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

[Chanwoo Choi <cw00.choi@samsung.com>]

Hi,

On 19. 6. 6. 오전 4:00, Ezequiel Garcia wrote:
> 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.")
> 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");

I don't agree to remove the exception handling. Even if request_module()
returns positive value, it have to be handled the negative code for error case.

>  		else
> -			err = request_module("governor_%s", name);
> -		/* Restore previous state before return */
> +			request_module("governor_%s", name);

ditto.

>  		mutex_lock(&devfreq_list_lock);
> -		if (err)> -			return ERR_PTR(err);

You better to modify it as following:

if (err < 0)
	return ERR_PTR(err);
else if (err > 0)
	return ERR_PTR(-EINVAL);


>  
>  		governor = find_devfreq_governor(name);
>  	}
> 


-- 
Best Regards,
Chanwoo Choi
Samsung Electronics