Re: [PATCH]cpuidle: preventive check in cpuidle_select against crash

[gaurav jindal <gauravjindal1104@gmail.com>]

On Wed, Dec 27, 2017 at 03:30:02AM +0100, Rafael J. Wysocki wrote:
> On Wed, Dec 27, 2017 at 2:57 AM, gaurav jindal
> <gauravjindal1104@gmail.com> wrote:
> > On Wed, Dec 27, 2017 at 01:42:58AM +0100, Rafael J. Wysocki wrote:
> >> On Tue, Dec 26, 2017 at 8:26 AM, gaurav jindal
> >> <gauravjindal1104@gmail.com> wrote:
> >> > When selecting the idle state using cpuidle_select, there is no
> >> > check on cpuidle_curr_governor. In cpuidle_switch_governor,
> >> > cpuidle_currr_governor can be set to NULL to specify "disabled".
> >>
> >> How exactly?
> >
> > In cpuidle_switch_governor:
> >
> > /**
> >  * cpuidle_switch_governor - changes the governor
> >  * @gov: the new target governor
> >  *
> >  * NOTE: "gov" can be NULL to specify disabled
> >  * Must be called with cpuidle_lock acquired.
> >  */
> > int cpuidle_switch_governor(struct cpuidle_governor *gov)
> > {
> >         struct cpuidle_device *dev;
> >
> >         if (gov == cpuidle_curr_governor)
> >                 return 0;
> >
> >         cpuidle_uninstall_idle_handler();
> >
> >         if (cpuidle_curr_governor) {
> >                 list_for_each_entry(dev, &cpuidle_detected_devices, device_list)
> >                         cpuidle_disable_device(dev);
> >         }
> >
> >         cpuidle_curr_governor = gov;
> >
> > This allows to set the cpuidle_switch_governor as NULL. Although there is no
> > current code flow leading here, but it has a potential for bug in future. So
> > may be better to have prevention.
> 
> Or maybe not.
> 
> Why don't you make cpuidle_switch_governor() check the argument
> against NULL instead?

If we check gov (argument passed in  cpuidle_switch_governor())against
NULL in cpuidle_switch_governor, can be a problem in a case where it 
is called as
cpuidle_switch_governor(NULL);

If cpuidle_curr_governor is not NULL, first the device is disabled.

if (cpuidle_curr_governor) {
	list_for_each_entry(dev, &cpuidle_detected_devices, device_list)
	cpuidle_disable_device(dev);
}

after this cpuidle_curr_governor is set to gov, which is NULL in this case.

cpuidle_curr_governor = gov;
/* if is not updated by inserting a check, it will have an oudated value*/

Now, if gov is not NULL (which it is in this case), cpuidle device is enabled

if (gov) {
	list_for_each_entry(dev, &cpuidle_detected_devices, device_list)
	cpuidle_enable_device(dev);
	cpuidle_install_idle_handler();
	printk(KERN_INFO "cpuidle: using governor %s\n", gov->name);
}
If we check for gov against NULL in this function, it will produce
dangling pointers and resource leaks.