linux-kernel - 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 08:01:03 -0700 (PDT)
From:   Liran Alon <liran.alon@...cle.com>
To:     <mrv@...atatu.com>
Cc:     <netdev@...r.kernel.org>, <shmulik.ladkani@...il.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


----- mrv@...atatu.com wrote:

> Liran Alon <liran.alon@...cle.com> writes:
> 
> 
> [...]
> 
> >> Overall I think it might be nice to not need scrubbing skb in such
> >> cases,
> >> although my concern would be that this has potential to break
> >> existing
> >> setups when they would expect mark being zero on other veth peer
> in
> >> any
> >> case since it's the behavior for a long time already. The safer
> >> option
> >> would be to have some sort of explicit opt-in e.g. on link creation
> to
> >> let
> >> the skb->mark pass through unscrubbed. This would definitely be a
> >> useful
> >> option e.g. when mark is set in the netns facing veth via
> >> clsact/egress
> >> on xmit and when the container is unprivileged anyway.
> >> 
> >> Thanks,
> >> Daniel
> >
> > I see your point in regards to backwards comparability.
> > However, not scrubbing skb when it cross netns via some kernel
> functions compared to
> > others is basically a bug which could easily break with a little bit
> of more refactoring.
> > Therefore, it seems a bit weird to me to from now on, we will force
> > every user on link creation to consider that once there was a bug
> leading
> > to this weird behavior on specific netdevs.
> > Thus, I suggest to maybe control this via a global /proc/sys/net
> file instead.
> 
> One valid use case could be preserving a source namespace nsid in
> skb->mark when a packet crosses netns.

Before and after this commit, veth peers that crosses netns zero skb->mark.
Therefore, what you suggest is a new feature unrelated to the
issue fixed by this commit.

I still think that default behavior should be to zero skb->mark only when skb
cross netdevs in different netns. And for backwards comparability, we can
consider adding a /proc/sys/net/core file to let dev_forward_skb() to always
scrub packet regardless if it crosses netdevs or not.