Some properties of the inner map is used in the verification time.
When an inner map is inserted to an outer map at runtime,
bpf_map_meta_equal() is currently used to ensure those properties
of the inserting inner map stays the same as the verification
time.

In particular, the current bpf_map_meta_equal() checks max_entries which
turns out to be too restrictive for most of the maps which do not use
max_entries during the verification time.  It limits the use case that
wants to replace a smaller inner map with a larger inner map.  There are
some maps do use max_entries during verification though.  For example,
the map_gen_lookup in array_map_ops uses the max_entries to generate
the inline lookup code.

To accommodate differences between maps, the map_meta_equal is added
to bpf_map_ops.  Each map-type can decide what to check when its
map is used as an inner map during runtime.

Also, some map types cannot be used as an inner map and they are
currently black listed in bpf_map_meta_alloc() in map_in_map.c.
It is not unusual that the new map types may not aware that such
blacklist exists.  This patch enforces an explicit opt-in
and only allows a map to be used as an inner map if it has
implemented the map_meta_equal ops.  It is based on the
discussion in [1].

All maps that support inner map has its map_meta_equal points
to bpf_map_meta_equal in this patch.  A later patch will
relax the max_entries check for most maps.  bpf_types.h
counts 28 map types.  This patch adds 23 ".map_meta_equal"
by using coccinelle.  -5 for
	BPF_MAP_TYPE_PROG_ARRAY
	BPF_MAP_TYPE_(PERCPU)_CGROUP_STORAGE
	BPF_MAP_TYPE_STRUCT_OPS
	BPF_MAP_TYPE_ARRAY_OF_MAPS
	BPF_MAP_TYPE_HASH_OF_MAPS

The "if (inner_map->inner_map_meta)" check in bpf_map_meta_alloc()
is moved such that the same error is returned.

[1]: https://lore.kernel.org/bpf/20200522022342.899756-1-kafai@fb.com/Signed-off-by: Martin KaFai Lau <kafai@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20200828011806.1970400-1-kafai@fb.com