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

On 24/06/2021 18:26, Nicolas Dichtel wrote:

Hi Eyal,

Le 24/06/2021 à 14:16, Eyal Birger a écrit :

quoted

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!

quoted

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 :/

I agree, and was also surprised that ebpf redirection scrubs the mark in 
the same ns - and only on ingress! - so I think keeping the mark should 
have been the default behavior. As noted in the thread though it is not 
clear whether this would break existing deployments both for ebpf and 
veth pairs on the same ns.

Eyal.