Support for maps of maps

[alban]

Maps of maps such as BPF_MAP_TYPE_ARRAY_OF_MAPS are created with bpf() using the flag inner_map_fd.

The kernel bpf library (samples/bpf/bpf_load.h) uses the following bpf_map_def:

struct bpf_map_def {
        unsigned int type;
        unsigned int key_size;
        unsigned int value_size;
        unsigned int max_entries;
        unsigned int map_flags;
        unsigned int inner_map_idx;
        unsigned int numa_node;
};      

inner_map_idx refers to the map index in the same ELF object.

Gobpf does not have any way in its own version of struct bpf_map_def to use inner_map_fd:

typedef struct bpf_map_def {
	unsigned int type;
	unsigned int key_size;
	unsigned int value_size;
	unsigned int max_entries;
	unsigned int map_flags;
	unsigned int pinning;
	char namespace[BUF_SIZE_MAP_NS];
} bpf_map_def;

The kernel uses inner_map_fd to know the type of the inner map in order to let the verifier check that it's used correctly.

iproute2 has support for inner maps but uses a different struct bpf_elf_map with an indirect index:

/* ELF map definition */
struct bpf_elf_map {
	__u32 type;
	__u32 size_key;
	__u32 size_value;
	__u32 max_elem;
	__u32 flags;
	__u32 id;
	__u32 pinning;
	__u32 inner_id;
	__u32 inner_idx;
};