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

[Enric Balletbo Serra <eballetbo@gmail.com>]

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.

> 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 ...

> -                       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.

Thanks,
 Enric

>
>                 governor = find_devfreq_governor(name);
>         }
> --
> 2.20.1
>