linux-kernel - Re: [PATCH] kernel/power/hibernate.c: be sure of 'start' is not bigger than 'stop'

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Date:	Mon, 21 Apr 2014 22:43:10 +0200
From:	"Rafael J. Wysocki" <rjw@...ysocki.net>
To:	Pavel Machek <pavel@....cz>
Cc:	Chen Gang <gang.chen.5i5j@...il.com>,
	Guan Xuetao <gxt@...c.pku.edu.cn>, len.brown@...el.com,
	"linux-pm@...r.kernel.org" <linux-pm@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] kernel/power/hibernate.c: be sure of 'start' is not bigger than 'stop'

On Monday, April 21, 2014 10:09:30 PM Pavel Machek wrote:
> On Mon 2014-04-21 21:40:27, Chen Gang wrote:
> > For do_div(), it need 'u64' type, which means the outside must be sure
> > of 'start' is not bigger than 'stop', or it will report warning.
> > 
> > The related warning (with allmodconfig for unicore32):
> > 
> >     CC      kernel/power/hibernate.o
> >   kernel/power/hibernate.c: In function ‘swsusp_show_speed’:
> >   kernel/power/hibernate.c:237: warning: comparison of distinct pointer types lacks a cast
> > 
> > Signed-off-by: Chen Gang <gang.chen.5i5j@...il.com>
> 
> >  {
> > -	s64 elapsed_centisecs64;
> > +	u64 elapsed_centisecs64;
> >  	int centisecs;
> >  	int k;
> >  	int kps;
> >  
> >  	elapsed_centisecs64 = timeval_to_ns(stop) - timeval_to_ns(start);
> > +	if ((s64)elapsed_centisecs64 < 0) {
> > +		printk(KERN_ERR "PM: stop: %llu < start %llu\n",
> > +			timeval_to_ns(stop), timeval_to_ns(start));
> > +		return;
> > +	}
> >  	do_div(elapsed_centisecs64, NSEC_PER_SEC / 100);
> >  	centisecs = elapsed_centisecs64;
> >  	if (centisecs == 0)
> 
> Quite a lot of code to shut up warning in mostly debugging
> function... is there alternative method of shutting it up? Have you
> seen < 0 values in wild?

No, I don't think so.  This is a purely theoretical problem that never
happens in practice, because all of the callers do the right thing.

Rafael 

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