netdev - Re: [PATCH bpf-next v3] libbpf: fix compatibility for kernels without need

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Date:   Thu, 31 Oct 2019 13:39:12 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Jiri Olsa <jolsa@...hat.com>
Cc:     Toke Høiland-Jørgensen <toke@...hat.com>,
        Björn Töpel <bjorn.topel@...il.com>,
        Magnus Karlsson <magnus.karlsson@...il.com>,
        Magnus Karlsson <magnus.karlsson@...el.com>,
        Björn Töpel <bjorn.topel@...el.com>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Network Development <netdev@...r.kernel.org>,
        Jonathan Lemon <jonathan.lemon@...il.com>,
        bpf <bpf@...r.kernel.org>, degeneloy@...il.com,
        John Fastabend <john.fastabend@...il.com>
Subject: Re: [PATCH bpf-next v3] libbpf: fix compatibility for kernels without need_wakeup

On Thu, Oct 31, 2019 at 12:18 PM Jiri Olsa <jolsa@...hat.com> wrote:
> >
> > yes. older vmlinux and newer installed libbpf.so
> > or any version of libbpf.a that is statically linked into apps
> > is something that libbpf code has to support.
> > The server can be rebooted into older than libbpf kernel and
> > into newer than libbpf kernel. libbpf has to recognize all these
> > combinations and work appropriately.
> > That's what backward and forward compatibility is.
> > That's what makes libbpf so difficult to test, develop and code review.
> > What that particular server has in /usr/include is irrelevant.
>
> sure, anyway we can't compile following:
>
>         tredaell@...ebaran ~ $ echo "#include <bpf/xsk.h>" | gcc -x c -
>         In file included from <stdin>:1:
>         /usr/include/bpf/xsk.h: In function ‘xsk_ring_prod__needs_wakeup’:
>         /usr/include/bpf/xsk.h:82:21: error: ‘XDP_RING_NEED_WAKEUP’ undeclared (first use in this function)
>            82 |  return *r->flags & XDP_RING_NEED_WAKEUP;
>         ...
>
>         XDP_RING_NEED_WAKEUP is defined in kernel v5.4-rc1 (77cd0d7b3f257fd0e3096b4fdcff1a7d38e99e10).
>         XSK_UNALIGNED_BUF_ADDR_MASK and XSK_UNALIGNED_BUF_OFFSET_SHIFT are defined in kernel v5.4-rc1 (c05cd3645814724bdeb32a2b4d953b12bdea5f8c).
>
> with:
>   kernel-headers-5.3.6-300.fc31.x86_64
>   libbpf-0.0.5-1.fc31.x86_64
>
> if you're saying this is not supported, I guess we could be postponing
> libbpf rpm releases until we have the related fedora kernel released

why? github/libbpf is the source of truth for building packages
and afaik it builds fine.

> or how about inluding uapi headers in libbpf-devel.. but that might
> actualy cause more confusion

Libraries (libbpf or any other) should not install headers that
typically go into /usr/include/
if_xdp.h case is not unique.
We'll surely add another #define, enum, etc to uapi/linux/bpf.h tomorrow.
And we will not copy paste these constants and types into tools/lib/bpf/.
In kernel tree libbpf development is using kernel tree headers.
No problem there for libbpf developers.
Packages are built out of github/libbpf that has a copy of uapi headers
necessary to create packages.
No problem there for package builders either.
But libbpf package is not going to install those uapi headers.
libbpf package installs only libbpf own headers (like libbpf.h)
The users that want to build against the latest libbpf package need
to install corresponding uapi headers package.
I don't think such dependency is specified in rpm scripts.
May be it is something to fix? Or may be not.
Some folks might not want to update all of /usr/include to bring libbpf-devel.
Then it would be their responsibility to get fresh /usr/include headers.