linux-kernel - Re: [PATCH v2 3/3] drivers: base: Free devm resources when unregistering a device

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Message-ID: <CABVgOSmBcSA69SXEOh8_A7=aSigv5vztkhPYt9TBSNnxhiRSYA@mail.gmail.com>
Date:   Wed, 19 Jul 2023 17:13:58 +0800
From:   David Gow <davidgow@...gle.com>
To:     Maxime Ripard <mripard@...nel.org>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Brendan Higgins <brendan.higgins@...ux.dev>,
        linux-kselftest@...r.kernel.org, kunit-dev@...glegroups.com,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 3/3] drivers: base: Free devm resources when
 unregistering a device

On Wed, 28 Jun 2023 at 17:50, Maxime Ripard <mripard@...nel.org> wrote:
>
> From: David Gow <davidgow@...gle.com>
>
> In the current code, devres_release_all() only gets called if the device
> has a bus and has been probed.
>
> This leads to issues when using bus-less or driver-less devices where
> the device might never get freed if a managed resource holds a reference
> to the device. This is happening in the DRM framework for example.
>
> We should thus call devres_release_all() in the device_del() function to
> make sure that the device-managed actions are properly executed when the
> device is unregistered, even if it has neither a bus nor a driver.
>
> This is effectively the same change than commit 2f8d16a996da ("devres:
> release resources on device_del()") that got reverted by commit
> a525a3ddeaca ("driver core: free devres in device_release") over
> use-after-free concerns.
>
> It's not clear whether those concerns are legitimate though, but I would
> expect drivers not to register new resources in their device-managed
> actions.

It might be clearer to notice that this patch effectively combines the
two patches above, freeing _both_ on device_del() and
device_release(). This should give us the best of both worlds. I'm not
aware of a use-after-free issue that could result here, though it's
possible there's a double free I'm missing now that we are freeing
things twice. My understanding is that commit a525a3ddeaca ("driver
core: free devres in device_release") was more to avoid a leak than a
use-after-free, but I could be wrong.


> Fixes: a525a3ddeaca ("driver core: free devres in device_release")
> Signed-off-by: Maxime Ripard <mripard@...nel.org>
> ---

Signed-off-by: David Gow <davidgow@...gle.com>

Personally, I feel that this is the right way to go, but I'm
definitely not an expert, so I'll let someone else review it in case
there's something I'm missing.

Cheers,
-- David



>  drivers/base/core.c                      | 11 +++++++++++
>  drivers/base/test/platform-device-test.c |  2 --
>  drivers/base/test/root-device-test.c     |  2 --
>  3 files changed, 11 insertions(+), 4 deletions(-)
>
> diff --git a/drivers/base/core.c b/drivers/base/core.c
> index 3dff5037943e..6ceaf50f5a67 100644
> --- a/drivers/base/core.c
> +++ b/drivers/base/core.c
> @@ -3817,6 +3817,17 @@ void device_del(struct device *dev)
>         device_platform_notify_remove(dev);
>         device_links_purge(dev);
>
> +       /*
> +        * If a device does not have a driver attached, we need to clean
> +        * up any managed resources. We do this in device_release(), but
> +        * it's never called (and we leak the device) if a managed
> +        * resource holds a reference to the device. So release all
> +        * managed resources here, like we do in driver_detach(). We
> +        * still need to do so again in device_release() in case someone
> +        * adds a new resource after this point, though.
> +        */
> +       devres_release_all(dev);
> +
>         bus_notify(dev, BUS_NOTIFY_REMOVED_DEVICE);
>         kobject_uevent(&dev->kobj, KOBJ_REMOVE);
>         glue_dir = get_glue_dir(dev);
> diff --git a/drivers/base/test/platform-device-test.c b/drivers/base/test/platform-device-test.c
> index b6ebf1dcdffb..1ae5ce8bd366 100644
> --- a/drivers/base/test/platform-device-test.c
> +++ b/drivers/base/test/platform-device-test.c
> @@ -87,8 +87,6 @@ static void platform_device_devm_register_get_unregister_with_devm_test(struct k
>         struct test_priv *priv = test->priv;
>         int ret;
>
> -       kunit_skip(test, "This needs to be fixed in the core.");
> -
>         pdev = platform_device_alloc(DEVICE_NAME, PLATFORM_DEVID_NONE);
>         KUNIT_ASSERT_NOT_ERR_OR_NULL(test, pdev);
>
> diff --git a/drivers/base/test/root-device-test.c b/drivers/base/test/root-device-test.c
> index 9a3e6cccae13..780d07455f57 100644
> --- a/drivers/base/test/root-device-test.c
> +++ b/drivers/base/test/root-device-test.c
> @@ -78,8 +78,6 @@ static void root_device_devm_register_get_unregister_with_devm_test(struct kunit
>         struct test_priv *priv = test->priv;
>         int ret;
>
> -       kunit_skip(test, "This needs to be fixed in the core.");
> -
>         priv->dev = root_device_register(DEVICE_NAME);
>         KUNIT_ASSERT_NOT_ERR_OR_NULL(test, priv->dev);
>
>
> --
> 2.40.0
>

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