[net-next,2/2] netlink: specify netlink packet direction for nlmon

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, "overload" the meaning of these
flags for netlink skbs on nlmon devices, thus it can be picked up
via sll_pkttype in struct sockaddr_ll. We have now:

 - PACKET_USER    ->  to user space
 - PACKET_KERNEL  ->  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>
---
 include/uapi/linux/if_packet.h | 3 +++
 net/netlink/af_netlink.c       | 2 ++
 2 files changed, 5 insertions(+)

From: Daniel Borkmann <dborkman@redhat.com>
Date: Thu, 19 Dec 2013 02:29:39 +0100

> @@ -29,6 +29,9 @@ struct sockaddr_ll {
>  /* These ones are invisible by user level */
>  #define PACKET_LOOPBACK		5		/* MC/BRD frame looped back */
>  #define PACKET_FASTROUTE	6		/* Fastrouted frame	*/
> +/* These ones are for nlmon devices */
> +#define PACKET_USER		0		/* To user space */
> +#define PACKET_KERNEL		1		/* To kernel space */

I know it is tempting to do so, but please do not reuse values
like this.

THanks.
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On 12/23/2013 12:56 AM, David Miller wrote:
> From: Daniel Borkmann <dborkman@redhat.com>
> Date: Thu, 19 Dec 2013 02:29:39 +0100
>
>> @@ -29,6 +29,9 @@ struct sockaddr_ll {
>>   /* These ones are invisible by user level */
>>   #define PACKET_LOOPBACK		5		/* MC/BRD frame looped back */
>>   #define PACKET_FASTROUTE	6		/* Fastrouted frame	*/
>> +/* These ones are for nlmon devices */
>> +#define PACKET_USER		0		/* To user space */
>> +#define PACKET_KERNEL		1		/* To kernel space */
>
> I know it is tempting to do so, but please do not reuse values
> like this.

Hm, ok. As we only have 3 bits for all this, are you okay with
doing the following ...

#define PACKET_USER	6	/* To user space   */
#define PACKET_KERNEL	7	/* To kernel space */

PACKET_FASTROUTE isn't used anywhere in the tree, only defined
in this header file. Then, by doing this, we would still fit.

> THanks.
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