linux-kernel - Re: [PATCH v3 2/2] gpiolib: protect the GPIO device against being dropped while in use by user-space

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Message-ID: <Y4Y5BjTwVCF5bAn5@smile.fi.intel.com>
Date:   Tue, 29 Nov 2022 18:53:26 +0200
From:   Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
To:     Bartosz Golaszewski <brgl@...ev.pl>
Cc:     Kent Gibson <warthog618@...il.com>,
        Linus Walleij <linus.walleij@...aro.org>,
        linux-gpio@...r.kernel.org, linux-kernel@...r.kernel.org,
        Bartosz Golaszewski <bartosz.golaszewski@...aro.org>
Subject: Re: [PATCH v3 2/2] gpiolib: protect the GPIO device against being
 dropped while in use by user-space

On Tue, Nov 29, 2022 at 01:35:53PM +0100, Bartosz Golaszewski wrote:
> From: Bartosz Golaszewski <bartosz.golaszewski@...aro.org>
> 
> While any of the GPIO cdev syscalls is in progress, the kernel can call
> gpiochip_remove() (for instance, when a USB GPIO expander is disconnected)
> which will set gdev->chip to NULL after which any subsequent access will
> cause a crash.
> 
> To avoid that: use an RW-semaphore in which the syscalls take it for
> reading (so that we don't needlessly prohibit the user-space from calling
> syscalls simultaneously) while gpiochip_remove() takes it for writing so
> that it can only happen once all syscalls return.

...

I would do

typedef __poll_t (*poll_fn)(struct file *, struct poll_table_struct *);

and so on and use that one in the respective parameters.

BUT. Since it's a fix, up to you which one to choose.

> +static __poll_t call_poll_locked(struct file *file,
> +				 struct poll_table_struct *wait,
> +				 struct gpio_device *gdev,
> +				 __poll_t (*func)(struct file *,
> +						  struct poll_table_struct *))
> +{
> +	__poll_t ret;
> +
> +	down_read(&gdev->sem);
> +	ret = func(file, wait);
> +	up_read(&gdev->sem);
> +
> +	return ret;
> +}

...

> +	down_write(&gdev->sem);

+ Blank line?

>  	/* FIXME: should the legacy sysfs handling be moved to gpio_device? */
>  	gpiochip_sysfs_unregister(gdev);
>  	gpiochip_free_hogs(gc);

...

>  	gcdev_unregister(gdev);

+ Blank line ?

> +	up_write(&gdev->sem);
>  	put_device(&gdev->dev);

-- 
With Best Regards,
Andy Shevchenko