Subject: Re: xdp_redirect ifindex vs port. Was: best API for returning/setting egress port?

From: John Fastabend <[email-masked]>
Date: Tue, 25 Apr 2017 20:07:34 -0700

On 17-04-25 05:26 PM, Alexei Starovoitov wrote:
> On Tue, Apr 25, 2017 at 11:34:53AM +0200, Jesper Dangaard Brouer wrote:
>>> Note the very first bpf patchset years ago contained the port table
>>> abstraction. ovs has concept of vports as well. These two very
>>> different projects needed port table to provide a layer of
>>> indirection between ifindex==netdev and virtual port number.
>>> This is still the case and I'd like to see this port table to be
>>> implemented for both cls_bpf and xdp. In that sense xdp is not
>>> special.
>>
>> Glad to hear you want to see this implemented, I will start coding on
>> this then.  Good point with cls_bpf, I was planning to make this port
>> table strongly connected to XDP, guess I should also think of cls_bpf.
> 
> perfect.
> I think we should try to make all additions to bpf networking world
> to be usable for both tc and xdp, since both are actively used and
> it wouldn't be great to have cool feature for one, but not the other.
> I think port table is an excellent candidate that applies to both.

+1

Jesper, I was working up the code for the redirect piece for ixgbe and
virtio, please use this as a base for your virtual port number table. I'll
push an update onto github tomorrow. I think the table should drop in fairly
nicely.

One piece that isn't clear to me is how do you plan to instantiate and
program this table. Is it a new static bpf map that is created any time we see
the redirect command? I think this would be preferred.

> 
>> I'm not worried about the DROP case, I agree that is fine (as you also
>> say).  The problem is unintentionally sending a packet to a wrong
>> ifindex.  This is clearly an eBPF program error, BUT with XDP this
>> becomes a very hard to debug program error.  With TC-redirect/cls_bpf
>> we can tcpdump the packets, with XDP there is no visibility into this
>> happening (the NSA is going to love this "feature").  Maybe we could add
>> yet-another tracepoint to allow debugging this.  My proposal that we
>> simply remove the possibility for such program errors, by as you say
>> move the validation from run-time into static insertion-time, via a
>> port table.
> 
> I think lack of tcpdump-like debugging in xdp is a separate issue.
> As I was saying in the other thread we have trivial 'xdpdump' kern+user
> app that emits pcap file, but it's too specific to how we use
> tail_calls+prog_array in our xdp setup. I'm working on the program
> chaining that will be generic and allow us transparently add multiple
> xdp or tc progs to the same attachment point and will allow us to
> do 'xdpdump' at any point of this pipeline, so debugging of what
> happened to the packet will be easier and done in the same way
> for both tc and xdp.
> btw in our experience working with both tc and xdp the tc+bpf was
> actually harder to use and more bug prone.
> 

Nice, the tcpdump-like debugging looks interesting.