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Commit abcce733 authored by Jesper Dangaard Brouer's avatar Jesper Dangaard Brouer Committed by Daniel Borkmann
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samples/bpf: xdp_fwd explain bpf_fib_lookup return codes 



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: default avatarAnton Protopopov <a.s.protopopov@gmail.com>
Signed-off-by: default avatarJesper Dangaard Brouer <brouer@redhat.com>
Reviewed-by: default avatarDavid Ahern <dsahern@gmail.com>
Acked-by: default avatarYonghong Song <yhs@fb.com>
Reviewed-by: default avatarToke Høiland-Jørgensen <toke@redhat.com>
Signed-off-by: default avatarDaniel Borkmann <daniel@iogearbox.net>
parent a32a32cb

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).