From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753317AbbIZSJK (ORCPT ); Sat, 26 Sep 2015 14:09:10 -0400 Received: from mail-yk0-f180.google.com ([209.85.160.180]:34644 "EHLO mail-yk0-f180.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753060AbbIZSJI (ORCPT ); Sat, 26 Sep 2015 14:09:08 -0400 Date: Sat, 26 Sep 2015 14:09:03 -0400 From: Tejun Heo To: Herbert Xu Cc: David Miller , cwang@twopensource.com, tom@herbertland.com, kafai@fb.com, kernel-team@fb.com, linux-kernel@vger.kernel.org, netdev@vger.kernel.org, torvalds@linux-foundation.org, jiri@resnulli.us, nicolas.dichtel@6wind.com, tgraf@suug.ch, sfeldma@gmail.com Subject: Re: netlink: Add netlink_bound helper and use it in netlink_getname Message-ID: <20150926180903.GD3572@htj.duckdns.org> References: <20150921133415.GA1740@gondor.apana.org.au> <20150921182022.GB13263@mtj.duckdns.org> <20150922033856.GA7851@gondor.apana.org.au> <20150924.121142.870602292135442487.davem@davemloft.net> <20150924200510.GE25415@mtj.duckdns.org> <20150925014327.GA3725@gondor.apana.org.au> <20150925150113.GD4449@mtj.duckdns.org> <20150926131621.GA16724@gondor.apana.org.au> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20150926131621.GA16724@gondor.apana.org.au> User-Agent: Mutt/1.5.23 (2014-03-12) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Hello, Herbert. On Sat, Sep 26, 2015 at 09:16:21PM +0800, Herbert Xu wrote: > You misunderstood what I wrote. I was not basing this on whether > user-space transitions contained a barrier, but on the fact that > the next syscall must recheck nlk->bound before using nlk->portid. But that isn't what you wrote in the comment. /* No need for barriers here as we return to user-space without * using any of the bound attributes. */ > In fact thanks to your email I now realise that my fix to the > getsockname problem is wrong. Instead of adding a barrier to > netlink_connect I should be adding a nlk->bound check to getname. I don't know, man. This thread almost feels surreal at this point. > @@ -1628,7 +1632,7 @@ static int netlink_getname(struct socket *sock, struct sockaddr *addr, > nladdr->nl_pid = nlk->dst_portid; > nladdr->nl_groups = netlink_group_mask(nlk->dst_group); > } else { > - nladdr->nl_pid = nlk->portid; > + nladdr->nl_pid = netlink_bound(nlk) ? nlk->portid : 0; > nladdr->nl_groups = nlk->groups ? nlk->groups[0] : 0; > } > return 0; So, this is really weird because netlink_getname() doens't participate in the autobind race and thus it's perfectly fine for it to not worry about whether ->bound is set or the memory barrier - whoever its caller may be, the caller is of course responsible for ensuring that the port is bound and visible if it expects to read back the number - ie. if the caller doesn't know (in memory ordering sense) that bind/connect/sendmsg succeeded, it of course can't expect to reliably read back the port number. getname never needed the barrier. The above is shifting synchronization from the source to its users. This is a bad thing to do. Thanks. -- tejun