Re: bpf_redirect and xdpgeneric

[David Ahern <dsahern@gmail.com>]

On 9/18/20 12:42 PM, Thomas Ptacek wrote:
> The setup is pretty simple. There's an eno1 (igb driver), to which our
> default route points. On the same box are several VMs. There's a tap
> interface (for each VM, call it tapX). Traffic for a VM flows in from
> the Internet on eno1 and is directed to tapX; the response traffic
> flows in the other direction.
> 
> I'm deliberately simplifying here:
> 
> eno1 runs an XDP program that does some lightweight IP rewriting from
> anycast addresses to internal VM addresses on ingress. eno1's XDP
> program currently XDP_PASS's rewritten packets to the IP stack, where
> they're routed to the VM's tap. This works fine.
> 
> tapX runs an XDP program that does the same rewriting in reverse.
> Right now, it also XDP_PASS's packets to the stack, which also works
> --- the stack routes response traffic out eno1.
> 
> I'm playing with XDP_REDIRECT'ing instead of XDP_PASS'ing.
> 
> I have the ifindexes and MAC addresses (and those of IP neighbors) in
> a map --- a normal HASH map, not a DEVMAP. Using that map, I can
> successfully redirect traffic from tapX to arbitrary other tap
> interfaces. What I can't do is redirect packets from tapX to eno1,
> which is what the system actually needs to do.
> 

XDP_REDIRECT sends the packet to a devices ndo_xdp_xmit function. tap
implements it hence eno1 -> tap works; igb does not meaning tap -> eno1
fails.

xdpgeneric does not work in the Tx path.