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

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Date:   Wed, 16 Aug 2023 13:37:32 +0200
From:   Bartosz Golaszewski <brgl@...ev.pl>
To:     Linus Walleij <linus.walleij@...aro.org>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Kent Gibson <warthog618@...il.com>
Cc:     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 9:30 PM Bartosz Golaszewski <brgl@...ev.pl> 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
>
> Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@...aro.org>
> ---
> v1 -> v2:
> - add a comment about why we can't use VALIDATE_DESC_VOID()
>
> v2 -> v3:
> - we must drop the reference to the owner module before we drop the one
>   to the gpio_device as the latter may be removed if this is the last
>   reference and we'll end up calling module_put() on freed memory
>
>  drivers/gpio/gpiolib.c | 16 +++++++++++-----
>  1 file changed, 11 insertions(+), 5 deletions(-)
>
> diff --git a/drivers/gpio/gpiolib.c b/drivers/gpio/gpiolib.c
> index 251c875b5c34..76e0c38026c3 100644
> --- a/drivers/gpio/gpiolib.c
> +++ b/drivers/gpio/gpiolib.c
> @@ -2167,12 +2167,18 @@ static bool gpiod_free_commit(struct gpio_desc *desc)
>
>  void gpiod_free(struct gpio_desc *desc)
>  {
> -       if (desc && desc->gdev && gpiod_free_commit(desc)) {
> -               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);
>  }
>
>  /**
> --
> 2.39.2
>

Queued for fixes.

Bart