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

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Date:   Tue, 18 Jan 2022 17:17:56 +0100
From:   Toke Høiland-Jørgensen <toke@...hat.com>
To:     Arnaldo Carvalho de Melo <acme@...nel.org>
Cc:     Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...nel.org>, netdev@...r.kernel.org,
        bpf@...r.kernel.org
Subject: Re: [PATCH bpf] libbpf: define BTF_KIND_* constants in btf.h to
 avoid compilation errors

Arnaldo Carvalho de Melo <acme@...nel.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@...hat.com>

Thanks!

-Toke