samples/bpf: Fix xdp_redirect_map egress devmap prog

LLVM compiler optimized out the memcpy in xdp_redirect_map_egress,
which caused the Ethernet source MAC-addr to always be zero
when enabling the devmap egress prog via cmdline --load-egress.

Issue observed with LLVM version 14.0.0
 - Shipped with Fedora 36 on target: x86_64-redhat-linux-gnu.

In verbose mode print the source MAC-addr in case xdp_devmap_attached
mode is used.

Signed-off-by: Jesper Dangaard Brouer <brouer@redhat.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/165754826292.575614.5636444052787717159.stgit@firesoul

samples/bpf/xdp_redirect_map.bpf.c

@@ -33,7 +33,7 @@ struct {
 } tx_port_native SEC(".maps");
 
 /* store egress interface mac address */
-const volatile char tx_mac_addr[ETH_ALEN];
+const volatile __u8 tx_mac_addr[ETH_ALEN];
 
 static __always_inline int xdp_redirect_map(struct xdp_md *ctx, void *redirect_map)
 {
@@ -73,6 +73,7 @@ int xdp_redirect_map_egress(struct xdp_md *ctx)
 {
 	void *data_end = (void *)(long)ctx->data_end;
 	void *data = (void *)(long)ctx->data;
+	u8 *mac_addr = (u8 *) tx_mac_addr;
 	struct ethhdr *eth = data;
 	u64 nh_off;
 
@@ -80,7 +81,8 @@ int xdp_redirect_map_egress(struct xdp_md *ctx)
 	if (data + nh_off > data_end)
 		return XDP_DROP;
 
-	__builtin_memcpy(eth->h_source, (const char *)tx_mac_addr, ETH_ALEN);
+	barrier_var(mac_addr); /* prevent optimizing out memcpy */
+	__builtin_memcpy(eth->h_source, mac_addr, ETH_ALEN);
 
 	return XDP_PASS;
 }

samples/bpf/xdp_redirect_map_user.c

@@ -40,6 +40,8 @@ static const struct option long_options[] = {
 	{}
 };
 
+static int verbose = 0;
+
 int main(int argc, char **argv)
 {
 	struct bpf_devmap_val devmap_val = {};
@@ -79,6 +81,7 @@ int main(int argc, char **argv)
 			break;
 		case 'v':
 			sample_switch_mode();
+			verbose = 1;
 			break;
 		case 's':
 			mask |= SAMPLE_REDIRECT_MAP_CNT;
@@ -134,6 +137,12 @@ int main(int argc, char **argv)
 			ret = EXIT_FAIL;
 			goto end_destroy;
 		}
+		if (verbose)
+			printf("Egress ifindex:%d using src MAC %02x:%02x:%02x:%02x:%02x:%02x\n",
+			       ifindex_out,
+			       skel->rodata->tx_mac_addr[0], skel->rodata->tx_mac_addr[1],
+			       skel->rodata->tx_mac_addr[2], skel->rodata->tx_mac_addr[3],
+			       skel->rodata->tx_mac_addr[4], skel->rodata->tx_mac_addr[5]);
 	}
 
 	skel->rodata->from_match[0] = ifindex_in;