linux-kernel - Re: [PATCH] PM: QoS: Add check to make sure CPU freq is non-negative

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Date:   Wed, 13 Jul 2022 19:50:14 +0200
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     Shivnandan Kumar <quic_kshivnan@...cinc.com>
Cc:     "Rafael J. Wysocki" <rafael@...nel.org>,
        Len Brown <len.brown@...el.com>, Pavel Machek <pavel@....cz>,
        Linux PM <linux-pm@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] PM: QoS: Add check to make sure CPU freq is non-negative

On Wed, Jul 13, 2022 at 10:37 AM Shivnandan Kumar
<quic_kshivnan@...cinc.com> wrote:
>
> Hi Rafael,
>
>
> Thanks for taking the time to review my patch and providing feedback.
>
> Please find answer inline.
>
> Thanks,
>
> Shivnandan
>
> On 7/13/2022 12:07 AM, Rafael J. Wysocki wrote:
> > On Thu, Jun 23, 2022 at 8:47 AM Shivnandan Kumar
> > <quic_kshivnan@...cinc.com> wrote:
> >>          CPU frequency should never be negative.
> > Do you mean "always be non-negative"?
> Yes,corrected subject now.
> >
> >>          If some client driver calls freq_qos_update_request with some
> >>          value greater than INT_MAX, then it will set max CPU freq at
> >>          fmax but it will add plist node with some negative priority.
> >>          plist node has priority from INT_MIN (highest) to INT_MAX
> >>          (lowest). Once priority is set as negative, another client
> >>          will not be able to reduce max CPU frequency. Adding check
> >>          to make sure CPU freq is non-negative will fix this problem.
> >> Signed-off-by: Shivnandan Kumar <quic_kshivnan@...cinc.com>
> >>
> >> ---
> >>   kernel/power/qos.c | 6 ++++--
> >>   1 file changed, 4 insertions(+), 2 deletions(-)
> >>
> >> diff --git a/kernel/power/qos.c b/kernel/power/qos.c
> >> index ec7e1e85923e..41e96fe34bfd 100644
> >> --- a/kernel/power/qos.c
> >> +++ b/kernel/power/qos.c
> >> @@ -531,7 +531,8 @@ int freq_qos_add_request(struct freq_constraints *qos,
> >>   {
> >>          int ret;
> >>
> >> -       if (IS_ERR_OR_NULL(qos) || !req)
> >> +       if (IS_ERR_OR_NULL(qos) || !req || value < FREQ_QOS_MIN_DEFAULT_VALUE
> >> +               || value > FREQ_QOS_MAX_DEFAULT_VALUE)
> > Why do you check against the defaults?
> Want to make sure to guard against negative value.
> >
> >>                  return -EINVAL;
> >>
> >>          if (WARN(freq_qos_request_active(req),
> >> @@ -563,7 +564,8 @@ EXPORT_SYMBOL_GPL(freq_qos_add_request);
> >>    */
> >>   int freq_qos_update_request(struct freq_qos_request *req, s32 new_value)
> >>   {
> >> -       if (!req)
> >> +       if (!req || new_value < FREQ_QOS_MIN_DEFAULT_VALUE ||
> >> +               new_value > FREQ_QOS_MAX_DEFAULT_VALUE)
> >>                  return -EINVAL;
> >>
> >>          if (WARN(!freq_qos_request_active(req),
> >> --
> > I agree that it should guard against adding negative values, but I
> > don't see why s32 can be greater than INT_MAX.
> yes, checking against negative values will be sufficient.
> I will share patch v2 with only check against negative values.
> >
> > Also why don't you put the guard into freq_qos_apply() instead of
> > duplicating it in the callers of that function?
> Because function  freq_qos_remove_request calls freq_qos_apply with
> PM_QOS_DEFAULT_VALUE which is actually negative.
> So I do not want to break that.

OK