Subject: Re: [PATCH v2 net-next 01/14] net: Clear skb->tstamp only on the forwarding path

From: Eric Dumazet <[email-masked]>
Date: Fri, 13 Jul 2018 10:35:15 -0700

On 07/03/2018 03:42 PM, Jesus Sanchez-Palencia wrote:
> This is done in preparation for the upcoming time based transmission
> patchset. Now that skb->tstamp will be used to hold packet's txtime,
> we must ensure that it is being cleared when traversing namespaces.
> Also, doing that from skb_scrub_packet() before the early return would
> break our feature when tunnels are used.
> 
> Signed-off-by: Jesus Sanchez-Palencia <[email-masked]>
> ---
>  net/core/skbuff.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/net/core/skbuff.c b/net/core/skbuff.c
> index 1357f36c8a5e..c4e24ac27464 100644
> --- a/net/core/skbuff.c
> +++ b/net/core/skbuff.c
> @@ -4898,7 +4898,6 @@ EXPORT_SYMBOL(skb_try_coalesce);
>   */
>  void skb_scrub_packet(struct sk_buff *skb, bool xnet)
>  {
> -	skb->tstamp = 0;
>  	skb->pkt_type = PACKET_HOST;
>  	skb->skb_iif = 0;
>  	skb->ignore_df = 0;
> @@ -4912,6 +4911,7 @@ void skb_scrub_packet(struct sk_buff *skb, bool xnet)
>  
>  	ipvs_reset(skb);
>  	skb->mark = 0;
> +	skb->tstamp = 0;
>  }
>  EXPORT_SYMBOL_GPL(skb_scrub_packet);
>  
> 

I believe we had some misunderstanding here.

What I meant by forwarding is the following case :

- We receive a packet.
- netstamp_wanted is >0 (because at least one packet capture is active)
- __net_timestamp() is called and does :
    skb->tstamp = ktime_get_real();

Then this skb is forwarded into an interface where EDT is taken into
consideration by either a qdisc or a device.

Since CLOCK_TAI is a different base than CLOCK_REALTIME, we might have a problem.

Solutions for this problem :

1) Convert all our skb->tstamp usages to CLOCK_TAI base.

or

2) clear skb->tstamp in forwarding paths, including the ones not scrubbing the packet.

My preference is 1), even if it is a bit more work.