linux-kernel - Re: [PATCH v5 6/8] gpio: cdev: put emitting the line state events on a workqueue

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Message-ID: <77968553c766aaa0240c4c9b81852fb262bc1942.camel@siemens.com>
Date: Fri, 14 Nov 2025 11:57:00 +0000
From: "Sverdlin, Alexander" <alexander.sverdlin@...mens.com>
To: "brgl@...ev.pl" <brgl@...ev.pl>
CC: "bartosz.golaszewski@...aro.org" <bartosz.golaszewski@...aro.org>,
	"warthog618@...il.com" <warthog618@...il.com>, "linux-gpio@...r.kernel.org"
	<linux-gpio@...r.kernel.org>, "linux-kernel@...r.kernel.org"
	<linux-kernel@...r.kernel.org>, "linus.walleij@...aro.org"
	<linus.walleij@...aro.org>
Subject: Re: [PATCH v5 6/8] gpio: cdev: put emitting the line state events on
 a workqueue

Hi Bartosz,

On Fri, 2025-11-14 at 02:11 -0800, Bartosz Golaszewski wrote:
> > > > > In order to allow line state notifications to be emitted from atomic
> > > > > context (for instance: from gpiod_direction_input/output()), we must
> > > > > stop calling any sleeping functions in lineinfo_changed_notify(). To
> > > > > that end let's use the new workqueue.
> > > > > 
> > > > > Let's atomically allocate small structures containing the required data
> > > > > and fill it with information immediately upon being notified about the
> > > > > change except for the pinctrl state which will be retrieved later from
> > > > > process context. We can pretty reliably do this as pin functions are
> > > > > typically set once per boot.
> > > > > 
> > > > > Let's make sure to bump the reference count of GPIO device and the GPIO
> > > > > character device file descriptor to keep both alive until the event was
> > > > > queued.
> > > > > 
> > > > > Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@...aro.org>
> > > > 
> > > > starting from this patch up to the current linux-next (v6.18-rcX)
> > > > I see the following refcnt warnings + KASAN UAF reports on either reboot
> > > > (when gpio-manager is being stopped) or
> > > > `systemctl kill --signal=SIGKILL gpio-manager` (if some GPIOs are being
> > > > requested from (owned by) gpio-manager prior to kill):
> > 
> > []
> > 
> > > Thanks for the bug report. I confirm it's reproducible on my side too.
> > > It never occurred to me to try and SIGKILL gpio-manager. On normal
> > > exit, nothing bad happens. Let me look into it.
> > 
> > In my case it happens also on every reboot/shutdown, however if I
> > `systemctl stop gpio-manager`, I don't see the issue... not sure yet,
> > what is the difference... and I'm not telling SIGKILL is how one
> > should stop gpio-manager, but I'm happy SIGKILL now allows you to
> > reproduce the issue in the kernel code!
> > 
> 
> When the process is killed, it seems the character device's file struct is no
> longer valid even though chrdev_release() was not yet called - it's called
> after we try to send out this notification. I think it's best to just check
> if the file is active and not do anything if it isn't.
> 
> Can you test the following change before I submit a formal patch?

thank you for the quick fix!
I've tested the below patch (on top of v6.18-rc4 with quite some debug options
and even KASAN) and I don't get any issues, neither killing gpio-manager,
nor during reboot.

Feel free to add my
Tested-by: Alexander Sverdlin <alexander.sverdlin@...mens.com>

> diff --git a/drivers/gpio/gpiolib-cdev.c b/drivers/gpio/gpiolib-cdev.c
> index e6a289fa0f8f..0f1103a679b5 100644
> --- a/drivers/gpio/gpiolib-cdev.c
> +++ b/drivers/gpio/gpiolib-cdev.c
> @@ -2548,10 +2548,17 @@ static int lineinfo_changed_notify(struct
> notifier_block *nb,
>  		container_of(nb, struct gpio_chardev_data, lineinfo_changed_nb);
>  	struct lineinfo_changed_ctx *ctx;
>  	struct gpio_desc *desc = data;
> +	struct file *fp;
> 
>  	if (!test_bit(gpio_chip_hwgpio(desc), cdev->watched_lines))
>  		return NOTIFY_DONE;
> 
> +	/* Keep the file descriptor alive for the duration of the notification. */
> +	fp = get_file_active(&cdev->fp);
> +	if (!fp)
> +		/* Chardev file descriptor was already released. */
> +		return NOTIFY_DONE;
> +
>  	/*
>  	 * If this is called from atomic context (for instance: with a spinlock
>  	 * taken by the atomic notifier chain), any sleeping calls must be done
> @@ -2575,8 +2582,6 @@ static int lineinfo_changed_notify(struct
> notifier_block *nb,
>  	/* Keep the GPIO device alive until we emit the event. */
>  	ctx->gdev = gpio_device_get(desc->gdev);
>  	ctx->cdev = cdev;
> -	/* Keep the file descriptor alive too. */
> -	get_file(ctx->cdev->fp);
> 
>  	INIT_WORK(&ctx->work, lineinfo_changed_func);
>  	queue_work(ctx->gdev->line_state_wq, &ctx->work);

-- 
Alexander Sverdlin
Siemens AG
www.siemens.com