff --git a/drivers/iio/iio_core.h b/drivers/iio/iio_core.h
> index a527a66be9e5..34c3e19229d8 100644
> --- a/drivers/iio/iio_core.h
> +++ b/drivers/iio/iio_core.h
> @@ -17,6 +17,20 @@ struct iio_dev;
>
> extern struct device_type iio_device_type;
>
> +#define IIO_IOCTL_UNHANDLED 1
> +struct iio_ioctl_handler {
> + struct list_head entry;
> + long (*ioctl)(struct iio_dev *indio_dev, struct file *filp,
> + unsigned int cmd, unsigned long arg);
> +};
> +
> +long iio_device_ioctl(struct iio_dev *indio_dev, struct file *filp,
> + unsigned int cmd, unsigned long arg);
> +
> +void iio_device_ioctl_handler_register(struct iio_dev *indio_dev,
> + struct iio_ioctl_handler *h);
> +void iio_device_ioctl_handler_unregister(struct iio_ioctl_handler *h);
> +
> int __iio_add_chan_devattr(const char *postfix,
> struct iio_chan_spec const *chan,
> ssize_t (*func)(struct device *dev,
> diff --git a/drivers/iio/industrialio-core.c b/drivers/iio/industrialio-core.c
> index 32e045c7f0c1..5df3af5e7dcb 100644
> --- a/drivers/iio/industrialio-core.c
> +++ b/drivers/iio/industrialio-core.c
> @@ -1534,6 +1534,7 @@ struct iio_dev *iio_device_alloc(int sizeof_priv)
> }
> dev_set_name(&dev->dev, "iio:device%d", dev->id);
> INIT_LIST_HEAD(&dev->buffer_list);
> + INIT_LIST_HEAD(&dev->ioctl_handlers);
>
> return dev;
> }
> @@ -1587,6 +1588,33 @@ struct iio_dev *devm_iio_device_alloc(struct device *dev, int sizeof_priv)
> }
> EXPORT_SYMBOL_GPL(devm_iio_device_alloc);
>
> +void iio_device_ioctl_handler_register(struct iio_dev *indio_dev,
> + struct iio_ioctl_handler *h)
> +{
> + /* this assumes that all ioctl() handlers are statically allocated */
> + list_add_tail(&h->entry, &indio_dev->ioctl_handlers);
> +}
> +
> +long iio_device_ioctl(struct iio_dev *indio_dev, struct file *filp,
> + unsigned int cmd, unsigned long arg)
> +{
> + struct iio_ioctl_handler *h;
> + int ret;
> +
> + if (!indio_dev->info)
> + return -ENODEV;

The locking is gone?
> +
> + list_for_each_entry(h, &indio_dev->ioctl_handlers, entry) {
> + ret = h->ioctl(indio_dev, filp, cmd, arg);
> + if (ret == 0)
> + return 0;
> + if (ret != IIO_IOCTL_UNHANDLED)
> + return ret;
> + }
> +
> + return -EINVAL;
> +}
> +
> static int iio_check_unique_scan_index(struct iio_dev *indio_dev)
> {
> int i, j;
> @@ -1722,6 +1750,8 @@ EXPORT_SYMBOL(__iio_device_register);
> **/
> void iio_device_unregister(struct iio_dev *indio_dev)
> {
> + struct iio_ioctl_handler *h, *t;
> +
> cdev_device_del(indio_dev->chrdev, &indio_dev->dev);
> iio_device_free_chrdev_id(&indio_dev->dev);
>
> @@ -1731,6 +1761,9 @@ void iio_device_unregister(struct iio_dev *indio_dev)
>
> iio_disable_all_buffers(indio_dev);
>
> + list_for_each_entry_safe(h, t, &indio_dev->ioctl_handlers, entry)
> + list_del(&h->entry);
> +
> indio_dev->info = NULL;
>
> iio_device_wakeup_eventset(indio_dev);
> diff --git a/include/linux/iio/iio.h b/include/linux/iio/iio.h
> index 52992be44e9e..b6ca8d85629e 100644
> --- a/include/linux/iio/iio.h
> +++ b/include/linux/iio/iio.h
> @@ -488,6 +488,7 @@ struct iio_buffer_setup_ops {
> * @currentmode: [DRIVER] current operating mode
> * @dev: [DRIVER] device structure, should be assigned a parent
> * and owner
> + * @ioctl_handlers: [INTERN] list of registered ioctl handlers
> * @event_interface: [INTERN] event chrdevs associated with interrupt lines
> * @buffer: [DRIVER] any buffer present
> * @buffer_list: [INTERN] list of all buffers currently attached
> @@ -529,6 +530,7 @@ struct iio_dev {
> int modes;
> int currentmode;
> struct device dev;
> + struct list_head ioctl_handlers;
>
> struct iio_event_interface *event_interface;
>