From mboxrd@z Thu Jan  1 00:00:00 1970
From: Eric Dumazet <eric.dumazet@gmail.com>
Subject: Re: [PATCH v2 net-next 01/14] net: Clear skb->tstamp only on the
 forwarding path
Date: Fri, 13 Jul 2018 10:35:15 -0700
Message-ID: <1e52c128-59f4-43ae-3487-059a84ae61c3@gmail.com>
References: <20180703224300.25300-1-jesus.sanchez-palencia@intel.com>
 <20180703224300.25300-2-jesus.sanchez-palencia@intel.com>
Mime-Version: 1.0
Content-Type: text/plain; charset=utf-8
Content-Transfer-Encoding: 7bit
Cc: tglx@linutronix.de, jan.altenberg@linutronix.de,
        vinicius.gomes@intel.com, kurt.kanzenbach@linutronix.de,
        henrik@austad.us, richardcochran@gmail.com,
        ilias.apalodimas@linaro.org, ivan.khoronzhuk@linaro.org,
        mlichvar@redhat.com, willemb@google.com, jhs@mojatatu.com,
        xiyou.wangcong@gmail.com, jiri@resnulli.us, eric.dumazet@gmail.com,
        jeffrey.t.kirsher@intel.com
To: Jesus Sanchez-Palencia <jesus.sanchez-palencia@intel.com>,
        netdev@vger.kernel.org
Return-path: <netdev-owner@vger.kernel.org>
Received: from mail-pf0-f195.google.com ([209.85.192.195]:46483 "EHLO
        mail-pf0-f195.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org
        with ESMTP id S1729963AbeGMRuw (ORCPT
        <rfc822;netdev@vger.kernel.org>); Fri, 13 Jul 2018 13:50:52 -0400
Received: by mail-pf0-f195.google.com with SMTP id l123-v6so23002518pfl.13
        for <netdev@vger.kernel.org>; Fri, 13 Jul 2018 10:35:17 -0700 (PDT)
In-Reply-To: <20180703224300.25300-2-jesus.sanchez-palencia@intel.com>
Content-Language: en-US
Sender: netdev-owner@vger.kernel.org
List-ID: <netdev.vger.kernel.org>



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 <jesus.sanchez-palencia@intel.com>
> ---
>  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.