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

From: Marco Vedovati <[email-masked]>
Date: Wed, 23 Feb 2022 20:18:09 +0000

From: Andrii Nakryiko <[email-masked]>
Sent: Thursday, February 17, 2022 11:08 PM
To: Marco Vedovati
Cc: [email-masked]; [email-masked]; [email-masked]; [email-masked]; Martin Kelly; [email-masked]; [email-masked]; [email-masked]; Andrii Nakryiko
Subject: [External] Re: Clarifications on linux/types.h used with libbpf
    
> On Tue, Feb 15, 2022 at 4:58 AM Marco Vedovati
> <[email-masked]> 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.

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