Allow static initialization for BPF_MAP_TYPE_HASH_OF_MAPS map with integer as key_size

[shpalani]

Description

1. In ebpf_native.c : Check map-in-map is Hash of Maps with integer as key_size. If true, allow initialization.
2. Added test cases in libbpf_tests.cpp.

Testing

_Do any existing tests cover this change? No
Are new tests needed? Yes

Manual test:

PS C:\Temp> netsh ebpf add program .\hash_of_map_in_array_map.sys
Loaded with ID 196609

PS C:\Temp> dir .\hash_of_map_in_array_map.sys


    Directory: C:\Temp


Mode                LastWriteTime         Length Name
----                -------------         ------ ----
-a----        10/5/2023   9:09 AM          11664 hash_of_map_in_array_map.sys


PS C:\Temp> netsh ebpf show programs

    ID  Pins  Links  Mode       Type           Name
======  ====  =====  =========  =============  ====================
196609     1      1  NATIVE     xdp            lookup

PS C:\Temp> netsh ebpf show maps

                              Key  Value      Max  Inner
     ID            Map Type  Size   Size  Entries     ID  Pins  Name
=======  ==================  ====  =====  =======  =====  ====  ========
 131073        hash_of_maps     2      4    65536  65537     0  outer_map

PS C:\Temp>

Documentation

_Is there any documentation impact for this change? No

Installation

_Is there any installer impact for this change? No

[shankarseal]

what if key size is uint8_t and max_entries exceeds 256 etc.? Does this code handle that? @Alan-Jowett can you confirm if Linux mandates key_size to be strictly 4-bytes in this case?

[shpalani]

Can you please clarify the concern on the max_entries (number of entries in the hash table) and key_size (size of the key) relation?

[shpalani]

@shankarseal : Could you please help clarify the discussed suggestion for the value_size?

For static initialization of hash-of-maps,

1. Add key_size == sizeof(int) or sizeof(long) (so, it is only 4 bytes allowed).
2. value_size == ???

[shpalani]

With the suggested change of key_size == sizeof(int), adding value into the inner map array is failing. Hence the suggestion does not work.

[shankarseal]

If the key size is x, there can be at most 2^8x entries in the map which must be checked.
Also, it is not enough to only check the length of the key; but the type should be an integer. Not sure if/how that can be verified here.

[saxena-anurag]

If bpf2c has that info (from parsing btf data), I think we need to change bpf2c to add that information in native driver, and then this code can check it.

[shankarseal]

Static initialization of hashmap of maps is not possible unless its a BTF map.

Make change in bpf2c to fail converstion if static initialized map in maps dont use integer as key type. Similar changes need to be made for the JIT-ed case as well.

[Alan-Jowett]

@shpalani What is the status of this PR? Has it been abandoned? If so, please close. If not, please address any comments and mark as ready for review.

[shpalani]

Closing this PR, as it is committed through
#3211