linux-kernel - Re: [PATCH v2 2/2] gpiolib: protect the GPIO device against being dropped while in use by user-space

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Date:   Mon, 28 Nov 2022 21:21:51 +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 v2 2/2] gpiolib: protect the GPIO device against being
 dropped while in use by user-space

On Mon, Nov 28, 2022 at 06:52:14PM +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.

Missed also Dependency (the previous change).

> Fixes: d7c51b47ac11 ("gpio: userspace ABI for reading/writing GPIO lines")
> Fixes: 3c0d9c635ae2 ("gpiolib: cdev: support GPIO_V2_GET_LINE_IOCTL and GPIO_V2_LINE_GET_VALUES_IOCTL")
> Fixes: aad955842d1c ("gpiolib: cdev: support GPIO_V2_GET_LINEINFO_IOCTL and GPIO_V2_GET_LINEINFO_WATCH_IOCTL")
> Fixes: a54756cb24ea ("gpiolib: cdev: support GPIO_V2_LINE_SET_CONFIG_IOCTL")
> Fixes: 7b8e00d98168 ("gpiolib: cdev: support GPIO_V2_LINE_SET_VALUES_IOCTL")

...

> +	down_read(&gdev->sem);
>  
> -	if (!gdev->chip)
> +	if (!gdev->chip) {
> +		up_read(&gdev->sem);
>  		return -ENODEV;
> +	}

Wouldn't be easier to wrap existing functions (with their renaming) into a new
ones with semaphore?

You can even start that in the previous patch.


-- 
With Best Regards,
Andy Shevchenko