Re: [PATCH bpf] libbpf: define BTF_KIND_* constants in btf.h to avoid compilation errors

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

Arnaldo Carvalho de Melo <acme@kernel.org> writes:

> Em Tue, Jan 18, 2022 at 03:13:27PM +0100, Toke Høiland-Jørgensen escreveu:
>> The btf.h header included with libbpf contains inline helper functions to
>> check for various BTF kinds. These helpers directly reference the
>> BTF_KIND_* constants defined in the kernel header, and because the header
>> file is included in user applications, this happens in the user application
>> compile units.
>> 
>> This presents a problem if a user application is compiled on a system with
>> older kernel headers because the constants are not available. To avoid
>> this, add #defines of the constants directly in btf.h before using them.
>> 
>> Since the kernel header moved to an enum for BTF_KIND_*, the #defines can
>> shadow the enum values without any errors, so we only need #ifndef guards
>> for the constants that predates the conversion to enum. We group these so
>> there's only one guard for groups of values that were added together.
>> 
>>   [0] Closes: https://github.com/libbpf/libbpf/issues/436
>
> The coexistence of enums with the defines (in turn #ifndef guarded) as
> something I hadn't considered, clever.

Me neither - that bit was Andrii's idea :)

> Should fix lots of build errors in my test containers :-)
>
> FWIW:
>
> Acked-by: Arnaldo Carvalho de Melo <acme@redhat.com>

Thanks!

-Toke