Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S933143Ab1ERQvh (ORCPT ); Wed, 18 May 2011 12:51:37 -0400 Received: from mx1.redhat.com ([209.132.183.28]:52963 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932859Ab1ERQvg (ORCPT ); Wed, 18 May 2011 12:51:36 -0400 Date: Wed, 18 May 2011 19:51:38 +0300 From: "Michael S. Tsirkin" To: Shirley Ma Cc: =?utf-8?B?TWljaGHFgiBNaXJvc8WCYXc=?= , Ben Hutchings , David Miller , Eric Dumazet , Avi Kivity , Arnd Bergmann , netdev@vger.kernel.org, kvm@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH V5 2/6 net-next] netdevice.h: Add zero-copy flag in netdevice Message-ID: <20110518165138.GD22001@redhat.com> References: <1305671318.10756.49.camel@localhost.localdomain> <20110518103819.GL7589@redhat.com> <20110518111734.GO7589@redhat.com> <1305729507.32080.6.camel@localhost.localdomain> <20110518154746.GA21378@redhat.com> <1305734857.32080.53.camel@localhost.localdomain> <20110518163633.GB22001@redhat.com> <1305737140.32080.59.camel@localhost.localdomain> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <1305737140.32080.59.camel@localhost.localdomain> User-Agent: Mutt/1.5.21 (2010-09-15) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 3723 Lines: 99 On Wed, May 18, 2011 at 09:45:40AM -0700, Shirley Ma wrote: > On Wed, 2011-05-18 at 19:36 +0300, Michael S. Tsirkin wrote: > > On Wed, May 18, 2011 at 09:07:37AM -0700, Shirley Ma wrote: > > > On Wed, 2011-05-18 at 18:47 +0300, Michael S. Tsirkin wrote: > > > > On Wed, May 18, 2011 at 07:38:27AM -0700, Shirley Ma wrote: > > > > > On Wed, 2011-05-18 at 13:40 +0200, Michał Mirosław wrote: > > > > > > >> >> Not more other restrictions, skb clone is OK. > > > > pskb_expand_head() > > > > > > looks > > > > > > >> >> OK to me from code review. > > > > > > >> > Hmm. pskb_expand_head calls skb_release_data while > > keeping > > > > > > >> > references to pages. How is that ok? What do I miss? > > > > > > >> It's making copy of the skb_shinfo earlier, so the pages > > > > refcount > > > > > > >> stays the same. > > > > > > > Exactly. But the callback is invoked so the guest thinks > > it's ok > > > > to > > > > > > > change this memory. If it does a corrupted packet will be > > sent > > > > out. > > > > > > > > > > > > Hmm. I tool a quick look at skb_clone(), and it looks like > > this > > > > > > sequence will break this scheme: > > > > > > > > > > > > skb2 = skb_clone(skb...); > > > > > > kfree_skb(skb) or pskb_expand_head(skb); /* callback called > > */ > > > > > > [use skb2, pages still referenced] > > > > > > kfree_skb(skb); /* callback called again */ > > > > > > > > > > > > This sequence is common in bridge, might be in other places. > > > > > > > > > > > > Maybe this ubuf thing should just track clones? This will make > > it > > > > work > > > > > > on all devices then. > > > > > > > > > > The callback was only invoked when last reference of skb was > > gone. > > > > > skb_clone does increase skb refcnt. I tested tcpdump on lower > > > > device, it > > > > > worked. > > > > > > > > Right, it will normally work, but two issues I think you miss: > > > > 1. malicious guest can change the memory between when it is sent > > out > > > > by > > > > device and consumed by tcpdump, so you will see different > > things > > > > (not sure how important this is). > > > > 2. if tcpdump stops consuming stuff from the packet socket (it's > > > > userspace, can't be trusted) then we won't get a callback for > > > > page potentially forever, guest networking will get blocked > > etc. > > > > > For the sequence of: > > > > > > > > > > skb_clone -> last refcnt + 1 > > > > > kfree_skb() or pskb_expand_head -> callback not called > > > > > kfree_skb() -> callback called > > > > > > > > > > I will check page refcount to see whether it's balanced. > > > > > > > > > > Thanks > > > > > shirley > > > > > > > > > > > > pskb_expand_head is a problem anyway I think as it > > > > can hang on to pages after it calls release_data. > > > > Then guest will modify these pages and you get trash there. > > > > > > This can be avoid by allowing pskb_expand_head in fastpath only, I > > > think. But not sure whether tcpdump can still work with this. > > > > > > Thanks > > > Shirley > > > > Yes, I agree. I think for tcpdump, we really need to copy the data > > anyway, to avoid guest changing it in between. So we do that and then > > use the copy everywhere, release the old one. Hmm? > > Yes. Old one use zerocopy, new one use copy data. > > Thanks > Shirley No, that's wrong, as they might become different with a malicious guest. As long as we copied already, lets realease the data and have everyone use the copy. -- MST -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/