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 14:08:22 +0100 [thread overview]
Message-ID: <52B835C6.3040800@6wind.com> (raw)
In-Reply-To: <52B81A53.9020305@redhat.com>
Le 23/12/2013 12:11, Daniel Borkmann a écrit :
> On 12/23/2013 12:03 PM, Nicolas Dichtel wrote:
>> 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.
>
> 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?
next prev parent reply other threads:[~2013-12-23 13:08 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
2013-12-23 11:11 ` Daniel Borkmann
2013-12-23 13:08 ` Nicolas Dichtel [this message]
2013-12-23 13:21 ` Daniel Borkmann
2013-12-31 18:48 ` David Miller
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