linux-kernel - Re: [PATCH v4 2/2] gpiolib: protect the GPIO device against being dropped while in use by user-space

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Message-ID: <Y4dGNy4vlDEUUFlw@smile.fi.intel.com>
Date:   Wed, 30 Nov 2022 14:01:59 +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 v4 2/2] gpiolib: protect the GPIO device against being
 dropped while in use by user-space

On Wed, Nov 30, 2022 at 10:05:56AM +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.

Bikeshedding below and one question.
(As per tag I'm fine with this version anyway)

...

> +typedef __poll_t (*poll_fn)(struct file *, struct poll_table_struct *);
> +typedef long (*ioctl_fn)(struct file *, unsigned int, unsigned long);

> +typedef ssize_t (*read_fn)(struct file *, char __user *,
> +			   size_t count, loff_t *);

<bikeshedding>
It's only 84 is on a single line.

Dunno if it's better to have typedef followed by wrapper pairs rather than
all typedefs and wrappers grouped.
</bikeshedding>

> +static __poll_t call_poll_locked(struct file *file,
> +				 struct poll_table_struct *wait,
> +				 struct gpio_device *gdev, poll_fn func)
> +{
> +	__poll_t ret;

> +	down_read(&gdev->sem);

Thinking more about this, wouldn't be better to actually

	ret = down_read_trylock(&gdev->sem);
	if (ret)
		return ret;

?

> +	ret = func(file, wait);
> +	up_read(&gdev->sem);
> +
> +	return ret;
> +}

-- 
With Best Regards,
Andy Shevchenko