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

[Jie Zhan <zhanjie9@hisilicon.com>]

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@kylinos.cn>
>>> ---
>>>   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.
>>
>> 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);