selftests/bpf: Use ping6 only if available in tc_redirect

In the tc_redirect test only use ping6 if it's available and
otherwise fall back to using "ping -6".
Signed-off-by: Jussi Maki <joamaki@gmail.com>

tools/testing/selftests/bpf/prog_tests/tc_redirect.c

@@ -13,15 +13,16 @@
 #define _GNU_SOURCE
 
 #include <arpa/inet.h>
+#include <linux/if.h>
+#include <linux/if_tun.h>
 #include <linux/limits.h>
 #include <linux/sysctl.h>
-#include <linux/if_tun.h>
-#include <linux/if.h>
 #include <sched.h>
 #include <stdbool.h>
 #include <stdio.h>
-#include <sys/stat.h>
 #include <sys/mount.h>
+#include <sys/stat.h>
+#include <unistd.h>
 
 #include "test_progs.h"
 #include "network_helpers.h"
@@ -389,11 +390,21 @@ static void test_tcp(int family, const char *addr, __u16 port)
 		close(client_fd);
 }
 
-static int test_ping(int family, const char *addr)
+static char *ping_command(int family)
 {
-	const char *ping = family == AF_INET6 ? "ping6" : "ping";
+	if (family == AF_INET6) {
+		/* On some systems 'ping' doesn't support IPv6, so use ping6 if it is present. */
+		if (!system("which ping6 >/dev/null 2>&1"))
+			return "ping6";
+		else
+			return "ping -6";
+	}
+	return "ping";
+}
 
-	SYS("ip netns exec " NS_SRC " %s " PING_ARGS " %s > /dev/null", ping, addr);
+static int test_ping(int family, const char *addr)
+{
+	SYS("ip netns exec " NS_SRC " %s " PING_ARGS " %s > /dev/null", ping_command(family), addr);
 	return 0;
 fail:
 	return -1;