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 10:35:42 -0400
From:   Roman Mashak <mrv@...atatu.com>
To:     Liran Alon <liran.alon@...cle.com>
Cc:     <daniel@...earbox.net>, <netdev@...r.kernel.org>,
        <shmulik.ladkani@...il.com>, <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

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.