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

diff --git a/samples/bpf/xdp_redirect_map.bpf.c b/samples/bpf/xdp_redirect_map.bpf.c
index 415bac1..8557c27 100644 (file)
--- a/samples/bpf/xdp_redirect_map.bpf.c
+++ b/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;
 }

diff --git a/samples/bpf/xdp_redirect_map_user.c b/samples/bpf/xdp_redirect_map_user.c
index b6e4fc8..c889a13 100644 (file)
--- a/samples/bpf/xdp_redirect_map_user.c
+++ b/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;