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

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Date:   Fri, 1 Nov 2019 13:41:23 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Toke Høiland-Jørgensen <toke@...hat.com>
Cc:     Jiri Olsa <jolsa@...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 Fri, Nov 1, 2019 at 12:36 PM Toke Høiland-Jørgensen <toke@...hat.com> wrote:
>
> Alexei Starovoitov <alexei.starovoitov@...il.com> writes:
>
> > On Fri, Nov 1, 2019 at 12:27 AM Jiri Olsa <jolsa@...hat.com> wrote:
> >>
> >> On Thu, Oct 31, 2019 at 01:39:12PM -0700, Alexei Starovoitov wrote:
> >> > 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.
> >>
> >> because we will get issues like above if there's no kernel
> >> avilable that we could compile libbpf against
> >
> > what is the issue again?
> > bpf-next builds fine. github/libbpf builds fine.
> > If distro is doing something else it's distro's mistake.
>
> With that you're saying that distros should always keep their kernel
> headers and libbpf version in sync. Which is fine in itself; they can
> certainly do that.

No. I'm not suggesting that.
distro is free to package whatever /usr/include headers.
kernel version is often != /usr/include headers

> The only concern with this is that without a flow of bugfixes into the
> 'bpf' tree (and stable), users may end up with buggy versions of libbpf.
> Which is in no one's interest. So how do we avoid that?

As I explained earlier. There is no 'bpf' tree for libbpf. It always
moves forward.