On 04/19/2017 02:00 PM, Jesper Dangaard Brouer wrote:
On Tue, 18 Apr 2017 13:54:45 -0700
John Fastabend <john.fastab...@gmail.com> wrote:
On 17-04-18 12:58 PM, Jesper Dangaard Brouer wrote:
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).
It's cleared in {skb,xdp}_do_redirect() right after fetching the
ifindex. I think this approach is just fine. The example described
above is a misuse of the API by a buggy program calling bpf_xdp_redirect()
and returning XDP_PASS while another time it returns XDP_REDIRECT
without the bpf_xdp_redirect() helper, sounds very exotic, but it's
as buggy as, say, a program doing the csum update wrong, a program
writing the wrong data to the packet, doing adjust head on the wrong
header offset, jumping into the wrong tail call entry and other things.
I think encoding this into an action code is rather limiting, f.e.
where would we place a flags argument if needed in future? Would
that mean, we need a XDP_REDIRECT2 return code that also allows for
encoding flags?
Thanks,
Daniel
