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

[Jie Zhan]

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.

and also:
if (!strncmp(devfreq->governor->name, governor->name,
	DEVFREQ_NAME_LEN))
	continue;
so we don't have to indent that much.

>  			/* 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);