selftests/bpf: test MSS value returned with bpf_tcp_gen_syncookie

One remaining difference between test_tcp_check_syncookie.sh and
btf_skc_cls_ingress is a small test on the mss value embedded in the
cookie generated with the eBPF helper.

Bring the corresponding test in btf_skc_cls_ingress.

Signed-off-by: Alexis Lothoré (eBPF Foundation) <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Martin KaFai Lau <[e-mail]>

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

···
 #define SERVER_ADDR_IPV4	"127.0.0.1"
 #define SERVER_ADDR_IPV6	"::1"
 #define SERVER_ADDR_DUAL	"::0"
+/* RFC791, 576 for minimal IPv4 datagram, minus 40 bytes of TCP header */
+#define MIN_IPV4_MSS		536
 
 static struct netns_obj *prepare_netns(struct test_btf_skc_cls_ingress *skel)
 {
···
 	skel->bss->recv_cookie = 0;
 	skel->bss->gen_cookie = 0;
 	skel->bss->linum = 0;
+	skel->bss->mss = 0;
 }
 
 static void print_err_line(struct test_btf_skc_cls_ingress *skel)
···
 			   "syncookie properly generated");
 		ASSERT_EQ(skel->bss->gen_cookie, skel->bss->recv_cookie,
 			  "matching syncookies on client and server");
+		ASSERT_GT(skel->bss->mss, MIN_IPV4_MSS,
+			  "MSS in cookie min value");
+		ASSERT_LT(skel->bss->mss, USHRT_MAX,
+			  "MSS in cookie max value");
 	}
 
 done:

tools/testing/selftests/bpf/progs/test_btf_skc_cls_ingress.c

···
 __u16 req_sk_sport = 0;
 __u32 recv_cookie = 0;
 __u32 gen_cookie = 0;
+__u32 mss = 0;
 __u32 linum = 0;
 
 #define LOG() ({ if (!linum) linum = __LINE__; })
···
 				LOG();
 		} else {
 			gen_cookie = (__u32)mss_cookie;
+			mss = mss_cookie >> 32;
 		}
 	} else if (gen_cookie) {
 		/* It was in cookie mode */