samples/bpf: xdp_fwd explain bpf_fib_lookup return codes (abcce733) · Commits · 戴 / test

Make it clear that this XDP program depend on the network stack to do the ARP resolution. This is connected with the BPF_FIB_LKUP_RET_NO_NEIGH return code from bpf_fib_lookup(). Another common mistake (seen via XDP-tutorial) is that users don't realize that sysctl net.ipv{4,6}.conf.all.forwarding setting is honored by bpf_fib_lookup. Reported-by:

Anton Protopopov <a.s.protopopov@gmail.com> Signed-off-by:

Jesper Dangaard Brouer <brouer@redhat.com> Reviewed-by:

David Ahern <dsahern@gmail.com> Acked-by:

Yonghong Song <yhs@fb.com> Reviewed-by:

Toke Høiland-Jørgensen <toke@redhat.com> Signed-off-by:

Daniel Borkmann <daniel@iogearbox.net>

samples/bpf/xdp_fwd_kern.c

+17 −2

Original line number	Diff line number	Diff line
		@@ -103,8 +103,23 @@ static __always_inline int xdp_fwd_flags(struct xdp_md *ctx, u32 flags)
		fib_params.ifindex = ctx->ingress_ifindex;

		rc = bpf_fib_lookup(ctx, &fib_params, sizeof(fib_params), flags);

		if (rc == 0) {
		/*
		* Some rc (return codes) from bpf_fib_lookup() are important,
		* to understand how this XDP-prog interacts with network stack.
		*
		* BPF_FIB_LKUP_RET_NO_NEIGH:
		* Even if route lookup was a success, then the MAC-addresses are also
		* needed. This is obtained from arp/neighbour table, but if table is
		* (still) empty then BPF_FIB_LKUP_RET_NO_NEIGH is returned. To avoid
		* doing ARP lookup directly from XDP, then send packet to normal
		* network stack via XDP_PASS and expect it will do ARP resolution.
		*
		* BPF_FIB_LKUP_RET_FWD_DISABLED:
		* The bpf_fib_lookup respect sysctl net.ipv{4,6}.conf.all.forwarding
		* setting, and will return BPF_FIB_LKUP_RET_FWD_DISABLED if not
		* enabled this on ingress device.
		*/
		if (rc == BPF_FIB_LKUP_RET_SUCCESS) {
		/* Verify egress index has been configured as TX-port.
		* (Note: User can still have inserted an egress ifindex that
		* doesn't support XDP xmit, which will result in packet drops).

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