selftests/bpf: Properly enable hwtstamp in xdp_hw_metadata

The existing timestamping_enable() is a no-op because it applies
to the socket-related path that we are not verifying here
anymore. (but still leaving the code around hoping we can
have xdp->skb path verified here as well)

  poll: 1 (0)
  xsk_ring_cons__peek: 1
  0xf64788: rx_desc[0]->addr=100000000008000 addr=8100 comp_addr=8000
  rx_hash: 3697961069
  rx_timestamp:  1674657672142214773 (sec:1674657672.1422)
  XDP RX-time:   1674657709561774876 (sec:1674657709.5618) delta sec:37.4196
  AF_XDP time:   1674657709561871034 (sec:1674657709.5619) delta
sec:0.0001 (96.158 usec)
  0xf64788: complete idx=8 addr=8000

Also, maybe something to archive here, see [0] for Jesper's note
about NIC vs host clock delta.

0: https://lore.kernel.org/bpf/f3a116dc-1b14-3432-ad20-a36179ef0608@redhat.com/

v2:
- Restore original value (Martin)

Fixes: 297a3f12 ("selftests/bpf: Simple program to dump XDP RX metadata")
Reported-by: Jesper Dangaard Brouer <jbrouer@redhat.com>
Tested-by: Jesper Dangaard Brouer <jbrouer@redhat.com>
Signed-off-by: Stanislav Fomichev <sdf@google.com>
Link: https://lore.kernel.org/r/20230126225030.510629-1-sdf@google.comSigned-off-by: Martin KaFai Lau <martin.lau@kernel.org>

tools/testing/selftests/bpf/xdp_hw_metadata.c

@@ -24,6 +24,7 @@
 #include <linux/net_tstamp.h>
 #include <linux/udp.h>
 #include <linux/sockios.h>
+#include <linux/net_tstamp.h>
 #include <sys/mman.h>
 #include <net/if.h>
 #include <poll.h>
@@ -278,13 +279,53 @@ static int rxq_num(const char *ifname)
 
 	ret = ioctl(fd, SIOCETHTOOL, &ifr);
 	if (ret < 0)
-		error(-1, errno, "socket");
+		error(-1, errno, "ioctl(SIOCETHTOOL)");
 
 	close(fd);
 
 	return ch.rx_count + ch.combined_count;
 }
 
+static void hwtstamp_ioctl(int op, const char *ifname, struct hwtstamp_config *cfg)
+{
+	struct ifreq ifr = {
+		.ifr_data = (void *)cfg,
+	};
+	strcpy(ifr.ifr_name, ifname);
+	int fd, ret;
+
+	fd = socket(AF_UNIX, SOCK_DGRAM, 0);
+	if (fd < 0)
+		error(-1, errno, "socket");
+
+	ret = ioctl(fd, op, &ifr);
+	if (ret < 0)
+		error(-1, errno, "ioctl(%d)", op);
+
+	close(fd);
+}
+
+static struct hwtstamp_config saved_hwtstamp_cfg;
+static const char *saved_hwtstamp_ifname;
+
+static void hwtstamp_restore(void)
+{
+	hwtstamp_ioctl(SIOCSHWTSTAMP, saved_hwtstamp_ifname, &saved_hwtstamp_cfg);
+}
+
+static void hwtstamp_enable(const char *ifname)
+{
+	struct hwtstamp_config cfg = {
+		.rx_filter = HWTSTAMP_FILTER_ALL,
+	};
+
+	hwtstamp_ioctl(SIOCGHWTSTAMP, ifname, &saved_hwtstamp_cfg);
+	saved_hwtstamp_ifname = strdup(ifname);
+	atexit(hwtstamp_restore);
+
+	hwtstamp_ioctl(SIOCSHWTSTAMP, ifname, &cfg);
+}
+
 static void cleanup(void)
 {
 	LIBBPF_OPTS(bpf_xdp_attach_opts, opts);
@@ -341,6 +382,8 @@ int main(int argc, char *argv[])
 
 	printf("rxq: %d\n", rxq);
 
+	hwtstamp_enable(ifname);
+
 	rx_xsk = malloc(sizeof(struct xsk) * rxq);
 	if (!rx_xsk)
 		error(-1, ENOMEM, "malloc");