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

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Date:   Wed, 23 Feb 2022 13:58:35 -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 Wed, Feb 23, 2022 at 12:18 PM Marco Vedovati
<marco.vedovati@...wdstrike.com> wrote:
>
> From: Andrii Nakryiko <andrii.nakryiko@...il.com>
> Sent: Thursday, February 17, 2022 11:08 PM
> To: Marco Vedovati
> Cc: bpf@...r.kernel.org; toke@...hat.com; netdev@...r.kernel.org; kernel-team@...com; Martin Kelly; ast@...nel.org; daniel@...earbox.net; davem@...emloft.net; Andrii Nakryiko
> Subject: [External] 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
> > [cut]
> >
> >
> > 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.
>
> Thank you for clarifying some of my confusions.
>
> So if I understood correctly, the only use of libbpf:include/linux/types.h
> is to allow building the library out of the kernel tree.
>
> An ambiguity I have found is about what version of linux/types.h to use
> use when building bpf source code (that includes <linux/bpf.h>).
> I saw 2 options:
>
> - do like libbpf-bootstrap C examples, that uses whatever linux/types.h
>   version available on the building host. This is however adding more
>   dependencies that are satisfied with extra "-idirafter" compiler options.
>
> - do like bpftool's makefile, that builds bpf source code by including
>   tools/include/uapi/. This does not require the "-idirafter" trick.

Applications shouldn't be building against Linux-internal
include/linux/types.h. It should always be resolved to
include/uapi/linux/types.h.

>
> Anyway, checking the history of "tools/include/uapi/linux/types.h", I
> believe that this file is mistakenly licensed as "GPL-2.0" instead of
> "GPL-2.0 WITH Linux-syscall-note". I may come up with a patch to fix it.
>
> >
> > Thanks,
> > Marco
>