[PATCH bpf-next 5/6] selftests/bpf: test MSS value returned with bpf_tcp_gen_syncookie

["Alexis Lothoré (eBPF Foundation)" <alexis.lothore@bootlin.com>]

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) <alexis.lothore@bootlin.com>
---
 tools/testing/selftests/bpf/prog_tests/btf_skc_cls_ingress.c | 7 +++++++
 tools/testing/selftests/bpf/progs/test_btf_skc_cls_ingress.c | 2 ++
 2 files changed, 9 insertions(+)

diff --git a/tools/testing/selftests/bpf/prog_tests/btf_skc_cls_ingress.c b/tools/testing/selftests/bpf/prog_tests/btf_skc_cls_ingress.c
index e0f8fe818f4230a1d5bf0118133c5a9fb50345e1..64601bafdd23e7fb13ebd496ac914660685f1efc 100644
--- a/tools/testing/selftests/bpf/prog_tests/btf_skc_cls_ingress.c
+++ b/tools/testing/selftests/bpf/prog_tests/btf_skc_cls_ingress.c
@@ -27,6 +27,8 @@
 #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)
 {
@@ -73,6 +75,7 @@ static void reset_test(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)
@@ -184,6 +187,10 @@ static void run_test(struct test_btf_skc_cls_ingress *skel, bool gen_cookies,
 			   "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:
diff --git a/tools/testing/selftests/bpf/progs/test_btf_skc_cls_ingress.c b/tools/testing/selftests/bpf/progs/test_btf_skc_cls_ingress.c
index cd528f8792ff2eb14683cbc13e8b0f3fd38329e9..3810ce98d3d54276f9a0130a887f59c79a21288e 100644
--- a/tools/testing/selftests/bpf/progs/test_btf_skc_cls_ingress.c
+++ b/tools/testing/selftests/bpf/progs/test_btf_skc_cls_ingress.c
@@ -15,6 +15,7 @@ __u16 listen_tp_sport = 0;
 __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__; })
@@ -46,6 +47,7 @@ static void test_syncookie_helper(void *iphdr, int iphdr_size,
 				LOG();
 		} else {
 			gen_cookie = (__u32)mss_cookie;
+			mss = mss_cookie >> 32;
 		}
 	} else if (gen_cookie) {
 		/* It was in cookie mode */

-- 
2.46.2