linux-kernel - Re: [PATCH] eeprom: at24: check the return value of nvmem

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Date:   Fri, 29 Dec 2017 00:05:01 +0100
From:   Heiner Kallweit <hkallweit1@...il.com>
To:     Bartosz Golaszewski <brgl@...ev.pl>,
        Johan Hovold <johan@...nel.org>
Cc:     linux-i2c <linux-i2c@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Srinivas Kandagatla <srinivas.kandagatla@...aro.org>
Subject: Re: [PATCH] eeprom: at24: check the return value of
 nvmem_unregister()

Am 28.12.2017 um 22:42 schrieb Bartosz Golaszewski:
> 2017-12-28 12:28 GMT+01:00 Johan Hovold <johan@...nel.org>:
>> On Wed, Dec 27, 2017 at 03:10:38PM +0100, Bartosz Golaszewski wrote:
>>> This function can fail with -EBUSY, but we don't check its return
>>> value in at24_remove(). Bail-out of remove() if nvmem_unregister()
>>> doesn't succeed.
>>>
>>> Signed-off-by: Bartosz Golaszewski <brgl@...ev.pl>
>>> ---
>>>  drivers/misc/eeprom/at24.c | 6 ++++--
>>>  1 file changed, 4 insertions(+), 2 deletions(-)
>>>
>>> diff --git a/drivers/misc/eeprom/at24.c b/drivers/misc/eeprom/at24.c
>>> index e79833d62284..fb21e1c45115 100644
>>> --- a/drivers/misc/eeprom/at24.c
>>> +++ b/drivers/misc/eeprom/at24.c
>>> @@ -684,11 +684,13 @@ static int at24_probe(struct i2c_client *client, const struct i2c_device_id *id)
>>>  static int at24_remove(struct i2c_client *client)
>>>  {
>>>       struct at24_data *at24;
>>> -     int i;
>>> +     int i, ret;
>>>
>>>       at24 = i2c_get_clientdata(client);
>>>
>>> -     nvmem_unregister(at24->nvmem);
>>> +     ret = nvmem_unregister(at24->nvmem);
>>> +     if (ret)
>>> +             return ret;
>>
>> I don't this makes much sense as a driver cannot refuse an unbind by
>> returning an errno from remove(). The return value is simply ignored,
>> remove() will never be called again, and you'd leave everything in an
>> inconsistent state.
>>
> 
> Cc: Srinivas
> 
> Hi Johan,
> 
> I blindly assumed that if there's a return value in remove() then
> someone cares about it. In that case all users of nvmem_unregister()
> that check the return value and bail-out of remove() on failure are
> wrong and in the (very unlikely) event that this routine fails, we
> leak all resources.
> 
>> It looks like the nvmem code grabs a reference to the owning module
>> in __nvmem_device_get() which would at least prevent a module unload
>> while another driver is using the device. And the (sysfs) userspace
>> interface should be fine as device removal is handled by the kernfs
>> code.
> 
> Indeed. I believe we should remove the -EBUSY return case from
> nvmem_register() and just do what gpiolib does - scream loud
> (dev_crit()) when someone forces a module unload or otherwise
> unregisters the device if some cells are still requested. This would
> also allow us to eventually add a devres variant for nvmem_register().
> 

Please see also discussion here, it touches the same topic.
https://lkml.org/lkml/2017/6/4/77

Rgds, Heiner

> What do you think Srinivas?
> 
> Best regards,
> Bartosz Golaszewski
>