netdev - Re: [PATCH net] dev_forward_skb: do not scrub skb mark within the same name space

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Date:   Thu, 24 Jun 2021 17:26:59 +0200
From:   Nicolas Dichtel <nicolas.dichtel@...nd.com>
To:     Eyal Birger <eyal.birger@...il.com>
Cc:     netdev@...r.kernel.org, davem@...emloft.net, kuba@...nel.org,
        liran.alon@...cle.com, shmulik.ladkani@...il.com,
        daniel@...earbox.net
Subject: Re: [PATCH net] dev_forward_skb: do not scrub skb mark within the
 same name space

Hi Eyal,

Le 24/06/2021 à 14:16, Eyal Birger a écrit :
> Hi Nicholas,
> 
> On 24/06/2021 11:05, Nicolas Dichtel wrote:
>> The goal is to keep the mark during a bpf_redirect(), like it is done for
>> legacy encapsulation / decapsulation, when there is no x-netns.
>> This was initially done in commit 213dd74aee76 ("skbuff: Do not scrub skb
>> mark within the same name space").
>>
>> When the call to skb_scrub_packet() was added in dev_forward_skb() (commit
>> 8b27f27797ca ("skb: allow skb_scrub_packet() to be used by tunnels")), the
>> second argument (xnet) was set to true to force a call to skb_orphan(). At
>> this time, the mark was always cleanned up by skb_scrub_packet(), whatever
>> xnet value was.
>> This call to skb_orphan() was removed later in commit
>> 9c4c325252c5 ("skbuff: preserve sock reference when scrubbing the skb.").
>> But this 'true' stayed here without any real reason.
>>
>> Let's correctly set xnet in ____dev_forward_skb(), this function has access
>> to the previous interface and to the new interface.
> 
> This change was suggested in the past [1] and I think one of the main concerns
> was breaking existing callers which assume the mark would be cleared [2].
Thank you for the pointers!

> 
> Personally, I think the suggestion made in [3] adding a flag to bpf_redirect()
> makes a lot of sense for this use case.
I began with this approach, but actually, as I tried to explain in the commit
log, this looks more like a bug. This function is called almost everywhere in
the kernel (except for openvswitch and wireguard) with the xnet argument
reflecting a netns change. In other words, the behavior is different between
legacy encapsulation and the one made with ebpf :/


Regards,
Nicolas

> 
> Eyal.
> 
> [1]
> https://lore.kernel.org/netdev/1520953642-8145-1-git-send-email-liran.alon@oracle.com/
> 
> 
> [2] https://lore.kernel.org/netdev/20180315112150.58586758@halley/
> 
> [3]
> https://lore.kernel.org/netdev/cd0b73e3-2cde-1442-4312-566c69571e8a@iogearbox.net/