bpf: Add hash of maps support

This patch adds hash of maps support (hashmap->bpf_map).
BPF_MAP_TYPE_HASH_OF_MAPS is added.

A map-in-map contains a pointer to another map and lets call
this pointer 'inner_map_ptr'.

Notes on deleting inner_map_ptr from a hash map:

1. For BPF_F_NO_PREALLOC map-in-map, when deleting
   an inner_map_ptr, the htab_elem itself will go through
   a rcu grace period and the inner_map_ptr resides
   in the htab_elem.

2. For pre-allocated htab_elem (!BPF_F_NO_PREALLOC),
   when deleting an inner_map_ptr, the htab_elem may
   get reused immediately.  This situation is similar
   to the existing prealloc-ated use cases.

   However, the bpf_map_fd_put_ptr() calls bpf_map_put() which calls
   inner_map->ops->map_free(inner_map) which will go
   through a rcu grace period (i.e. all bpf_map's map_free
   currently goes through a rcu grace period).  Hence,
   the inner_map_ptr is still safe for the rcu reader side.

This patch also includes BPF_MAP_TYPE_HASH_OF_MAPS to the
check_map_prealloc() in the verifier.  preallocation is a
must for BPF_PROG_TYPE_PERF_EVENT.  Hence, even we don't expect
heavy updates to map-in-map, enforcing BPF_F_NO_PREALLOC for map-in-map
is impossible without disallowing BPF_PROG_TYPE_PERF_EVENT from using
map-in-map first.

Signed-off-by: Martin KaFai Lau <kafai@fb.com>
Acked-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 7 insertions, 1 deletions

diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
index 6e24fdf1f373..c35ebfe6d84d 100644
--- a/kernel/bpf/syscall.c
+++ b/kernel/bpf/syscall.c
@@ -352,7 +352,8 @@ static int map_lookup_elem(union bpf_attr *attr)
                 err = bpf_percpu_array_copy(map, key, value);
         } else if (map->map_type == BPF_MAP_TYPE_STACK_TRACE) {
                 err = bpf_stackmap_copy(map, key, value);
-        } else if (map->map_type == BPF_MAP_TYPE_ARRAY_OF_MAPS) {
+        } else if (map->map_type == BPF_MAP_TYPE_ARRAY_OF_MAPS ||
+                   map->map_type == BPF_MAP_TYPE_HASH_OF_MAPS) {
                 err = -ENOTSUPP;
         } else {
                 rcu_read_lock();
@@ -446,6 +447,11 @@ static int map_update_elem(union bpf_attr *attr)
                 err = bpf_fd_array_map_update_elem(map, f.file, key, value,
                                                    attr->flags);
                 rcu_read_unlock();
+        } else if (map->map_type == BPF_MAP_TYPE_HASH_OF_MAPS) {
+                rcu_read_lock();
+                err = bpf_fd_htab_map_update_elem(map, f.file, key, value,
+                                                  attr->flags);
+                rcu_read_unlock();
         } else {
                 rcu_read_lock();
                 err = map->ops->map_update_elem(map, key, value, attr->flags);