[PATCH bpf-next 3/3] selftests/bpf: add path_fd-based BPF_OBJ_PIN and BPF_OBJ_GET tests

[Andrii Nakryiko <andrii@kernel.org>]

Add a selftest demonstrating using detach-mounted BPF FS using new mount
APIs, and pinning and getting BPF map using such mount. This
demonstrates how something like container manager could setup BPF FS,
pin and adjust all the necessary objects in it, all before exposing BPF
FS to a particular mount namespace.

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
---
 .../bpf/prog_tests/bpf_obj_pinning.c          | 110 ++++++++++++++++++
 1 file changed, 110 insertions(+)
 create mode 100644 tools/testing/selftests/bpf/prog_tests/bpf_obj_pinning.c

diff --git a/tools/testing/selftests/bpf/prog_tests/bpf_obj_pinning.c b/tools/testing/selftests/bpf/prog_tests/bpf_obj_pinning.c
new file mode 100644
index 000000000000..4624f58e7b8a
--- /dev/null
+++ b/tools/testing/selftests/bpf/prog_tests/bpf_obj_pinning.c
@@ -0,0 +1,110 @@
+// SPDX-License-Identifier: GPL-2.0
+/* Copyright (c) 2023 Meta Platforms, Inc. and affiliates. */
+
+#include <test_progs.h>
+#include <bpf/btf.h>
+#include <fcntl.h>
+#include <unistd.h>
+#include <linux/unistd.h>
+#include <linux/mount.h>
+#include <sys/syscall.h>
+
+static inline int sys_fsopen(const char *fsname, unsigned flags)
+{
+	return syscall(__NR_fsopen, fsname, flags);
+}
+
+static inline int sys_fsconfig(int fs_fd, unsigned cmd, const char *key, const void *val, int aux)
+{
+	return syscall(__NR_fsconfig, fs_fd, cmd, key, val, aux);
+}
+
+static inline int sys_fsmount(int fs_fd, unsigned flags, unsigned ms_flags)
+{
+	return syscall(__NR_fsmount, fs_fd, flags, ms_flags);
+}
+
+static inline int sys_move_mount(int from_dfd, const char *from_path,
+			         int to_dfd, const char *to_path,
+			         unsigned int ms_flags)
+{
+	return syscall(__NR_move_mount, from_dfd, from_path, to_dfd, to_path, ms_flags);
+}
+
+void test_bpf_obj_pinning(void)
+{
+	LIBBPF_OPTS(bpf_obj_pin_opts, pin_opts);
+	LIBBPF_OPTS(bpf_obj_get_opts, get_opts);
+	int fs_fd = -1, mnt_fd = -1;
+	int map_fd = -1, map_fd2 = -1;
+	int zero = 0, src_value, dst_value, err;
+	const char *map_name = "fsmount_map";
+
+	/* A bunch of below UAPI calls are constructed based on reading:
+	 * https://brauner.io/2023/02/28/mounting-into-mount-namespaces.html
+	 */
+
+	/* create VFS context */
+	fs_fd = sys_fsopen("bpf", 0);
+	if (!ASSERT_GE(fs_fd, 0, "fs_fd"))
+		goto cleanup;
+
+	/* instantiate FS object */
+	err = sys_fsconfig(fs_fd, FSCONFIG_CMD_CREATE, NULL, NULL, 0);
+	if (!ASSERT_OK(err, "fs_create"))
+		goto cleanup;
+
+	/* create O_PATH fd for detached mount */
+	mnt_fd = sys_fsmount(fs_fd, 0, 0);
+	if (!ASSERT_GE(mnt_fd, 0, "mnt_fd"))
+		goto cleanup;
+
+	/* If we wanted to expose detached mount in the file system, we'd do
+	 * something like below. But the whole point is that we actually don't
+	 * even have to expose BPF FS in the file system to be able to work
+	 * (pin/get objects) with it.
+	 *
+	 * err = sys_move_mount(mnt_fd, "", -EBADF, mnt_path, MOVE_MOUNT_F_EMPTY_PATH);
+	 * if (!ASSERT_OK(err, "move_mount"))
+	 *	goto cleanup;
+	 */
+
+	/* create BPF map to pin */
+	map_fd = bpf_map_create(BPF_MAP_TYPE_ARRAY, map_name, 4, 4, 1, NULL);
+	if (!ASSERT_GE(map_fd, 0, "map_fd"))
+		goto cleanup;
+
+	/* pin BPF map into detached BPF FS through mnt_fd */
+	pin_opts.path_fd = mnt_fd;
+	err = bpf_obj_pin_opts(map_fd, map_name, &pin_opts);
+	if (!ASSERT_OK(err, "map_pin"))
+		goto cleanup;
+
+	/* get BPF map from detached BPF FS through mnt_fd */
+	get_opts.path_fd = mnt_fd;
+	map_fd2 = bpf_obj_get_opts(map_name, &get_opts);
+	if (!ASSERT_GE(map_fd2, 0, "map_get"))
+		goto cleanup;
+
+	/* update map through one FD */
+	src_value = 0xcafebeef;
+	err = bpf_map_update_elem(map_fd, &zero, &src_value, 0);
+	ASSERT_OK(err, "map_update");
+
+	/* check values written/read through different FDs do match */
+	dst_value = 0;
+	err = bpf_map_lookup_elem(map_fd2, &zero, &dst_value);
+	ASSERT_OK(err, "map_lookup");
+	ASSERT_EQ(dst_value, src_value, "map_value_eq1");
+	ASSERT_EQ(dst_value, 0xcafebeef, "map_value_eq2");
+
+cleanup:
+	if (map_fd >= 0)
+		ASSERT_OK(close(map_fd), "close_map_fd");
+	if (map_fd2 >= 0)
+		ASSERT_OK(close(map_fd2), "close_map_fd2");
+	if (fs_fd >= 0)
+		ASSERT_OK(close(fs_fd), "close_fs_fd");
+	if (mnt_fd >= 0)
+		ASSERT_OK(close(mnt_fd), "close_mnt_fd");
+}
-- 
2.34.1