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

From: Chanwoo Choi
Date: Tue Dec 27 2016 - 00:05:40 EST

Hi Hans,

Thanks for your report.
I'll check this problem and try to resolve it.

On 2016ë 12ì 22ì 00:54, 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
> Regards,
> Hans

Chanwoo Choi