Re: [Patch 4/5] Network Drop Monitor: Adding drop monitor implementation & Netlink protocol

[David Miller <davem@davemloft.net>]

From: Herbert Xu <herbert@gondor.apana.org.au>
Date: Thu, 2 Apr 2009 17:39:52 +0800

> (Catching up with old emails)
> 
> David Miller <davem@davemloft.net> wrote:
> > From: Patrick McHardy <kaber@trash.net>
> > Date: Wed, 04 Mar 2009 11:06:41 +0100
> > 
> >> It should also be noted that netlink attributes only provide 4 byte
> >> alignment. Not sure what the current status of handling unaligned
> >> accesses on all architectures is, but something that might have to
> >> be taken into consideration.
> > 
> > Indeed, we have hacks in libnl to work around some of these
> > issues :-(
> 
> Requiring the use of aligned_u64 instead of __u64 in netlink
> structs should help with new data structures, right?

I lied.

It doesn't help, because the problem is that attributes can
only ever be 4 byte aligned, so no matter what attributes
you tag the types with they will be only 4-byte aligned.

It has to do with how the netlink attributes get packed together
into the messages.  It's this crap:

#define NLMSG_ALIGNTO	4
#define NLMSG_ALIGN(len) ( ((len)+NLMSG_ALIGNTO-1) & ~(NLMSG_ALIGNTO-1) )
 ...
#define NLMSG_NEXT(nlh,len)	 ((len) -= NLMSG_ALIGN((nlh)->nlmsg_len), \
				  (struct nlmsghdr*)(((char*)(nlh)) + NLMSG_ALIGN((nlh)->nlmsg_len)))

Thus, nothing in an nlmsg can ever be more than 4 byte aligned.
And this is hard coded into the protocol.