Re: [RFC PATCH v3 net-next 3/4] net: Set skb->mono_delivery_time and clear it when delivering locally

[Julian Anastasov <ja@ssi.bg>]

	Hello,

On Thu, 20 Jan 2022, Martin KaFai Lau wrote:

> This patch sets the skb->mono_delivery_time to flag the skb->tstamp
> is used as the mono delivery_time (EDT) instead of the (rcv) timestamp.
> 
> skb_clear_delivery_time() is added to clear the delivery_time and set
> back to the (rcv) timestamp if needed when the skb is being delivered
> locally (to a sk).  skb_clear_delivery_time() is called in
> ip_local_deliver() and ip6_input().  In most of the regular ingress
> cases, the skb->tstamp should already have the (rcv) timestamp.
> For the egress loop back to ingress cases, the marking of the (rcv)
> timestamp is postponed from dev.c to ip_local_deliver() and
> ip6_input().
> 
> Another case needs to clear the delivery_time is the network
> tapping (e.g. af_packet by tcpdump).  Regardless of tapping at the ingress
> or egress,  the tapped skb is received by the af_packet socket, so
> it is ingress to the af_packet socket and it expects
> the (rcv) timestamp.
> 
> When tapping at egress, dev_queue_xmit_nit() is used.  It has already
> expected skb->tstamp may have delivery_time,  so it does
> skb_clone()+net_timestamp_set() to ensure the cloned skb has
> the (rcv) timestamp before passing to the af_packet sk.
> This patch only adds to clear the skb->mono_delivery_time
> bit in net_timestamp_set().
> 
> When tapping at ingress, it currently expects the skb->tstamp is either 0
> or has the (rcv) timestamp.  Meaning, the tapping at ingress path
> has already expected the skb->tstamp could be 0 and it will get
> the (rcv) timestamp by ktime_get_real() when needed.
> 
> There are two cases for tapping at ingress:
> 
> One case is af_packet queues the skb to its sk_receive_queue.  The skb
> is either not shared or new clone created.  The skb_clear_delivery_time()
> is called to clear the delivery_time (if any) before it is queued to the
> sk_receive_queue.
> 
> Another case, the ingress skb is directly copied to the rx_ring
> and tpacket_get_timestamp() is used to get the (rcv) timestamp.
> skb_tstamp() is used in tpacket_get_timestamp() to check
> the skb->mono_delivery_time bit before returning skb->tstamp.
> As mentioned earlier, the tapping@ingress has already expected
> the skb may not have the (rcv) timestamp (because no sk has asked
> for it) and has handled this case by directly calling ktime_get_real().
> 
> In __skb_tstamp_tx, it clones the egress skb and queues the clone to the
> sk_error_queue.  The outgoing skb may have the mono delivery_time while
> the (rcv) timestamp is expected for the clone, so the
> skb->mono_delivery_time bit is also cleared from the clone.
> 
> Signed-off-by: Martin KaFai Lau <kafai@fb.com>
> ---
>  include/linux/skbuff.h | 27 +++++++++++++++++++++++++--
>  net/core/dev.c         |  4 +++-
>  net/core/skbuff.c      |  6 ++++--
>  net/ipv4/ip_input.c    |  1 +
>  net/ipv6/ip6_input.c   |  1 +
>  net/packet/af_packet.c |  4 +++-
>  6 files changed, 37 insertions(+), 6 deletions(-)
> 

> diff --git a/net/ipv4/ip_input.c b/net/ipv4/ip_input.c
> index 3a025c011971..35311ca75496 100644
> --- a/net/ipv4/ip_input.c
> +++ b/net/ipv4/ip_input.c
> @@ -244,6 +244,7 @@ int ip_local_deliver(struct sk_buff *skb)
>  	 */
>  	struct net *net = dev_net(skb->dev);
>  
> +	skb_clear_delivery_time(skb);

	Is it safe to move this line into ip_local_deliver_finish ?

>  	if (ip_is_fragment(ip_hdr(skb))) {
>  		if (ip_defrag(net, skb, IP_DEFRAG_LOCAL_DELIVER))
>  			return 0;
> diff --git a/net/ipv6/ip6_input.c b/net/ipv6/ip6_input.c
> index 80256717868e..84f93864b774 100644
> --- a/net/ipv6/ip6_input.c
> +++ b/net/ipv6/ip6_input.c
> @@ -469,6 +469,7 @@ static int ip6_input_finish(struct net *net, struct sock *sk, struct sk_buff *sk
>  
>  int ip6_input(struct sk_buff *skb)
>  {
> +	skb_clear_delivery_time(skb);

	Is it safe to move this line into ip6_input_finish?
The problem for both cases is that IPVS hooks at LOCAL_IN and
can decide to forward the packet by returning NF_STOLEN and
avoiding the _finish code. In short, before reaching the
_finish code it is still not decided that packet reaches the
sockets.

>  	return NF_HOOK(NFPROTO_IPV6, NF_INET_LOCAL_IN,
>  		       dev_net(skb->dev), NULL, skb, skb->dev, NULL,
>  		       ip6_input_finish);

Regards

--
Julian Anastasov <ja@ssi.bg>