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 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

Signed-off-by: Mickaël Salaün <mic@digikod.net>
Cc: Alexei Starovoitov <ast@kernel.org>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Daniel Borkmann <daniel@iogearbox.net>
Cc: David S. Miller <davem@davemloft.net>
Cc: Kees Cook <redacted>
Link: https://lkml.kernel.org/r/CALCETrWwTiz3kZTkEgOW24-DvhQq6LftwEXh77FD2G5o71yD7g@mail.gmail.com
---
 include/linux/bpf.h      |  14 ++++
 include/uapi/linux/bpf.h |  18 +++++
 kernel/bpf/arraymap.c    | 203 +++++++++++++++++++++++++++++++++++++++++++++++
 kernel/bpf/verifier.c    |  12 ++-
 4 files changed, 246 insertions(+), 1 deletion(-)

diff --git a/include/linux/bpf.h b/include/linux/bpf.h
index fa9a988400d9..eae4ce4542c1 100644
--- a/include/linux/bpf.h
+++ b/include/linux/bpf.h

@@ -13,6 +13,10 @@
 #include <linux/percpu.h>
 #include <linux/err.h>
 
+#ifdef CONFIG_SECURITY_LANDLOCK
+#include <linux/fs.h> /* struct file */
+#endif /* CONFIG_SECURITY_LANDLOCK */
+
 struct perf_event;
 struct bpf_map;
 

@@ -38,6 +42,7 @@ struct bpf_map_ops {
 struct bpf_map {
 	atomic_t refcnt;
 	enum bpf_map_type map_type;
+	enum bpf_map_array_type map_array_type;
 	u32 key_size;
 	u32 value_size;
 	u32 max_entries;

@@ -187,6 +192,9 @@ struct bpf_array {
 	 */
 	enum bpf_prog_type owner_prog_type;
 	bool owner_jited;
+#ifdef CONFIG_SECURITY_LANDLOCK
+	u32 n_entries;	/* number of entries in a handle array */
+#endif /* CONFIG_SECURITY_LANDLOCK */
 	union {
 		char value[0] __aligned(8);
 		void *ptrs[0] __aligned(8);

@@ -194,6 +202,12 @@ struct bpf_array {
 	};
 };
 
+#ifdef CONFIG_SECURITY_LANDLOCK
+struct map_landlock_handle {
+	u32 type; /* enum bpf_map_handle_type */
+};
+#endif /* CONFIG_SECURITY_LANDLOCK */
+
 #define MAX_TAIL_CALL_CNT 32
 
 struct bpf_event_entry {

diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
index 7cd36166f9b7..b68de57f7ab8 100644
--- a/include/uapi/linux/bpf.h
+++ b/include/uapi/linux/bpf.h

@@ -87,6 +87,15 @@ enum bpf_map_type {
 	BPF_MAP_TYPE_PERCPU_ARRAY,
 	BPF_MAP_TYPE_STACK_TRACE,P_TYPE_CGROUP_ARRAY
 	BPF_MAP_TYPE_CGROUP_ARRAY,
+	BPF_MAP_TYPE_LANDLOCK_ARRAY,
+};
+
+enum bpf_map_array_type {
+	BPF_MAP_ARRAY_TYPE_UNSPEC,
+};
+
+enum bpf_map_handle_type {
+	BPF_MAP_HANDLE_TYPE_UNSPEC,
 };