netdev - Re: [RFD] First draft of RDNSS-in-RA support for IPv6 DNS autoconfiguration

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Message-Id: <1182564699.23976.7.camel@xo-13-A4-25.localdomain>
Date:	Fri, 22 Jun 2007 22:11:39 -0400
From:	Dan Williams <dcbw@...hat.com>
To:	"C. Scott Ananian" <cscott@...top.org>
Cc:	Michael Buesch <mb@...sch.de>, netdev@...r.kernel.org
Subject: Re: [RFD] First draft of RDNSS-in-RA support for IPv6 DNS
	autoconfiguration

On Fri, 2007-06-22 at 20:09 -0400, C. Scott Ananian wrote:
> > > diff -ruHpN -X dontdiff linux-2.6.22-rc5-orig/include/net/ip6_rdnss.h
> > > linux-2.6.22-rc5/include/net/ip6_rdnss.h
> > > --- linux-2.6.22-rc5-orig/include/net/ip6_rdnss.h1969-12-31
> > > 19:00:00.000000000 -0500
> > > +++ linux-2.6.22-rc5/include/net/ip6_rdnss.h2007-06-21
> > > 18:16:33.000000000 -0400 @@ -0,0 +1,58 @@
> > > +#ifndef _NET_IP6_RDNSS_H
> > > +#define _NET_IP6_RDNSS_H
> > > +
> > > +#ifdef __KERNEL__
> > > +
> > > +#include <linux/in6.h>
> > > +
> > > +struct nd_opt_rdnss {
> > > +__u8type;
> > > +__u8length;
> > > +#if defined(__BIG_ENDIAN_BITFIELD)
> > > +__u8priority:4,
> > > +open:1,
> > > +reserved1:3;
> > > +#elif defined(__LITTLE_ENDIAN_BITFIELD)
> > > +__u8reserved1:3,
> > > +open:1,
> > > +priority:4;
> > > +#else
> > > +# error not little or big endian
> > > +#endif
> >
> > That is not endianess-safe. Don't use foo:x at all
> > for stuff where a specific endianess is needed. The
> > compiler doesn't make any guarantee about it.
> 
> This was copied directly from include/net/ip6_route.h.  I believe that
> it does in fact work, and I (for one) find this much more readable
> than the alternative.  If it is in fact broken, then
> include/net/ip6_route.h (and the 35 other files which use this #ifdef
> in this manner) should be fixed.

Though in general, we shouldn't be using bitfields, FYI.  They are known
to generate really crappy code on many architectures, and patches that
contain them have been smacked down quite hard by people we all know are
better hackers than us :)

Dan


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