Subject: Re: [PATCH bpf-next v3] libbpf: fix compatibility for kernels without need_wakeup

From: Toke Høiland-Jørgensen <[email-masked]>
Date: Fri, 01 Nov 2019 20:36:04 +0100

Alexei Starovoitov <[email-masked]> writes:

> On Fri, Nov 1, 2019 at 12:27 AM Jiri Olsa <[email-masked]> 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 <[email-masked]> 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@aldebaran ~ $ 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.

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?

-Toke