Subject: Re: [PATCH net-next v3 2/2] netlink: specify netlink packet direction for nlmon

From: Nicolas Dichtel <[email-masked]>
Date: Mon, 23 Dec 2013 18:46:51 +0100

Le 23/12/2013 14:35, 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 not supported anywhere, 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 <[email-masked]>
> Signed-off-by: Jakub Zawadzki <[email-masked]>
> ---
>   v1->v2:
>    - let PACKET_* values not overlap as requested by Dave
>   v2->v3:
>    - fixed typo in comment spotted by Nicolas, thanks
>
>   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	*/
> +#define PACKET_KERNEL		7		/* To kernel space	*/
> +/* Unused, PACKET_FASTROUTE and PACKET_LOOPBACK are invisible to user space */
>   #define PACKET_FASTROUTE	6		/* Fastrouted frame	*/
Sorry to insist, I just try to understand. Why not removing the definition of
PACKET_FASTROUTE?
Or have a name like PACKET_NL_USER to document the difference between both
cases?