On Tue, 18 Apr 2017 13:54:45 -0700
John Fastabend [off-list ref] wrote:

On 17-04-18 12:58 PM, Jesper Dangaard Brouer wrote:

quoted

As I argued in NetConf presentation[1] (from slide #9) we need a port
mapping table (instead of using ifindex'es).  Both for supporting
other "port" types than net_devices (think sockets), and for
sandboxing what XDP can bypass.

I want to create a new XDP action called XDP_REDIRECT, that instruct
XDP to send the xdp_buff to another "port" (get translated into a
net_device, or something else depending on internal port type).

Looking at the userspace/eBPF interface, I'm wondering what is the
best API for "returning" this port number from eBPF?

The options I see is:

1) Split-up the u32 action code, and e.g let the high-16-bit be the
   port number and lower-16bit the (existing) action verdict.

 Pros: Simple API
 Cons: Number of ports limited to 64K

2) Extend both xdp_buff + xdp_md to contain a (u32) port number, allow
   eBPF to update xdp_md->port.

 Pros: Larger number of ports.
 Cons: This require some ebpf translation steps between xdp_buff <-> xdp_md.
       (see xdp_convert_ctx_access)

3) Extend only xdp_buff and create bpf_helper that set port in xdp_buff.

 Pros: Hides impl details, and allows helper to give eBPF code feedback
       (on e.g. if port doesn't exist any longer)
 Cons: Helper function call likely slower?

  

How about doing this the same way redirect is done in the tc case? I have this
patch under test,

 https://github.com/jrfastab/linux/commit/e78f5425d5e3c305b4170ddd85c61c2e15359fee

I have been looking at this approach, which is close to option #3 above.

The problem with your implementation that you use a per-cpu store.
This creates the problem of storing state between packets. First packet
can call helper bpf_xdp_redirect() setting an ifindex, but program can
still return XDP_PASS.  Next packet can call XDP_REDIRECT and use the
ifindex set from the first packet.  IMHO this is a problematic API to
expose.

I do see that the TC interface that uses the same approach, via helper
bpf_redirect().  Maybe it have the same API problem?  Looking at
sch_handle_ingress() I don't see this is handled (e.g. by always
clearing this_cpu_ptr(redirect_info)->ifindex = 0).

that should give you some idea. It just needs a port mapping table in the
bpf_tx_xdp() call.

I'll take a closer look. I don't think we need the per-cpu-store
approach for XDP, as we might as well store the port info in xdp_buff,
or return it directly option #1.

(TC redirect need the per-cpu-store to avoid extending the SKB).

quoted

(Cc'ed xdp-newbies as end-users might have an opinion on UAPI?)

I would still like people to comment on the above options?

-- 
Best regards,
  Jesper Dangaard Brouer
  MSc.CS, Principal Kernel Engineer at Red Hat
  LinkedIn: http://www.linkedin.com/in/brouer