selftests/bpf: Add an iterator selftest for bpf_sk_storage_get

The eBPF program iterates over all files and tasks. For all socket
files, it stores the tgid of the last task it encountered with a handle
to that socket. This is a heuristic for finding the "owner" of a socket
similar to what's done by lsof, ss, netstat or fuser. Potentially, this
information could be used from a cgroup_skb/*gress hook to try to
associate network traffic with processes.

The test makes sure that a socket it created is tagged with prog_tests's
pid.
Signed-off-by: Florent Revest <revest@google.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Yonghong Song <yhs@fb.com>
Acked-by: Martin KaFai Lau <kafai@fb.com>
Link: https://lore.kernel.org/bpf/20201204113609.1850150-5-revest@google.com

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

@@ -975,6 +975,44 @@ static void test_bpf_sk_storage_delete(void)
 	bpf_iter_bpf_sk_storage_helpers__destroy(skel);
 }
 
+/* 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.
+ */
+static void test_bpf_sk_storage_get(void)
+{
+	struct bpf_iter_bpf_sk_storage_helpers *skel;
+	int err, map_fd, val = -1;
+	int sock_fd = -1;
+
+	skel = bpf_iter_bpf_sk_storage_helpers__open_and_load();
+	if (CHECK(!skel, "bpf_iter_bpf_sk_storage_helpers__open_and_load",
+		  "skeleton open_and_load failed\n"))
+		return;
+
+	sock_fd = socket(AF_INET6, SOCK_STREAM, 0);
+	if (CHECK(sock_fd < 0, "socket", "errno: %d\n", errno))
+		goto out;
+
+	map_fd = bpf_map__fd(skel->maps.sk_stg_map);
+
+	err = bpf_map_update_elem(map_fd, &sock_fd, &val, BPF_NOEXIST);
+	if (CHECK(err, "bpf_map_update_elem", "map_update_failed\n"))
+		goto close_socket;
+
+	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",
+	      "map value wasn't set correctly (expected %d, got %d, err=%d)\n",
+	      getpid(), val, err);
+
+close_socket:
+	close(sock_fd);
+out:
+	bpf_iter_bpf_sk_storage_helpers__destroy(skel);
+}
+
 static void test_bpf_sk_storage_map(void)
 {
 	DECLARE_LIBBPF_OPTS(bpf_iter_attach_opts, opts);
@@ -1131,6 +1169,8 @@ void test_bpf_iter(void)
 		test_bpf_sk_storage_map();
 	if (test__start_subtest("bpf_sk_storage_delete"))
 		test_bpf_sk_storage_delete();
+	if (test__start_subtest("bpf_sk_storage_get"))
+		test_bpf_sk_storage_get();
 	if (test__start_subtest("rdonly-buf-out-of-bound"))
 		test_rdonly_buf_out_of_bound();
 	if (test__start_subtest("buf-neg-offset"))

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

@@ -21,3 +21,27 @@ int delete_bpf_sk_storage_map(struct bpf_iter__bpf_sk_storage_map *ctx)
 
 	return 0;
 }
+
+SEC("iter/task_file")
+int fill_socket_owner(struct bpf_iter__task_file *ctx)
+{
+	struct task_struct *task = ctx->task;
+	struct file *file = ctx->file;
+	struct socket *sock;
+	int *sock_tgid;
+
+	if (!task || !file)
+		return 0;
+
+	sock = bpf_sock_from_file(file);
+	if (!sock)
+		return 0;
+
+	sock_tgid = bpf_sk_storage_get(&sk_stg_map, sock->sk, 0, 0);
+	if (!sock_tgid)
+		return 0;
+
+	*sock_tgid = task->tgid;
+
+	return 0;
+}