Re: [PATCH RESEND 1/4] PM / devfreq: Fix possible null pointer issue in devfreq_add_governor()

[Jie Zhan]

On 3/31/2026 5:17 PM, Yaxiong Tian wrote:
> 
> 在 2026/3/31 16:49, Jie Zhan 写道:
>>
>> On 3/31/2026 3:49 PM, Yaxiong Tian wrote:
>>> 在 2026/3/31 15:04, Jie Zhan 写道:
>>>> On 3/19/2026 5:16 PM, Yaxiong Tian wrote:
>>>>> When a user removes a governor using devfreq_remove_governor(), if
>>>>> the current device is using this governor, devfreq->governor will
>>>>> be set to NULL. When the user registers any governor
>>>>> using devfreq_add_governor(), since devfreq->governor is NULL, a
>>>>> null pointer error occurs in strncmp().
>>>>>
>>>>> For example: A user loads the userspace gov through a module, then
>>>>> a device selects userspace. When unloading the userspace module and
>>>>> then loading it again, the null pointer error occurs:
>>>>>
>>>>> Unable to handle kernel NULL pointer dereference at virtual address
>>>>> 0000000000000010
>>>>> Mem abort info:
>>>>> ESR = 0x0000000096000004
>>>>> EC = 0x25: DABT (current EL), IL = 32 bits
>>>>> *******************skip *********************
>>>>> Call trace:
>>>>> __pi_strncmp+0x20/0x1b8
>>>>> devfreq_userspace_init+0x1c/0xff8 [governor_userspace]
>>>>> do_one_initcall+0x4c/0x278
>>>>> do_init_module+0x5c/0x218
>>>>> load_module+0x1f1c/0x1fc8
>>>>> init_module_from_file+0x8c/0xd0
>>>>> __arm64_sys_finit_module+0x220/0x3d8
>>>>> invoke_syscall+0x48/0x110
>>>>> el0_svc_common.constprop.0+0xbc/0xe8
>>>>> do_el0_svc+0x20/0x30
>>>>> el0_svc+0x24/0xb8
>>>>> el0t_64_sync_handler+0xb8/0xc0
>>>>> el0t_64_sync+0x14c/0x150
>>>>>
>>>>> To fix this issue, modify the relevant logic in devfreq_add_governor():
>>>>> Only check whether the new governor matches the existing one when
>>>>> devfreq->governor exists. When devfreq->governor is NULL, directly
>>>>> select the new governor and perform the DEVFREQ_GOV_START operation.
>>>>>
>>>>> Fixes: 1b5c1be2c88e ("PM / devfreq: map devfreq drivers to governor using name")
>>>>> Signed-off-by: Yaxiong Tian <tianyaxiong@xxxxxxxxxx>
>>>>> ---
>>>>>    drivers/devfreq/devfreq.c | 24 +++++++++++-------------
>>>>>    1 file changed, 11 insertions(+), 13 deletions(-)
>>>>>
>>>>> diff --git a/drivers/devfreq/devfreq.c b/drivers/devfreq/devfreq.c
>>>>> index 54f0b18536db..63ce6e25abe2 100644
>>>>> --- a/drivers/devfreq/devfreq.c
>>>>> +++ b/drivers/devfreq/devfreq.c
>>>>> @@ -1288,23 +1288,21 @@ int devfreq_add_governor(struct devfreq_governor *governor)
>>>>>            int ret = 0;
>>>>>            struct device *dev = devfreq->dev.parent;
>>>>>    -        if (!strncmp(devfreq->governor->name, governor->name,
>>>>> +        if (devfreq->governor && !strncmp(devfreq->governor->name, governor->name,
>>>>>                     DEVFREQ_NAME_LEN)) {
>>>> Probaly do:
>>>> if (!devfreq->governor)
>>>>      continue;
>>>> so as to keep the line short and the "else if (!devfreq->governor) {" below
>>>> can be removed.
>>> If that's the case, a dev without a governor would not be able to add a new governor.
>> It's not about adding.
>>
>> The whole point here (the block inside list_for_each_entry()) is a very
>> rare safe check that you're adding a governor of the same name of a
>> governor used by an existing devfreq device.  If so, stop the old one and
>> start the new one.
>>
>> Therefore, it doesn't have to start/restart it if devfreq->governor is
>> NULL.
> In the current patch, it directly start when devfreq->governor is NULL, and only follows the same-name logic you mentioned when it is non-NULL.
> 
> There isn't the issue you mentioned.
> 
Ok, so create_sysfs_files() is skipped...
For example, setting 'userspace' as the governor won't create its belonging
files.

I'd recommend sticking with the existing logic here and avoid adding a new
block."

>>>> and also:
>>>> if (!strncmp(devfreq->governor->name, governor->name,
>>>>      DEVFREQ_NAME_LEN))
>>>>      continue;
>>>> so we don't have to indent that much.
>>> Currently, governor->name can be set arbitrarily, so duplicate names are possible.
>>>
>>> But they could still be two different governors. Leaving it unchanged seems fine to me.
>>>
>> Sorry, my mistake!
>>
>> I mean:
>> if (strncmp(devfreq->governor->name, governor->name,
>>     DEVFREQ_NAME_LEN))
>>     continue;
>>
>>>>>                /* The following should never occur */
>>>>> -            if (devfreq->governor) {
>>>>> +            dev_warn(dev,
>>>>> +                 "%s: Governor %s already present\n",
>>>>> +                 __func__, devfreq->governor->name);
>>>>> +            ret = devfreq->governor->event_handler(devfreq,
>>>>> +                        DEVFREQ_GOV_STOP, NULL);
>>>>> +            if (ret) {
>>>>>                    dev_warn(dev,
>>>>> -                     "%s: Governor %s already present\n",
>>>>> -                     __func__, devfreq->governor->name);
>>>>> -                ret = devfreq->governor->event_handler(devfreq,
>>>>> -                            DEVFREQ_GOV_STOP, NULL);
>>>>> -                if (ret) {
>>>>> -                    dev_warn(dev,
>>>>> -                         "%s: Governor %s stop = %d\n",
>>>>> -                         __func__,
>>>>> -                         devfreq->governor->name, ret);
>>>>> -                }
>>>>> -                /* Fall through */
>>>>> +                     "%s: Governor %s stop = %d\n",
>>>>> +                     __func__,
>>>>> +                     devfreq->governor->name, ret);
>>>>>                }
>>>>> +        } else if (!devfreq->governor) {
>>>>>                devfreq->governor = governor;
>>>>>                ret = devfreq->governor->event_handler(devfreq,
>>>>>                            DEVFREQ_GOV_START, NULL);