linux-kernel - Re: [PATCH 0/8] counter: Remove struct counter

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Message-ID: <20211221113542.rl4aburbzzrgs3km@pengutronix.de>
Date:   Tue, 21 Dec 2021 12:35:42 +0100
From:   Uwe Kleine-König <u.kleine-koenig@...gutronix.de>
To:     Lars-Peter Clausen <lars@...afoo.de>
Cc:     Alexandre Torgue <alexandre.torgue@...s.st.com>,
        David Lechner <david@...hnology.com>,
        Fabrice Gasnier <fabrice.gasnier@...s.st.com>,
        Jarkko Nikula <jarkko.nikula@...ux.intel.com>,
        Kamel Bouhara <kamel.bouhara@...tlin.com>,
        Maxime Coquelin <mcoquelin.stm32@...il.com>,
        Oleksij Rempel <linux@...pel-privat.de>,
        Patrick Havelange <patrick.havelange@...ensium.com>,
        Syed Nayyar Waris <syednwaris@...il.com>,
        William Breathitt Gray <vilhelm.gray@...il.com>,
        linux-arm-kernel@...ts.infradead.org, linux-iio@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        linux-stm32@...md-mailman.stormreply.com
Subject: Re: [PATCH 0/8] counter: Remove struct counter_device::priv

Hello Lars,

On Tue, Dec 21, 2021 at 12:12:12PM +0100, Lars-Peter Clausen wrote:
> On 12/21/21 11:45 AM, Uwe Kleine-König wrote:
> > similar to patch
> > https://lore.kernel.org/r/4bde7cbd9e43a5909208102094444219d3154466.1640072891.git.vilhelm.gray@gmail.com
> > the usage of struct counter_device::priv can be replaced by
> > container_of which improves type safety and code size.
> > 
> > This series depends on above patch, converts the remaining drivers and
> > finally drops struct counter_device::priv.
> 
> Not sure if this is such a good idea. struct counter_device should not be
> embedded in the drivers state struct in the first place.

Just to mention it: My patch series didn't change this, this was already
broken before.

> struct counter_device contains a struct device, which is a reference counted
> object. But by embedding it in the driver state struct the life time of both
> the struct counter_device and and struct device are bound to the life time
> of the driver state struct.
> 
> Which means the struct device memory can get freed before the last reference
> is dropped, which leads to a use-after-free and undefined behavior.

Well, the driver struct is allocated using devm_kzalloc for all drivers.
So I think it's not *very* urgent to fix. Still you're right, this
should be addressed.
 
> The framework should be changed to rather then embedding the struct
> counter_device in the state struct to just have a pointer to it. With the
> struct counter_device having its own allocation that will be freed when the
> last reference to the struct device is dropped.

My favourite would be to implement a counter_device_alloc /
counter_device_add approach, similar to what spi_alloc_controller and
alloc_etherdev do. The downside is that this isn't typesafe either :-\

Best regards
Uwe

-- 
Pengutronix e.K.                           | Uwe Kleine-König            |
Industrial Linux Solutions                 | https://www.pengutronix.de/ |

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