should eBPF for Windows allow multi-level nesting in map-in-map?

[shpalani]

Describe the bug

Request: ebpf-for-windows should not allow multi-level nesting for map_of_maps. The inner map should not be another map-in-map.

Example of multi-level nesting:
Outer map: BPF_MAP_TYPE_ARRAY_OF_MAPS/BPF_MAP_TYPE_HASH_OF_MAPS
Inner map: BPF_MAP_TYPE_ARRAY_OF_MAPS/BPF_MAP_TYPE_HASH_OF_MAPS

Compatibility with Linux eBPF:
As per Linux doc https://docs.kernel.org/bpf/map_of_maps.html

• Multi-level nesting is not supported.

OS information

Windows 10 and above

Steps taken to reproduce bug

Map-in-map sample:

struct {
		__uint(type, BPF_MAP_TYPE_HASH_OF_MAPS);
		__uint(key_size, sizeof(__u32));
		__uint(value_size, sizeof(__u32));
		__uint(max_entries, 1);
} LB6_MAP_INNER  SEC(".maps");

struct {
	__uint(type, BPF_MAP_TYPE_HASH_OF_MAPS);
	__type(key, __u16);
	__type(value, __u32);
	__uint(pinning, LIBBPF_PIN_BY_NAME);
	__uint(max_entries, 65535);
	__uint(map_flags, CONDITIONAL_PREALLOC);
	/* inner map definition */
	__array(values, LB6_MAP_INNER);
} LB6_MAP_OUTER SEC(".maps");

Expected behavior

bpf2c should flag an error, to unsupport more than 1 level of nesting.

Actual outcome

Currently, multi-level nesting is allowed.

Additional details

No response

[shankarseal]

It must support 1 level of nesting though.

[shankarseal]

At least investigation should be done on if verifier can handle 1+ level of nesting.

[shpalani]

1. JFYI, Linux eBPF code which disallows:
   https://github.com/torvalds/linux/blob/58720809f52779dc0f08e53e54b014209d13eebb/kernel/bpf/map_in_map.c#L22