Re: [PATCH 1/2] PM / devfreq: Fix governor module load failure

[Enric Balletbo i Serra <enric.balletbo@collabora.com>]

On 6/6/19 15:42, Ezequiel Garcia wrote:
> On Wed, 2019-06-05 at 23:46 +0200, Enric Balletbo Serra wrote:
>> Hi Ezequiel,
>>
>> Missatge de Ezequiel Garcia <ezequiel@collabora.com> del dia dc., 5 de
>> juny 2019 a les 21:06:
>>> A bit unexpectedly (but still documented), request_module may
>>> return a positive value, in case of a modprobe error.
>>> This is currently causing issues in the devfreq framework.
>>>
>>> When a request_module exits with a positive value, we currently
>>> return that via ERR_PTR. However, because the value is positive,
>>> it's not a ERR_VALUE proper, and is therefore treated as a
>>> valid struct devfreq_governor pointer, leading to a kernel oops.
>>>
>>> The right way to fix this is hinted in __request_module documentation:
>>>
>>> """
>>> [snip] The function returns
>>> zero on success or a negative errno code or positive exit code from
>>> "modprobe" on failure. Note that a successful module load does not mean
>>> the module did not then unload and exit on an error of its own. Callers
>>> must check that the service they requested is now available not blindly
>>> invoke it.
>>> """
>>>
>>> Therefore, drop the return value check, which is not useful, and instead
>>> just re-try to find the (hopefully now loaded) governor.
>>>
>>> Fixes: 23c7b54ca1cd1 ("PM / devfreq: Fix devfreq_add_device() when drivers are built as modules.")
>>
>> I think that what you really fixed is a bug introduced by:
>>
>> b53b0128052ff ("PM / devfreq: Fix static checker warning in
>> try_then_request_governor")
>>
>> not the above commit.
>>
> 
> Oh, you are right of course. I looked for the commit introducing the
> request_module usage, and thought it was the culprit.
> 
>>> Signed-off-by: Ezequiel Garcia <ezequiel@collabora.com>
>>> ---
>>>  drivers/devfreq/devfreq.c | 8 ++------
>>>  1 file changed, 2 insertions(+), 6 deletions(-)
>>>
>>> diff --git a/drivers/devfreq/devfreq.c b/drivers/devfreq/devfreq.c
>>> index 6b6991f0e873..8868ad9472d2 100644
>>> --- a/drivers/devfreq/devfreq.c
>>> +++ b/drivers/devfreq/devfreq.c
>>> @@ -236,7 +236,6 @@ static struct devfreq_governor *find_devfreq_governor(const char *name)
>>>  static struct devfreq_governor *try_then_request_governor(const char *name)
>>>  {
>>>         struct devfreq_governor *governor;
>>> -       int err = 0;
>>>
>>>         if (IS_ERR_OR_NULL(name)) {
>>>                 pr_err("DEVFREQ: %s: Invalid parameters\n", __func__);
>>> @@ -251,13 +250,10 @@ static struct devfreq_governor *try_then_request_governor(const char *name)
>>>
>>>                 if (!strncmp(name, DEVFREQ_GOV_SIMPLE_ONDEMAND,
>>>                              DEVFREQ_NAME_LEN))
>>> -                       err = request_module("governor_%s", "simpleondemand");
>>> +                       request_module("governor_%s", "simpleondemand");
>>>                 else
>>> -                       err = request_module("governor_%s", name);
>>> -               /* Restore previous state before return */
>>> +                       request_module("governor_%s", name);
>>>                 mutex_lock(&devfreq_list_lock);
>>> -               if (err)
>>
>> If you remove this check you'll iterate always over the full devfreq
>> list of governors, I know should be quick and is not too long but ...
>>
> 
> Keep in mind that when the request_module succeeds, we need
> to iterate anyways to find the governor.
> 

Well, the error path will be a micro-bit faster :-)

>>> -                       return ERR_PTR(err);
>>
>> The fix can be simply:
>>
>> return ERR_PTR(-EINVAL);
>>
>> I don't think overlap the real error is a problem here.
>>
> 
> Yeah, I also thought about this, but somehow thought this
> was simpler.
> 
> I don't have a strong opinion, so whatever you prefer is fine.
> 

Me neither, I think is more up to the maintainer :-)

BTW, with the Fixes tag fixed

Reviewed-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>

> Thanks,
> Eze
> 
>> Thanks,
>>  Enric
>>
>>>                 governor = find_devfreq_governor(name);
>>>         }
>>> --
>>> 2.20.1
>>>
> 
>