linux-kernel - Re: Unregistering extcon providers while they are in use leads to kernel crashes

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Date:   Wed, 21 Dec 2016 16:55:13 +0100
From:   Hans de Goede <hdegoede@...hat.com>
To:     Chanwoo Choi <cw00.choi@...sung.com>
Cc:     myungjoo.ham@...sung.com,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: Unregistering extcon providers while they are in use leads to
 kernel crashes

Hi,

On 21-12-16 16:54, Hans de Goede wrote:
> Hi,
>
> With the recent extcon work I've been doing I noticed that
> if I want to rmmod and then insmod say extcon_axp288 I can
> do so without problems even if axp288_charger is holding
> a reference to the extcon device returned by extcon_get_extcon_dev.
>
> The problem is that extcon_get_extcon_dev simply looks up
> the extcon-device in the list of current registered extcon-s
> and then returns a pointer to it, without any reference
> counting.
>
> The rmmod scenario can be fixed by doing a module_get from
> extcon_get_extcon_dev, but that still leaves the same problem
> when root manually unbinds the driver through sysfs.
>
> A possible way fix this would be:
>
> 1) Make all extcon providers use devm_extcon_dev_allocate and document
> using this to allocate an extcon_dev mandatory
>
> 2) Add a refcount to struct extcon_dev and introduce extcon_dev_get
> and extcon_dev_put helpers which modify the refcount and only free
> the memory on the final put (and make the evm_extcon_dev_allocate
> cleanup function call extcon_dev_put)
>
> 3) On extcon_dev_unregister set a flag in the extcon_dev that it
> has been free-ed, make all extcon consumer functions which take
> an extcon_dev (extcon_get_state, extcon_register_notifier, etc.)
> check this flag and return -ENODEV when the extcon has been unregistered
>
> 4) Make extcon_get_extcon_dev call extcon_dev_get on the returned edev
> before returning it
>
> From here on we've fixed the crash, but we now leak the extcon_dev
> when the consumer gets unbound.
>
> 5) Add a devm_extcon_get_extcon_dev which calls extcon_dev_put as the devm
> cleanup function
>
> 6) Convert all extcon consumers to use devm_extcon_get_extcon_dev

p.s.

In case it was not clear, I noticed this, but I don't have the time to
fix it, which is why I wrote the above plan with the hope that someone
can use it as a base to actual fix this.

Regards,

Hans