bpf: Add bpf_map_of_map_fd_sys_lookup_elem() helper

bpf_map_of_map_fd_sys_lookup_elem() returns the map id as the value for
map-in-map. It will be used when userspace applications do lookup
operations for map-in-map.

Signed-off-by: Hou Tao <[email protected]>

Author: Hou Tao

kernel/bpf/map_in_map.c

@@ -190,3 +190,9 @@ void bpf_map_of_map_fd_put_ptr(void *ptr, bool need_defer)
 	else
 		bpf_inner_map_element_free_rcu(&element->rcu);
 }
+
+u32 bpf_map_of_map_fd_sys_lookup_elem(void *ptr)
+{
+	rcu_read_lock_held();
+	return ((struct bpf_inner_map_element *)ptr)->map->id;
+}

kernel/bpf/map_in_map.h

@@ -22,5 +22,6 @@ u32 bpf_map_fd_sys_lookup_elem(void *ptr);
 
 void *bpf_map_of_map_fd_get_ptr(struct bpf_map *map, struct file *map_file, int ufd);
 void bpf_map_of_map_fd_put_ptr(void *ptr, bool need_defer);
+u32 bpf_map_of_map_fd_sys_lookup_elem(void *ptr);
 
 #endif