linux-kernel - Re: [PATCH v3] gpiolib: fix reference leaks when removing GPIO chips still in use

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Message-ID: <ZNtKQlnQxFediB0J@smile.fi.intel.com>
Date:   Tue, 15 Aug 2023 12:49:54 +0300
From:   Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
To:     Bartosz Golaszewski <brgl@...ev.pl>
Cc:     Linus Walleij <linus.walleij@...aro.org>,
        Kent Gibson <warthog618@...il.com>, linux-gpio@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Bartosz Golaszewski <bartosz.golaszewski@...aro.org>
Subject: Re: [PATCH v3] gpiolib: fix reference leaks when removing GPIO chips
 still in use

On Fri, Aug 11, 2023 at 09:30:34PM +0200, Bartosz Golaszewski wrote:
> From: Bartosz Golaszewski <bartosz.golaszewski@...aro.org>
> 
> After we remove a GPIO chip that still has some requested descriptors,
> gpiod_free_commit() will fail and we will never put the references to the
> GPIO device and the owning module in gpiod_free().
> 
> Rework this function to:
> - not warn on desc == NULL as this is a use-case on which most free
>   functions silently return
> - put the references to desc->gdev and desc->gdev->owner unconditionally
>   so that the release callback actually gets called when the remaining
>   references are dropped by external GPIO users

...

> -	if (desc && desc->gdev && gpiod_free_commit(desc)) {

The commit message doesn't explain disappearing of gdev check.

> -		module_put(desc->gdev->owner);
> -		gpio_device_put(desc->gdev);
> -	} else {
> +	/*
> +	 * We must not use VALIDATE_DESC_VOID() as the underlying gdev->chip
> +	 * may already be NULL but we still want to put the references.
> +	 */
> +	if (!desc)
> +		return;
> +
> +	if (!gpiod_free_commit(desc))
>  		WARN_ON(extra_checks);
> -	}
> +
> +	module_put(desc->gdev->owner);
> +	gpio_device_put(desc->gdev);
>  }

So, if gdev can be NULL, you will get an Oops with new code.

To keep a status quo this needs to be rewritten.

-- 
With Best Regards,
Andy Shevchenko