netdev - Re: [PATCH] net: dev_forward_skb(): Scrub packet's per-netns info only when crossing netns

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Date:   Thu, 15 Mar 2018 10:14:01 -0700 (PDT)
From:   Liran Alon <liran.alon@...cle.com>
To:     <shmulik.ladkani@...il.com>
Cc:     <netdev@...r.kernel.org>, <mrv@...atatu.com>,
        <daniel@...earbox.net>, <davem@...emloft.net>,
        <linux-kernel@...r.kernel.org>, <yuval.shaia@...cle.com>,
        <idan.brown@...cle.com>
Subject: Re: [PATCH] net: dev_forward_skb(): Scrub packet's per-netns info
 only when crossing netns


----- shmulik.ladkani@...il.com wrote:

> On Thu, 15 Mar 2018 09:35:51 -0700 (PDT) Liran Alon
> <liran.alon@...cle.com> wrote:
> > ----- shmulik.ladkani@...il.com wrote:
> > 
> > > On Thu, 15 Mar 2018 08:01:03 -0700 (PDT) Liran Alon
> > > <liran.alon@...cle.com> wrote:  
> > > > 
> > > > I still think that default behavior should be to zero skb->mark
> only  
> > > when skb  
> > > > cross netdevs in different netns.  
> > > 
> > > But the previous default was scrub the mark in *both* xnet and
> > > non-xnet
> > > situations.
> > > 
> > > Therefore, there might be users which RELY on this (strange)
> default
> > > behavior in their same-netns-veth-pair setups.
> > > Meaning, changing the default behavior might break their apps
> relying
> > > on
> > > the former default behavior.
> > > 
> > > This is why the "disable mark scrubbing in non-xnet case" should
> be
> > > opt-in.  
> > 
> > We think the same.
> > The only difference is that I think this for now should be
> controllable
> > by a global /proc/sys/net/core file instead of giving a flexible
> per-netdev
> > control.
> > Because that is a larger change that could be done later.
> 
> A flags attribute to veth newlink is a very scoped change.
> User controls this per veth creation.
> This is way more neat than /proc/sys/net and provides the desired
> granular
> control.
> 
> Also, scoping this to veth has the advantage of not affecting the many
> other
> dev_forward_skb callers.

Agreed. But isn't this an issue also for the
many others (& future) callers of dev_forward_skb()?
This seems problematic to me.

This will kinda leave a kernel interface with broken default behavior
for backwards comparability.

A flag to netdev or /proc/sys/net/core to "fix" default behavior
will avoid this.

> 
> Regards,
> Shmulik