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 <redacted>
Cc: Alexei Starovoitov <redacted>
Cc: Andy Lutomirski <redacted>
Cc: Daniel Borkmann <redacted>
Cc: David S. Miller <davem-fT/PcQaiUtIeIZ0/mPfg9Q@public.gmane.org>
Cc: Kees Cook <redacted>
Link: https://lkml.kernel.org/r/CALCETrWwTiz3kZTkEgOW24-DvhQq6LftwEXh77FD2G5o71yD7g-JsoAwUIsXosN+BqQ9rBEUg@public.gmane.org
---
 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/kernel/bpf/arraymap.c b/kernel/bpf/arraymap.c
index a2ac051c342f..94256597eacd 100644
--- a/kernel/bpf/arraymap.c
+++ b/kernel/bpf/arraymap.c

[...]
+       /*
+        * Limit number of entries in an arraymap of handles to the maximum
+        * number of open files for the current process. The maximum number of
+        * handle entries (including all arraymaps) for a process is then
+        * (RLIMIT_NOFILE - 1) * RLIMIT_NOFILE. If the process' RLIMIT_NOFILE
+        * is 0, then any entry update is forbidden.
+        *
+        * An eBPF program can inherit all the arraymap FD. The worse case is
+        * to fill a bunch of arraymaps, create an eBPF program, close the
+        * arraymap FDs, and start again. The maximum number of arraymap
+        * entries can then be close to RLIMIT_NOFILE^3.
+        *
+        * FIXME: This should be improved... any idea?
+        */
+       if (unlikely(index >= rlimit(RLIMIT_NOFILE)))
+               return -EMFILE;