From: Nicolas Dichtel <nicolas.dichtel@6wind•com>
To: Daniel Borkmann <dborkman@redhat•com>
Cc: davem@davemloft•net, netdev@vger•kernel.org,
Jakub Zawadzki <darkjames-ws@darkjames•pl>
Subject: Re: [PATCH net-next v2 2/2] netlink: specify netlink packet direction for nlmon
Date: Mon, 23 Dec 2013 12:03:17 +0100 [thread overview]
Message-ID: <52B81875.4060201@6wind.com> (raw)
In-Reply-To: <52B81496.6060807@redhat.com>
Le 23/12/2013 11:46, Daniel Borkmann a écrit :
> On 12/23/2013 11:43 AM, Nicolas Dichtel wrote:
>> Le 23/12/2013 09:48, Daniel Borkmann a écrit :
>>> 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 (= 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-overlapping
>>> 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 (= 6) -> to user space
>>> - PACKET_KERNEL (= 7) -> to kernel space
>>>
>>> Partial `ip a` example strace for sa_family=AF_NETLINK with
>>> detected nl msg direction:
>>>
>>> syscall: direction:
>>> sendto(3, ...) = 40 /* to kernel */
>>> recvmsg(3, ...) = 3404 /* to user */
>>> recvmsg(3, ...) = 1120 /* to user */
>>> recvmsg(3, ...) = 20 /* to user */
>>> sendto(3, ...) = 40 /* to kernel */
>>> recvmsg(3, ...) = 168 /* to user */
>>> recvmsg(3, ...) = 144 /* to user */
>>> recvmsg(3, ...) = 20 /* to user */
>>>
>>> Signed-off-by: Daniel Borkmann <dborkman@redhat•com>
>>> Signed-off-by: Jakub Zawadzki <darkjames-ws@darkjames•pl>
>>> ---
>>> 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 allowed
to use existing API.
> 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.
next prev parent reply other threads:[~2013-12-23 11:03 UTC|newest]
Thread overview: 10+ messages / expand[flat|nested] mbox.gz Atom feed top
2013-12-23 8:48 [PATCH net-next v2 0/2] nlmon updates Daniel Borkmann
2013-12-23 8:48 ` [PATCH net-next v2 1/2] netlink: only do not deliver to tap when both sides are kernel sks Daniel Borkmann
2013-12-23 8:48 ` [PATCH net-next v2 2/2] netlink: specify netlink packet direction for nlmon Daniel Borkmann
2013-12-23 10:43 ` Nicolas Dichtel
2013-12-23 10:46 ` Daniel Borkmann
2013-12-23 11:03 ` Nicolas Dichtel [this message]
2013-12-23 11:11 ` Daniel Borkmann
2013-12-23 13:08 ` Nicolas Dichtel
2013-12-23 13:21 ` Daniel Borkmann
2013-12-31 18:48 ` David Miller
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