linux-kernel - Re: [PATCH] Enforce valid shift values in clocksource

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Date:	Wed, 26 Nov 2008 16:39:18 -0800
From:	John Stultz <johnstul@...ibm.com>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] Enforce valid shift values in clocksource_register()

On Thu, 2008-11-27 at 01:08 +0100, Thomas Gleixner wrote:
> On Wed, 26 Nov 2008, John Stultz wrote:
> 
> > Thomas noted that some of the timekeeping core assumes 
> > clocksource shift values will be less then 32. However, 
> > we do nothing to enforce such limits.
> > 
> > This patch prints a warning if a clocksource with an invalid
> > shift value is attempted to be registered, and will refuse to
> > register the clocksource
> > 
> > I could not find any clocksources that use 32 or higher for a 
> > shift value, so this should be only a preventive warning for
> > future clocksource writers.
> > 
> > Signed-off-by: John Stultz <johnstul@...ibm.com>
> > 
> > diff --git a/kernel/time/clocksource.c b/kernel/time/clocksource.c
> > index 9ed2eec..4a3bb13 100644
> > --- a/kernel/time/clocksource.c
> > +++ b/kernel/time/clocksource.c
> > @@ -318,7 +318,7 @@ static int clocksource_enqueue(struct clocksource *c)
> >   * clocksource_register - Used to install new clocksources
> >   * @t:		clocksource to be registered
> >   *
> > - * Returns -EBUSY if registration fails, zero otherwise.
> > + * Returns -EBUSY or -EINVAL if registration fails, zero otherwise.
> >   */
> >  int clocksource_register(struct clocksource *c)
> >  {
> > @@ -327,7 +327,14 @@ int clocksource_register(struct clocksource *c)
> >  
> >  	/* save mult_orig on registration */
> >  	c->mult_orig = c->mult;
> > -
> > +	if (c->shift >= 32) {
> > +		printk(KERN_WARNING "===============================\n");
> > +		printk(KERN_WARNING "WARNING: Cannot register %s clocksource\n",
> > +					c->name);
> > +		printk(KERN_WARNING "The shift value must be less then 32\n");
> > +		printk(KERN_WARNING "===============================\n");
> > +		return -EINVAL;
> 
> Just setting the shift value to 31 along with a WARN_ON() should be
> enough. We don't need to kill the clocksource in that case, as this
> can be nasty when it happens in the early boot code where we dont have
> any output.

Well, we can't tweak the shift value without changing the mult. And that
seemed a bit overreaching (but might be safe). I'll give it a shot.

Also early boot we should have the jiffies clocksource around, so unless
I'm forgetting something, I don't think it has early boot concerns.

thanks
-john

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