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authorJesper Dangaard Brouer <brouer@redhat.com>2019-08-08 18:17:47 +0200
committerDaniel Borkmann <daniel@iogearbox.net>2019-08-09 18:05:03 +0200
commitabcce733adb71013997fcb84142a5454ef133616 (patch)
tree28659aea47a6a2c3135b3f97a4ecb3dae929f2d1 /samples/bpf/xdp_fwd_kern.c
parenta32a32cb26eb6291125e4eb49b569874ca9a53b5 (diff)
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@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>
Diffstat (limited to 'samples/bpf/xdp_fwd_kern.c')
-rw-r--r--samples/bpf/xdp_fwd_kern.c19
1 files 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).