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

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Message-ID: <20131031045536.GS4126@linux.vnet.ibm.com>
Date:	Wed, 30 Oct 2013 21:55:36 -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 08:19:12PM +0000, Ben Hutchings wrote:
> On Wed, 2013-10-30 at 13:13 -0700, Paul E. McKenney wrote:
> > 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.
> 
> I realise there's an opportunity for optimisation, but if these cases
> are fixed on an ad-hoc basis, how will we know we're ready to make the
> switch?

I believe that there are some tools that check for code that relies on
signed integer overflow.  Probably not yet up to dealing with the
kernel.  In the meantime, fixing them as we come across them is not
a bad approach.

							Thanx, Paul

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