netdev - Re: Clarifications on linux/types.h used with libbpf

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Date:   Thu, 17 Feb 2022 14:08:35 -0800
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Marco Vedovati <marco.vedovati@...wdstrike.com>
Cc:     "bpf@...r.kernel.org" <bpf@...r.kernel.org>,
        "toke@...hat.com" <toke@...hat.com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "kernel-team@...com" <kernel-team@...com>,
        Martin Kelly <martin.kelly@...wdstrike.com>,
        "ast@...nel.org" <ast@...nel.org>,
        "daniel@...earbox.net" <daniel@...earbox.net>,
        "davem@...emloft.net" <davem@...emloft.net>,
        Andrii Nakryiko <andrii@...nel.org>
Subject: Re: Clarifications on linux/types.h used with libbpf

On Tue, Feb 15, 2022 at 4:58 AM Marco Vedovati
<marco.vedovati@...wdstrike.com> wrote:
>
> Hi,
>
> I have few questions about the linux/types.h file used to build bpf
> applications. This file gets included by both userspace applications using
> libbpf and by bpf programs. E.g., in a userspace application:
> foo.c
>   foo.skel.h
>     bpf/libbpf.h
>       linux/bpf.h
>         linux/types.h
>
> Or in a bpf program:
> foo.bpf.c
>   linux/bpf.h
>     linux/types.h
>
> libbpf provides its own copy of this file in include/linux/types.h.
> As I could understand from the Git history, it was initially copied from
> linux include/linux/types.h, but it is now maintained separately.
>
> Both linux bpftool and bpf selftests however are built using another
> types.h from tools/include/uapi/linux/types.h.
> Is there a reason why bpftool and selftests aren't built using the same
> types.h distributed by libbpf?
>
> I also see that the license of the three files differs:
> - (libbpf) include/linux/types.h is "LGPL-2.1 OR BSD-2-Clause"
> - (linux) include/linux/types.h is "GPL-2.0"
> - (linux) tools/include/uapi/linux/types.h is "GPL-2.0"
> Is there a reason why tools/include/uapi/linux/types.h isn't licensed as
> "GPL-2.0 WITH Linux-syscall-note"?
>
> Finally, would it make sense to also have libbpf use
> tools/include/uapi/linux/types.h instead of its own copy?
> The advantages would be:
> - consistency with linux use
> - the only architecture specific header included is "asm/bitsperlong.h",
>   instead of all the ones currently included.


include/uapi/linux/types.h (UAPI header) is different from
include/linux/types.h (kernel-internal header). Libbpf has to
reimplement minimum amount of declarations from kernel-internal
include/linux/types.h to build outside of the kernel. But short answer
is they are different headers, so I suspect that no, libbpf can't use
just UAPI version.

>
> Thanks,
> Marco