netdev - Re: [RESEND PATCH net-next-2.6 3/3] ipv6 sit: Set relay to 0.0.0.0 directly if relay

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Date:	Mon, 12 Oct 2009 17:50:54 +0900
From:	YOSHIFUJI Hideaki <yoshfuji@...ux-ipv6.org>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	davem@...emloft.net, netdev@...r.kernel.org
Subject: Re: [RESEND PATCH net-next-2.6 3/3] ipv6 sit: Set relay to 0.0.0.0
	directly if relay_prefixlen == 0.

Eric Dumazet wrote:
> YOSHIFUJI Hideaki a écrit :
> > ipv6 sit: Set relay to 0.0.0.0 directly if relay_prefixlen == 0.
> > 
> > Do not use bit-shift if relay_prefixlen == 0;
> > relay_prefix << 32 does not result in 0.
> > 
> > Signed-off-by: YOSHIFUJI Hideaki <yoshfuji@...ux-ipv6.org>
> > ---
> >  net/ipv6/sit.c |    9 ++++++---
> >  1 files changed, 6 insertions(+), 3 deletions(-)
> > 
> > diff --git a/net/ipv6/sit.c b/net/ipv6/sit.c
> > index 193d0c6..510d31f 100644
> > --- a/net/ipv6/sit.c
> > +++ b/net/ipv6/sit.c
> > @@ -1014,9 +1014,12 @@ ipip6_tunnel_ioctl (struct net_device *dev, struct ifreq *ifr, int cmd)
> >  					 ip6rd.prefixlen);
> >  			if (!ipv6_addr_equal(&prefix, &ip6rd.prefix))
> >  				goto done;
> > -			relay_prefix = ip6rd.relay_prefix &
> > -				       htonl(0xffffffffUL <<
> > -					     (32 - ip6rd.relay_prefixlen));
> > +			if (ip6rd.relay_prefixlen)
> > +				relay_prefix = ip6rd.relay_prefix &
> > +					       htonl(0xffffffffUL <<
> > +						     (32 - ip6rd.relay_prefixlen));
> > +			else
> > +				relay_prefix = 0;
> >  			if (relay_prefix != ip6rd.relay_prefix)
> >  				goto done;
> >  
> 
> 
> Sorry I dont get it
> 
> u32 val = any_value ;
> u32 relay_prefix = val & htonl(0xffffffffUL << 32) should give 0
> 
> If not, something is broken and should be fixed.

Unfortunately, on x86 architecture (80286 and later at least),
lower 5 bits (& 0x1f) are used for shift operation.
Thus, 0xffffffffUL << 32 gives 0xffffffffUL.

--yoshfuji

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