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

[Yaxiong Tian <tianyaxiong@kylinos.cn>]

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.
>
> 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.

>
>>   			/* 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);