linux-kernel - Re: [PATCH jiffies] Avoid undefined behavior from signed overflow

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Message-ID: <20130729142812.GN26694@linux.vnet.ibm.com>
Date:	Mon, 29 Jul 2013 07:28:12 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Kevin Easton <kevin@...rana.org>
Cc:	linux-kernel@...r.kernel.org, john.stultz@...aro.org,
	davem@...emloft.net, arnd@...db.de, mingo@...nel.org,
	torvalds@...ux-foundation.org
Subject: Re: [PATCH jiffies] Avoid undefined behavior from signed overflow

On Tue, Jul 30, 2013 at 12:01:03AM +1000, Kevin Easton wrote:
> Quoting "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>:
> 
> >On Mon, Jul 29, 2013 at 03:30:35PM +1000, caf@...rana.org wrote:
> >>Quoting "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>:
> >>
> ...
> >>>
> >>>Note that the C standard considers the cast from signed to
> >>>unsigned to be implementation-defined, see 6.3.1.3p3.
> ...
> >>
> >>Don't worry, the case from signed to unsigned is actually well-defined -
> >>the relevant part is 6.3.1.3p2 (in C99):
> >>
> >>>Otherwise, if the new type is unsigned, the value is converted by
> >>>repeatedly adding or subtracting one more than the maximum value that
> >>>can be represented in the new type until the value is in the range of
> >>>the new type.
> >
> >Yep, but we are going in the other direction, from unsigned to signed.
> 
> Ahh, there's an error in the commit message (it says signed to unsigned).

Good catch, fixed!

							Thanx, Paul

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