From mboxrd@z Thu Jan 1 00:00:00 1970 From: Daniel Borkmann Subject: Re: [PATCH net-next v2 2/2] netlink: specify netlink packet direction for nlmon Date: Mon, 23 Dec 2013 12:11:15 +0100 Message-ID: <52B81A53.9020305@redhat.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> 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: nicolas.dichtel@6wind.com Return-path: Received: from mx1.redhat.com ([209.132.183.28]:15578 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753416Ab3LWLLW (ORCPT ); Mon, 23 Dec 2013 06:11:22 -0500 In-Reply-To: <52B81875.4060201@6wind.com> Sender: netdev-owner@vger.kernel.org List-ID: 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 have >>>> set the new skb owner via netlink_skb_set_owner_r(), so we can use >>>> 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-overlappi= ng >>>> 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/i= f_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 b= ack */ >>>> +#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 allo= wed > 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! >> 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 t= o 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. Thanks !