On Wed, Oct 26, 2016 at 08:56:39AM +0200, Mickaël Salaün wrote:

This new arraymap looks like a set and brings new properties:
* strong typing of entries: the eBPF functions get the array type of
  elements instead of CONST_PTR_TO_MAP (e.g.
  CONST_PTR_TO_LANDLOCK_HANDLE_FS);
* force sequential filling (i.e. replace or append-only update), which
  allow quick browsing of all entries.

This strong typing is useful to statically check if the content of a map
can be passed to an eBPF function. For example, Landlock use it to store
and manage kernel objects (e.g. struct file) instead of dealing with
userland raw data. This improve efficiency and ensure that an eBPF
program can only call functions with the right high-level arguments.

The enum bpf_map_handle_type list low-level types (e.g.
BPF_MAP_HANDLE_TYPE_LANDLOCK_FS_FD) which are identified when
updating a map entry (handle). This handle types are used to infer a
high-level arraymap type which are listed in enum bpf_map_array_type
(e.g. BPF_MAP_ARRAY_TYPE_LANDLOCK_FS).

For now, this new arraymap is only used by Landlock LSM (cf. next
commits) but it could be useful for other needs.

Changes since v3:
* make handle arraymap safe (RCU) and remove buggy synchronize_rcu()
* factor out the arraymay walk

Changes since v2:
* add a RLIMIT_NOFILE-based limit to the maximum number of arraymap
  handle entries (suggested by Andy Lutomirski)
* remove useless checks

Changes since v1:
* arraymap of handles replace custom checker groups
* simpler userland API

[...]

+	case BPF_MAP_HANDLE_TYPE_LANDLOCK_FS_FD:
+		handle_file = fget(handle->fd);
+		if (IS_ERR(handle_file))
+			return ERR_CAST(handle_file);
+		/* check if the FD is tied to a user mount point */
+		if (unlikely(handle_file->f_path.mnt->mnt_flags & MNT_INTERNAL)) {
+			fput(handle_file);
+			return ERR_PTR(-EINVAL);
+		}
+		path_get(&handle_file->f_path);
+		ret = kmalloc(sizeof(*ret), GFP_KERNEL);
+		ret->path = handle_file->f_path;
+		fput(handle_file);

You can use fdget() and fdput() here because the reference to
handle_file is dropped before the end of the syscall.

+		break;
+	case BPF_MAP_HANDLE_TYPE_UNSPEC:
+	default:
+		return ERR_PTR(-EINVAL);
+	}
+	ret->type = handle_type;
+	return ret;
+}
+
+static void *nop_map_lookup_elem(struct bpf_map *map, void *key)
+{
+	return ERR_PTR(-EINVAL);
+}
+
+/* called from syscall or from eBPF program */
+static int landlock_array_map_update_elem(struct bpf_map *map, void *key,
+		void *value, u64 map_flags)
+{

This being callable from eBPF context is IMO pretty surprising and should
at least be well-documented. Also: What is the usecase here?