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

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Message-ID: <a4f9eefd-79fc-e9cf-88b8-efef424fb7c9@quicinc.com>
Date:   Wed, 13 Jul 2022 14:07:09 +0530
From:   Shivnandan Kumar <quic_kshivnan@...cinc.com>
To:     "Rafael J. Wysocki" <rafael@...nel.org>
CC:     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

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.