Re: [PATCH bpf-next] libbpf: add BTF-defined map-in-map support

["Toke Høiland-Jørgensen" <toke@redhat.com>]

Andrii Nakryiko <andriin@fb.com> writes:

> As discussed at LPC 2019 ([0]), this patch brings (a quite belated) support
> for declarative BTF-defined map-in-map support in libbpf. It allows to define
> ARRAY_OF_MAPS and HASH_OF_MAPS BPF maps without any user-space initialization
> code involved.
>
> Additionally, it allows to initialize outer map's slots with references to
> respective inner maps at load time, also completely declaratively.
>
> Despite a weak type system of C, the way BTF-defined map-in-map definition
> works, it's actually quite hard to accidentally initialize outer map with
> incompatible inner maps. This being C, of course, it's still possible, but
> even that would be caught at load time and error returned with helpful debug
> log pointing exactly to the slot that failed to be initialized.
>
> Here's the relevant part of libbpf debug log showing pretty clearly of what's
> going on with map-in-map initialization:
>
> libbpf: .maps relo #0: for 6 value 0 rel.r_offset 96 name 260 ('inner_map1')
> libbpf: .maps relo #0: map 'outer_arr' slot [0] points to map 'inner_map1'
> libbpf: .maps relo #1: for 7 value 32 rel.r_offset 112 name 249 ('inner_map2')
> libbpf: .maps relo #1: map 'outer_arr' slot [2] points to map 'inner_map2'
> libbpf: .maps relo #2: for 7 value 32 rel.r_offset 144 name 249 ('inner_map2')
> libbpf: .maps relo #2: map 'outer_hash' slot [0] points to map 'inner_map2'
> libbpf: .maps relo #3: for 6 value 0 rel.r_offset 176 name 260 ('inner_map1')
> libbpf: .maps relo #3: map 'outer_hash' slot [4] points to map 'inner_map1'
> libbpf: map 'inner_map1': created successfully, fd=4
> libbpf: map 'inner_map2': created successfully, fd=5
> libbpf: map 'outer_arr': created successfully, fd=7
> libbpf: map 'outer_arr': slot [0] set to map 'inner_map1' fd=4
> libbpf: map 'outer_arr': slot [2] set to map 'inner_map2' fd=5
> libbpf: map 'outer_hash': created successfully, fd=8
> libbpf: map 'outer_hash': slot [0] set to map 'inner_map2' fd=5
> libbpf: map 'outer_hash': slot [4] set to map 'inner_map1' fd=4
>
> See also included selftest with some extra comments explaining extra details
> of usage.

Could you please put an example of usage in the commit message as well?
Easier to find that way, especially if the selftests are not handy (such
as in the libbpf github repo).

-Toke