netdev - [bpf-next v3 PATCH 3/3] samples/bpf: xdp_fwd explain bpf_fib

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Date:   Thu, 08 Aug 2019 18:17:47 +0200
From:   Jesper Dangaard Brouer <brouer@...hat.com>
To:     netdev@...r.kernel.org, Daniel Borkmann <borkmann@...earbox.net>,
        Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc:     a.s.protopopov@...il.com, dsahern@...il.com,
        Toke Høiland-Jørgensen <toke@...e.dk>,
        ys114321@...il.com, Jesper Dangaard Brouer <brouer@...hat.com>
Subject: [bpf-next v3 PATCH 3/3] 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: Anton Protopopov <a.s.protopopov@...il.com>
Signed-off-by: Jesper Dangaard Brouer <brouer@...hat.com>
Reviewed-by: David Ahern <dsahern@...il.com>
Acked-by: Yonghong Song <yhs@...com>
---
 samples/bpf/xdp_fwd_kern.c |   19 +++++++++++++++++--
 1 file changed, 17 insertions(+), 2 deletions(-)

diff --git a/samples/bpf/xdp_fwd_kern.c b/samples/bpf/xdp_fwd_kern.c
index a43d6953c054..701a30f258b1 100644
--- a/samples/bpf/xdp_fwd_kern.c
+++ b/samples/bpf/xdp_fwd_kern.c
@@ -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).