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samples/bpf: xdp_fwd explain bpf_fib_lookup return codes

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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>
This commit is contained in:
Jesper Dangaard Brouer 2019-08-08 18:17:47 +02:00 committed by Daniel Borkmann
parent a32a32cb26
commit abcce733ad
1 changed files with 17 additions and 2 deletions
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19
samples/bpf/xdp_fwd_kern.c
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@ -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).