selftests/bpf: Test bpf_sk_storage_get in tcp iterators

This extends the existing bpf_sk_storage_get test where a socket is
created and tagged with its creator's pid by a task_file iterator.

A TCP iterator is now also used at the end of the test to negate the
values already stored in the local storage. The test therefore expects
-getpid() to be stored in the local storage.

Signed-off-by: Florent Revest <[e-mail]>
Signed-off-by: Daniel Borkmann <[e-mail]>
Acked-by: Yonghong Song <[e-mail]>
Acked-by: Martin KaFai Lau <[e-mail]>
Link: https://lore.kernel.org/bpf/[e-mail]

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

···
 /* This creates a socket and its local storage. It then runs a task_iter BPF
  * program that replaces the existing socket local storage with the tgid of the
  * only task owning a file descriptor to this socket, this process, prog_tests.
+ * It then runs a tcp socket iterator that negates the value in the existing
+ * socket local storage, the test verifies that the resulting value is -pid.
  */
 static void test_bpf_sk_storage_get(void)
 {
···
 	if (CHECK(sock_fd < 0, "socket", "errno: %d\n", errno))
 		goto out;
 
+	err = listen(sock_fd, 1);
+	if (CHECK(err != 0, "listen", "errno: %d\n", errno))
+		goto close_socket;
+
 	map_fd = bpf_map__fd(skel->maps.sk_stg_map);
 
 	err = bpf_map_update_elem(map_fd, &sock_fd, &val, BPF_NOEXIST);
···
 	do_dummy_read(skel->progs.fill_socket_owner);
 
 	err = bpf_map_lookup_elem(map_fd, &sock_fd, &val);
-	CHECK(err || val != getpid(), "bpf_map_lookup_elem",
+	if (CHECK(err || val != getpid(), "bpf_map_lookup_elem",
+	    "map value wasn't set correctly (expected %d, got %d, err=%d)\n",
+	    getpid(), val, err))
+		goto close_socket;
+
+	do_dummy_read(skel->progs.negate_socket_local_storage);
+
+	err = bpf_map_lookup_elem(map_fd, &sock_fd, &val);
+	CHECK(err || val != -getpid(), "bpf_map_lookup_elem",
 	      "map value wasn't set correctly (expected %d, got %d, err=%d)\n",
-	      getpid(), val, err);
+	      -getpid(), val, err);
 
 close_socket:
 	close(sock_fd);

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

···
 
 	return 0;
 }
+
+SEC("iter/tcp")
+int negate_socket_local_storage(struct bpf_iter__tcp *ctx)
+{
+	struct sock_common *sk_common = ctx->sk_common;
+	int *sock_tgid;
+
+	if (!sk_common)
+		return 0;
+
+	sock_tgid = bpf_sk_storage_get(&sk_stg_map, sk_common, 0, 0);
+	if (!sock_tgid)
+		return 0;
+
+	*sock_tgid = -*sock_tgid;
+
+	return 0;
+}