linux-kernel - Re: [PATCH 3/3] HID: wacom: Fix double free on managed resource

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Date:   Sat, 23 Apr 2022 13:42:12 +0200
From:   Hans de Goede <hdegoede@...hat.com>
To:     José Expósito <jose.exposito89@...il.com>,
        hadess@...ess.net
Cc:     dmitry.torokhov@...il.com, rydberg@...math.org, lains@...eup.net,
        jikos@...nel.org, benjamin.tissoires@...hat.com,
        linux-input@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/3] HID: wacom: Fix double free on managed resource

Hi,

On 4/22/22 18:17, José Expósito wrote:
> As described in the documentation for devm_input_allocate_device():
> 
>   Managed input devices do not need to be explicitly unregistered or
>   freed as it will be done automatically when owner device unbinds from
>   its driver (or binding fails).
> 
> However this driver was explicitly freeing the input device, allocated
> using devm_input_allocate_device() through wacom_allocate_input().
> 
> Remove the calls to input_free_device() to avoid a possible double free
> error.

Actually calling input_free_device() on a devm allocated input device
is fine. The input subsystem has chosen to not have a
separate devm_input_free_device(), instead input_free_device() knows
if a device is allocated through devm and then also frees the devres
tied to it:

void input_free_device(struct input_dev *dev)
{
        if (dev) {
                if (dev->devres_managed)
                        WARN_ON(devres_destroy(dev->dev.parent,
                                                devm_input_device_release,
                                                devm_input_device_match,
                                                dev));
                input_put_device(dev);
        }
}

So there is no need for this patch.

Regards,

Hans




> 
> Fixes: d2d13f18aaa51 ("Input: wacom - create a separate input device for pads")
> Signed-off-by: José Expósito <jose.exposito89@...il.com>
> ---
>  drivers/hid/wacom_sys.c | 3 ---
>  1 file changed, 3 deletions(-)
> 
> diff --git a/drivers/hid/wacom_sys.c b/drivers/hid/wacom_sys.c
> index 066c567dbaa2..164c0f7cb796 100644
> --- a/drivers/hid/wacom_sys.c
> +++ b/drivers/hid/wacom_sys.c
> @@ -2098,7 +2098,6 @@ static int wacom_register_inputs(struct wacom *wacom)
>  	error = wacom_setup_pen_input_capabilities(pen_input_dev, wacom_wac);
>  	if (error) {
>  		/* no pen in use on this interface */
> -		input_free_device(pen_input_dev);
>  		wacom_wac->pen_input = NULL;
>  		pen_input_dev = NULL;
>  	} else {
> @@ -2110,7 +2109,6 @@ static int wacom_register_inputs(struct wacom *wacom)
>  	error = wacom_setup_touch_input_capabilities(touch_input_dev, wacom_wac);
>  	if (error) {
>  		/* no touch in use on this interface */
> -		input_free_device(touch_input_dev);
>  		wacom_wac->touch_input = NULL;
>  		touch_input_dev = NULL;
>  	} else {
> @@ -2122,7 +2120,6 @@ static int wacom_register_inputs(struct wacom *wacom)
>  	error = wacom_setup_pad_input_capabilities(pad_input_dev, wacom_wac);
>  	if (error) {
>  		/* no pad in use on this interface */
> -		input_free_device(pad_input_dev);
>  		wacom_wac->pad_input = NULL;
>  		pad_input_dev = NULL;
>  	} else {