samples/bpf: xdp_redirect load XDP dummy prog on TX device

For supporting XDP_REDIRECT, a device driver must (obviously)
implement the "TX" function ndo_xdp_xmit().  An additional requirement
is you cannot TX out a device, unless it also have a xdp bpf program
attached. This dependency is caused by the driver code need to setup
XDP resources before it can ndo_xdp_xmit.

Update bpf samples xdp_redirect and xdp_redirect_map to automatically
attach a dummy XDP program to the configured ifindex_out device.  Use
the XDP flag XDP_FLAGS_UPDATE_IF_NOEXIST on the dummy load, to avoid
overriding an existing XDP prog on the device.

Signed-off-by: Jesper Dangaard Brouer <brouer@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 10 insertions, 1 deletions

diff --git a/samples/bpf/xdp_redirect_kern.c b/samples/bpf/xdp_redirect_kern.c
index a34ad457a684..1c90288d0203 100644
--- a/samples/bpf/xdp_redirect_kern.c
+++ b/samples/bpf/xdp_redirect_kern.c
@@ -26,6 +26,9 @@ struct bpf_map_def SEC("maps") tx_port = {
 	.max_entries = 1,
 };
 
+/* Count RX packets, as XDP bpf_prog doesn't get direct TX-success
+ * feedback.  Redirect TX errors can be caught via a tracepoint.
+ */
 struct bpf_map_def SEC("maps") rxcnt = {
 	.type = BPF_MAP_TYPE_PERCPU_ARRAY,
 	.key_size = sizeof(u32),
@@ -33,7 +36,6 @@ struct bpf_map_def SEC("maps") rxcnt = {
 	.max_entries = 1,
 };
 
-
 static void swap_src_dst_mac(void *data)
 {
 	unsigned short *p = data;
@@ -78,4 +80,11 @@ int xdp_redirect_prog(struct xdp_md *ctx)
 	return bpf_redirect(*ifindex, 0);
 }
 
+/* Redirect require an XDP bpf_prog loaded on the TX device */
+SEC("xdp_redirect_dummy")
+int xdp_redirect_dummy(struct xdp_md *ctx)
+{
+	return XDP_PASS;
+}
+
 char _license[] SEC("license") = "GPL";