From mboxrd@z Thu Jan 1 00:00:00 1970 From: Nicolas Dichtel Subject: Re: [PATCH net-next v2 2/2] netlink: specify netlink packet direction for nlmon Date: Mon, 23 Dec 2013 14:08:22 +0100 Message-ID: <52B835C6.3040800@6wind.com> References: <1387788519-17722-1-git-send-email-dborkman@redhat.com> <1387788519-17722-3-git-send-email-dborkman@redhat.com> <52B813E8.8010508@6wind.com> <52B81496.6060807@redhat.com> <52B81875.4060201@6wind.com> <52B81A53.9020305@redhat.com> Reply-To: nicolas.dichtel@6wind.com Mime-Version: 1.0 Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: QUOTED-PRINTABLE Cc: davem@davemloft.net, netdev@vger.kernel.org, Jakub Zawadzki To: Daniel Borkmann Return-path: Received: from mail-wi0-f175.google.com ([209.85.212.175]:38535 "EHLO mail-wi0-f175.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1757040Ab3LWNI0 (ORCPT ); Mon, 23 Dec 2013 08:08:26 -0500 Received: by mail-wi0-f175.google.com with SMTP id hi5so10882353wib.2 for ; Mon, 23 Dec 2013 05:08:25 -0800 (PST) In-Reply-To: <52B81A53.9020305@redhat.com> Sender: netdev-owner@vger.kernel.org List-ID: Le 23/12/2013 12:11, Daniel Borkmann a =C3=A9crit : > On 12/23/2013 12:03 PM, Nicolas Dichtel wrote: >> Le 23/12/2013 11:46, Daniel Borkmann a =C3=A9crit : >>> On 12/23/2013 11:43 AM, Nicolas Dichtel wrote: >>>> Le 23/12/2013 09:48, Daniel Borkmann a =C3=A9crit : >>>>> In order to facilitate development for netlink protocol dissector= , >>>>> fill the unused field skb->pkt_type of the cloned skb with a hint >>>>> of the address space of the new owner (receiver) socket in the >>>>> notion of "to kernel" resp. "to user". >>>>> >>>>> At the time we invoke __netlink_deliver_tap_skb(), we already hav= e >>>>> set the new skb owner via netlink_skb_set_owner_r(), so we can us= e >>>>> that for netlink_is_kernel() probing. >>>>> >>>>> In normal PF_PACKET network traffic, this field denotes if the >>>>> packet is destined for us (PACKET_HOST), if it's broadcast >>>>> (PACKET_BROADCAST), etc. >>>>> >>>>> As we only have 3 bit reserved, we can use the value (=3D 6) of >>>>> PACKET_FASTROUTE as it's _not used_ anywhere in the whole kernel >>>>> and packets of such type were never exposed to user space, so >>>>> there are no overlapping users of such kind. Thus, as wished, >>>>> that seems the only way to make both PACKET_* values non-overlapp= ing >>>>> and therefore device agnostic. >>>>> >>>>> By using those two flags for netlink skbs on nlmon devices, they >>>>> can be made available and picked up via sll_pkttype (previously >>>>> unused in netlink context) in struct sockaddr_ll. We now have >>>>> these two directions: >>>>> >>>>> - PACKET_USER (=3D 6) -> to user space >>>>> - PACKET_KERNEL (=3D 7) -> to kernel space >>>>> >>>>> Partial `ip a` example strace for sa_family=3DAF_NETLINK with >>>>> detected nl msg direction: >>>>> >>>>> syscall: direction: >>>>> sendto(3, ...) =3D 40 /* to kernel */ >>>>> recvmsg(3, ...) =3D 3404 /* to user */ >>>>> recvmsg(3, ...) =3D 1120 /* to user */ >>>>> recvmsg(3, ...) =3D 20 /* to user */ >>>>> sendto(3, ...) =3D 40 /* to kernel */ >>>>> recvmsg(3, ...) =3D 168 /* to user */ >>>>> recvmsg(3, ...) =3D 144 /* to user */ >>>>> recvmsg(3, ...) =3D 20 /* to user */ >>>>> >>>>> Signed-off-by: Daniel Borkmann >>>>> Signed-off-by: Jakub Zawadzki >>>>> --- >>>>> v1->v2: >>>>> - let PACKET_* values not overlap as wished by Dave >>>>> >>>>> include/uapi/linux/if_packet.h | 4 +++- >>>>> net/netlink/af_netlink.c | 2 ++ >>>>> 2 files changed, 5 insertions(+), 1 deletion(-) >>>>> >>>>> diff --git a/include/uapi/linux/if_packet.h b/include/uapi/linux/= if_packet.h >>>>> index e9d844c..06e2a28 100644 >>>>> --- a/include/uapi/linux/if_packet.h >>>>> +++ b/include/uapi/linux/if_packet.h >>>>> @@ -26,8 +26,10 @@ struct sockaddr_ll { >>>>> #define PACKET_MULTICAST 2 /* To group */ >>>>> #define PACKET_OTHERHOST 3 /* To someone else */ >>>>> #define PACKET_OUTGOING 4 /* Outgoing of any type= */ >>>>> -/* These ones are invisible by user level */ >>>>> #define PACKET_LOOPBACK 5 /* MC/BRD frame looped = back */ >>>>> +#define PACKET_USER 6 /* To user space */ >>>> Reusing this value is like changing the API. If some userland apps= and external >>>> modules rely on it, this patch may break them. >>> >>> Sorry, but I thought I made it clear in the commit message that >>> PACKET_FASTROUTE is *not* used anywhere in the whole kernel tree. >> Yes, it's why I talk about *external* modules, which in fact are all= owed >> to use existing API. > > Sorry, but we *never* cared about external out-of-tree modules! If > out-of-tree modules want to use kernel APIs and stay updated then > people should start submitting them to the kernel. I thought that > this is clear as this is the default policy here on netdev! Yes, this is perfectly clear. But I was thinking not changing/breaking = an API was a MUST too. > >>> And as the comment said as well, this type was never exposed to >>> user land. >> The fact is that the value is in include/uapi/*, hence it's exposed = to userland. > > Ok, let me explain once more ... no packet *what-so-ever* will ever > go up to user space with PACKET_FASTROUTE in sll_pkttype. 1) because > if you grep the kernel tree then you'll see that this is _not used > anywhere_, 2) as the comment says, skbs of such type were invisible > to user land, hence _never_ exposed through PF_PACKET in user space. Why keeping PACKET_FASTROUTE then?