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

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Date:   Tue, 15 Feb 2022 12:57:37 +0000
From:   Marco Vedovati <marco.vedovati@...wdstrike.com>
To:     "bpf@...r.kernel.org" <bpf@...r.kernel.org>,
        "toke@...hat.com" <toke@...hat.com>
CC:     "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: Clarifications on linux/types.h used with libbpf

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.

Thanks,
Marco