linux-kernel - Re: [PATCH V3] cpuidle: Add a sysfs entry to disable specific C state for debug purpose.

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Date:	Tue, 13 Mar 2012 09:18:00 +0800
From:	Yanmin Zhang <yanmin_zhang@...ux.intel.com>
To:	Henrique de Moraes Holschuh <hmh@....eng.br>
Cc:	ShuoX Liu <shuox.liu@...el.com>,
	Deepthi Dharwar <deepthi@...ux.vnet.ibm.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"Brown, Len" <len.brown@...el.com>, Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...or.com>,
	"linux-pm@...ts.linux-foundation.org" 
	<linux-pm@...ts.linux-foundation.org>
Subject: Re: [PATCH V3] cpuidle: Add a sysfs entry to disable specific C
 state for debug purpose.

On Mon, 2012-03-12 at 21:42 -0300, Henrique de Moraes Holschuh wrote:
> On Tue, 06 Mar 2012, Yanmin Zhang wrote:
> > On Mon, 2012-03-05 at 07:18 -0300, Henrique de Moraes Holschuh wrote:
> > > On Mon, 05 Mar 2012, ShuoX Liu wrote:
> > > > @@ -45,6 +46,7 @@ total 0
> > > >  /sys/devices/system/cpu/cpu0/cpuidle/state1:
> > > >  total 0
> > > >  -r--r--r-- 1 root root 4096 Feb  8 10:42 desc
> > > > +-rw-r--r-- 1 root root 4096 Feb  8 10:42 disable
> > > >  -r--r--r-- 1 root root 4096 Feb  8 10:42 latency
> > > >  -r--r--r-- 1 root root 4096 Feb  8 10:42 name
> > > >  -r--r--r-- 1 root root 4096 Feb  8 10:42 power
> > > 
> > > ...
> > > 
> > > > diff --git a/drivers/cpuidle/sysfs.c b/drivers/cpuidle/sysfs.c
> > > > index 3fe41fe..1eae29a 100644
> > > > --- a/drivers/cpuidle/sysfs.c
> > > > +++ b/drivers/cpuidle/sysfs.c
> > > > @@ -222,6 +222,9 @@ struct cpuidle_state_attr {
> > > >  #define define_one_state_ro(_name, show) \
> > > >  static struct cpuidle_state_attr attr_##_name = __ATTR(_name, 0444,
> > > > show, NULL)
> > > > 
> > > > +#define define_one_state_rw(_name, show, store) \
> > > > +static struct cpuidle_state_attr attr_##_name = __ATTR(_name, 0644,
> > > > show, store)
> > > > +
> > > >  #define define_show_state_function(_name) \
> > > >  static ssize_t show_state_##_name(struct cpuidle_state *state, \
> > > >  			 struct cpuidle_state_usage *state_usage, char *buf) \
> > > > @@ -229,6 +232,19 @@ static ssize_t show_state_##_name(struct
> > > > cpuidle_state *state, \
> > > >  	return sprintf(buf, "%u\n", state->_name);\
> > > >  }
> > > > 
> > > > +#define define_store_state_function(_name) \
> > > > +static ssize_t store_state_##_name(struct cpuidle_state *state, \
> > > > +		const char *buf, size_t size) \
> > > > +{ \
> > > > +	int value; \
> > > > +	sscanf(buf, "%d", &value); \
> > > > +	if (value) \
> > > > +		state->disable = 1; \
> > > > +	else \
> > > > +		state->disable = 0; \
> > > > +	return size; \
> > > > +}
> > > 
> > > Isn't this missing a check for capabilities?  Disabling cpuidle states is
> > > not something random Joe (and IMHO that does mean random capability-
> > > restricted Joe root) should be doing...
> > Sorry. Could you elaborate it?
> 
> Sure.  Should any user be able to disable a C state, therefore causing
> the system to consume more power?
Here we use the simple way to check access. Only root could change it.

> 
> I am pretty sure the answer is NO, in which case you should check for
> the appropriate user credentials before you allow a write to these
> "debug" controls to succeed.  "capability" here is one of the CAP_*
> capabilities tested through capable(), which are supposed to limit even
> root.
We would add below check.
 
        if (!capable(CAP_SYS_ADMIN))
                return -EPERM;

> 
> > > Also, maybe it would be best to use one of the lib helpers to parse that
> > > value, so that it will be less annoying to userspace (trim blanks, complain
> > > if there is trailing junk after trimming, etc)?
> > We would use strict_strtol to parse the value in next version.
> 
> Thanks!
> 


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