linux-kernel - Re: [PATCH v2 1/2] i8k: Autodetect maximal fan speed and fan RPM multiplier

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Message-ID: <20141219192808.GA8493@roeck-us.net>
Date:	Fri, 19 Dec 2014 11:28:08 -0800
From:	Guenter Roeck <linux@...ck-us.net>
To:	Pali Rohár <pali.rohar@...il.com>
Cc:	Arnd Bergmann <arnd@...db.de>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	linux-kernel@...r.kernel.org, Valdis.Kletnieks@...edu,
	Steven Honeyman <stevenhoneyman@...il.com>,
	Jean Delvare <jdelvare@...e.de>,
	Gabriele Mazzotta <gabriele.mzt@...il.com>,
	Jochen Eisinger <jochen@...guin-breeder.org>
Subject: Re: [PATCH v2 1/2] i8k: Autodetect maximal fan speed and fan RPM
 multiplier

On Fri, Dec 19, 2014 at 07:51:25PM +0100, Pali Rohár wrote:
> On Friday 19 December 2014 19:32:37 Guenter Roeck wrote:
> > > -static int i8k_fan_mult;
> > > -static int i8k_pwm_mult;
> > > -static int i8k_fan_max = I8K_FAN_HIGH;
> > > +static int i8k_fan_mult[2];
> > > +static int i8k_pwm_mult[2];
> > > +static int i8k_fan_max[2];
> > 
> > The rationale for this change is not explained in the commit
> > log.
> > 
> > Do you have any indication that those values would ever be
> > different for the two fans, ie that you actually need arrays
> > here ?
> > 
> 
> I do not know... But if we decide to use only single value for 
> multiplier and max value which fan to use for autodetection?
> 
That does not answer my question. That you can not decide which
fan to use for auto-detection does not mean that the result of
that auto-detection would be different for different fans.

> > > @@ -271,8 +274,25 @@ static int i8k_get_fan_speed(int fan)
> > > 
> > >  {
> > >  
> > >  	struct smm_regs regs = { .eax = I8K_SMM_GET_SPEED, };
> > > 
> > > +	if (fan < 0 || fan >= ARRAY_SIZE(i8k_fan_mult))
> > > +		return -EINVAL;
> > > +
> > 
> > This range check (and probably others) is still unnecessary.
> > 
> > Guenter
> > 
> 
> No, it is necessary. Function i8k_get_fan_speed is called from 
> ioctl callback with value which comes from userspace. If 
> userspace specify fan out of that array we can get kernel panic.
> 
Yes, but just because you introduced an array for various variables,
and you still have the unnecessary check for other callers.

If you want to return -EINVAL for bad ioctl parameters, add a range
check check there. But that would be a separate patch.

Guenter

> > >  	regs.ebx = fan & 0xff;
> > > 
> > > -	return i8k_smm(&regs) ? : (regs.eax & 0xffff) *
> > > i8k_fan_mult; +	return i8k_smm(&regs) ? : (regs.eax &
> > > 0xffff) * i8k_fan_mult[fan]; +}
> 
> -- 
> Pali Rohár
> pali.rohar@...il.com


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