selftests/bpf: Test updating flow_dissector link with same program

This case, while not particularly useful, is worth covering because we
expect the operation to succeed as opposed when re-attaching the same
program directly with PROG_ATTACH.

While at it, update the tests summary that fell out of sync when tests
extended to cover links.
Signed-off-by: Jakub Sitnicki <jakub@cloudflare.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Martin KaFai Lau <kafai@fb.com>
Link: https://lore.kernel.org/bpf/20200625141357.910330-5-jakub@cloudflare.com

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

 // SPDX-License-Identifier: GPL-2.0
 /*
- * Test that the flow_dissector program can be updated with a single
- * syscall by attaching a new program that replaces the existing one.
- *
- * Corner case - the same program cannot be attached twice.
+ * Tests for attaching, detaching, and replacing flow_dissector BPF program.
  */
 
 #define _GNU_SOURCE
@@ -308,6 +305,31 @@ static void test_link_update_replace_old_prog(int netns, int prog1, int prog2)
 	CHECK_FAIL(prog_is_attached(netns));
 }
 
+static void test_link_update_same_prog(int netns, int prog1, int prog2)
+{
+	DECLARE_LIBBPF_OPTS(bpf_link_create_opts, create_opts);
+	DECLARE_LIBBPF_OPTS(bpf_link_update_opts, update_opts);
+	int err, link;
+
+	link = bpf_link_create(prog1, netns, BPF_FLOW_DISSECTOR, &create_opts);
+	if (CHECK_FAIL(link < 0)) {
+		perror("bpf_link_create(prog1)");
+		return;
+	}
+	CHECK_FAIL(query_attached_prog_id(netns) != query_prog_id(prog1));
+
+	/* Expect success updating the prog with the same one */
+	update_opts.flags = 0;
+	update_opts.old_prog_fd = 0;
+	err = bpf_link_update(link, prog1, &update_opts);
+	if (CHECK_FAIL(err))
+		perror("bpf_link_update");
+	CHECK_FAIL(query_attached_prog_id(netns) != query_prog_id(prog1));
+
+	close(link);
+	CHECK_FAIL(prog_is_attached(netns));
+}
+
 static void test_link_update_invalid_opts(int netns, int prog1, int prog2)
 {
 	DECLARE_LIBBPF_OPTS(bpf_link_create_opts, create_opts);
@@ -571,6 +593,8 @@ static void run_tests(int netns)
 		  test_link_update_no_old_prog },
 		{ "link update with replace old prog",
 		  test_link_update_replace_old_prog },
+		{ "link update with same prog",
+		  test_link_update_same_prog },
 		{ "link update invalid opts",
 		  test_link_update_invalid_opts },
 		{ "link update invalid prog",