From mboxrd@z Thu Jan 1 00:00:00 1970 From: Thomas Graf Subject: Re: [PATCH net-next 0/7] Allow to monitor multicast cache event via rtnetlink Date: Thu, 6 Dec 2012 17:49:05 +0000 Message-ID: <20121206174905.GC16122@casper.infradead.org> References: <50BE56D3.2030704@6wind.com> <50BF29DA.7020903@6wind.com> <20121205.125453.1457654258131828976.davem@davemloft.net> <50C05AC2.1050504@6wind.com> Mime-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Transfer-Encoding: QUOTED-PRINTABLE Cc: David Miller , David.Laight@ACULAB.COM, netdev@vger.kernel.org To: Nicolas Dichtel Return-path: Received: from casper.infradead.org ([85.118.1.10]:43433 "EHLO casper.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932888Ab2LFRtM (ORCPT ); Thu, 6 Dec 2012 12:49:12 -0500 Content-Disposition: inline In-Reply-To: <50C05AC2.1050504@6wind.com> Sender: netdev-owner@vger.kernel.org List-ID: On 12/06/12 at 09:43am, Nicolas Dichtel wrote: > Le 05/12/2012 18:54, David Miller a =E9crit : > >From: "David Laight" > >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 sing= le > >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. We could use nla_type =3D 0 as it is a reserved value that should be available in all protocols. All readers (kernel and user space) must ignore such an attribute just like any other unknown attribute they encounter. We could easily extend nla_put_u64() and variants to automatically include such a padding attribute as needed. The only situation that I can think of where this would not work is if we have code like this: foo =3D nla_nest_start(); for ([..]) nla_put_u64([...]) nla_nest_end([...]) and a reader would stupidly do a nla_for_each_attr() in user space and assume all attributes found must be NLA_U64 without even checking the length of the attribute. I would say we take that risk and let such code die horribly.