netdev - Re: [net-next PATCH] net: codel: Avoid undefined behavior from signed overflow

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Message-ID: <20131030201327.GO4126@linux.vnet.ibm.com>
Date:	Wed, 30 Oct 2013 13:13:27 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Ben Hutchings <bhutchings@...arflare.com>
Cc:	Jesper Dangaard Brouer <brouer@...hat.com>, netdev@...r.kernel.org,
	Eric Dumazet <eric.dumazet@...il.com>,
	Dave Taht <dave.taht@...il.com>
Subject: Re: [net-next PATCH] net: codel: Avoid undefined behavior from
 signed overflow

On Wed, Oct 30, 2013 at 07:35:48PM +0000, Ben Hutchings wrote:
> On Wed, 2013-10-30 at 18:23 +0100, Jesper Dangaard Brouer wrote:
> > From: Jesper Dangaard Brouer <netoptimizer@...uer.com>
> > 
> > As described in commit 5a581b367 (jiffies: Avoid undefined
> > behavior from signed overflow), according to the C standard
> > 3.4.3p3, overflow of a signed integer results in undefined
> > behavior.
> [...]
> 
> According to the real processors that Linux runs on, signed arithmetic
> uses 2's complement representation and overflow wraps accordingly.  And
> we rely on that behaviour in many places, so we use
> '-fno-strict-overflow' to tell gcc not to assume we avoid signed
> overflow.  (There is also '-fwrapv' which tells gcc to assume the
> processor behaves this way, but shouldn't it already know how the target
> machine works?)

We should still fix them as we come across them.  There are a few types
of loops where '-fno-strict-overflow' results in more instructions
being generated.

							Thanx, Paul

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