netdev - RE: [PATCH net-next 0/7] Allow to monitor multicast cache event via rtnetlink

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Date:	Fri, 7 Dec 2012 10:38:37 -0000
From:	"David Laight" <David.Laight@...LAB.COM>
To:	"Thomas Graf" <tgraf@...g.ch>,
	"Nicolas Dichtel" <nicolas.dichtel@...nd.com>
Cc:	"David Miller" <davem@...emloft.net>, <netdev@...r.kernel.org>
Subject: RE: [PATCH net-next 0/7] Allow to monitor multicast cache event via rtnetlink

> On 12/06/12 at 09:43am, Nicolas Dichtel wrote:
> > Le 05/12/2012 18:54, David Miller a écrit :
> > >From: "David Laight" <David.Laight@...LAB.COM>
> > >Date: Wed, 5 Dec 2012 11:41:33 -0000
> > >
> > >>Probably worth commenting that the 64bit items might only be 32bit aligned.
> > >>Just to stop anyone trying to read/write them with pointer casts.
> > >
> > >Rather, let's not create this situation at all.
> > >
> > >It's totally inappropriate to have special code to handle every single
> > >time we want to put 64-bit values into netlink messages.
> > >
> > >We need a real solution to this issue.
> > >
> > The easiest way is to update *_ALIGNTO values (maybe we can keep
> > NLMSG_ALIGNTO to 4). But I think that many userland apps have these
> > values hardcoded and, the most important thing, this may increase
> > size of many netlink messages. Hence we need probably to find
> > something better.
> 
> We can't do this, as you say, ALIGNTO is compiled into all the
> binaries.
> 
> A simple backwards compatible workaround would be to include an
> unknown, empty padding attribute if needed. That would be 4 bytes
> in size and could be used to include padding as needed.

What are you going to align the data with respect to?
I doubt you can assume that the start of the netlink
message itself is 8 byte aligned - so any attempt
to 8 byte align an item is probably doomed to failure.

	David



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