Subject: Re: [PATCH net-next v2 2/2] netlink: specify netlink packet direction for nlmon

From: Daniel Borkmann <[email-masked]>
Date: Mon, 23 Dec 2013 11:46:46 +0100

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 <[email-masked]>
>> Signed-off-by: Jakub Zawadzki <[email-masked]>
>> ---
>>   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.
And as the comment said as well, this type was never exposed to
user land.

>> +#define PACKET_KERNEL        7        /* To kernel space    */
>> +/* Unused, PACKET_FASTROUTE and PACKET_LOOPBACK are invisble to user space */
> nitpicking: s/invisble/invisible
>
>>   #define PACKET_FASTROUTE    6        /* Fastrouted frame    */
>>
>>   /* Packet socket options */
>> diff --git a/net/netlink/af_netlink.c b/net/netlink/af_netlink.c
>> index 56e09d8..3f75f1c 100644
>> --- a/net/netlink/af_netlink.c
>> +++ b/net/netlink/af_netlink.c
>> @@ -204,6 +204,8 @@ static int __netlink_deliver_tap_skb(struct sk_buff *skb,
>>       if (nskb) {
>>           nskb->dev = dev;
>>           nskb->protocol = htons((u16) sk->sk_protocol);
>> +        nskb->pkt_type = netlink_is_kernel(sk) ?
>> +                 PACKET_KERNEL : PACKET_USER;
>>
>>           ret = dev_queue_xmit(nskb);
>>           if (unlikely(ret > 0))
>>
> --
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