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:

quoted

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/ (local)


[2] https://lore.kernel.org/netdev/20180315112150.58586758@halley/ (local)

[3]
https://lore.kernel.org/netdev/cd0b73e3-2cde-1442-4312-566c69571e8a@iogearbox.net/ (local)