Re: [PATCH] PM / devfreq: Check NULL governor in available_governors_show

[Leonard Crestez <leonard.crestez@nxp.com>]

On 2019-09-24 4:48 AM, Chanwoo Choi wrote:
> On 19. 9. 24. 오전 1:34, Leonard Crestez wrote:
>> The governor is initialized after sysfs attributes become visible so in
>> theory the governor field can be NULL here.
>>
>> Signed-off-by: Leonard Crestez <leonard.crestez@nxp.com>
>> ---
>>   drivers/devfreq/devfreq.c | 2 +-
>>   1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> Found this by hacking device core to call attribute "show" functions
>> from inside device_add.
>>
>> diff --git a/drivers/devfreq/devfreq.c b/drivers/devfreq/devfreq.c
>> index 00fc23fea5b2..896fb2312f2f 100644
>> --- a/drivers/devfreq/devfreq.c
>> +++ b/drivers/devfreq/devfreq.c
>> @@ -1322,11 +1322,11 @@ static ssize_t available_governors_show(struct device *d,
>>   
>>   	/*
>>   	 * The devfreq with immutable governor (e.g., passive) shows
>>   	 * only own governor.
>>   	 */
>> -	if (df->governor->immutable) {
>> +	if (df->governor && df->governor->immutable) {
>>   		count = scnprintf(&buf[count], DEVFREQ_NAME_LEN,
>>   				  "%s ", df->governor_name);
>>   	/*
>>   	 * The devfreq device shows the registered governor except for
>>   	 * immutable governors such as passive governor .
>>
> 
> As you mentioned, it create sysfs and then initialize the governor instance
> as following:
> 
> 	device_register()
> 		device_add()
> 			device_add_attrs()
> 				creating sysfs entries.
> 	governor = try_then_request_governor(...)
> 
> 
> Thanks for fix-up.
> Reviewed-by: Chanwoo Choi <cw00.choi@samsung.com>
> 
> Additionally, you have to add the following 'fixes' tag
> and then send it to stable mailing list(stable@vger.kernel.org).
> - Fixes: bcf23c79c4e46 ("PM / devfreq: Fix available_governor sysfs")

OK, but I'm not sure this meets the criteria for inclusion into linux 
stable:

* It must fix a real bug that bothers people (not a, "This could be a 
problem..." type thing).
* No "theoretical race condition" issues, unless an explanation of how 
the race can be exploited is also provided.

This patch fixes a theoretical race condition which has been there since 
the start.

--
Regards,
Leonard
_______________________________________________
linux-arm-kernel mailing list
linux-arm-kernel@lists.infradead.org
http://lists.infradead.org/mailman/listinfo/linux-arm-kernel