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

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Date:   Thu, 31 Oct 2019 09:02:56 +0100
From:   Björn Töpel <bjorn.topel@...il.com>
To:     Magnus Karlsson <magnus.karlsson@...il.com>
Cc:     Toke Høiland-Jørgensen <toke@...hat.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, 31 Oct 2019 at 08:17, Magnus Karlsson <magnus.karlsson@...il.com> wrote:
>
> On Wed, Oct 30, 2019 at 2:36 PM Toke Høiland-Jørgensen <toke@...hat.com> wrote:
> >
> > Magnus Karlsson <magnus.karlsson@...el.com> writes:
> >
> > > When the need_wakeup flag was added to AF_XDP, the format of the
> > > XDP_MMAP_OFFSETS getsockopt was extended. Code was added to the
> > > kernel to take care of compatibility issues arrising from running
> > > applications using any of the two formats. However, libbpf was
> > > not extended to take care of the case when the application/libbpf
> > > uses the new format but the kernel only supports the old
> > > format. This patch adds support in libbpf for parsing the old
> > > format, before the need_wakeup flag was added, and emulating a
> > > set of static need_wakeup flags that will always work for the
> > > application.
> >
> > Hi Magnus
> >
> > While you're looking at backwards compatibility issues with xsk: libbpf
> > currently fails to compile on a system that has old kernel headers
> > installed (this is with kernel-headers 5.3):
> >
> > $ 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;
> >       |                     ^~~~~~~~~~~~~~~~~~~~
> > /usr/include/bpf/xsk.h:82:21: note: each undeclared identifier is reported only once for each function it appears in
> > /usr/include/bpf/xsk.h: In function ‘xsk_umem__extract_addr’:
> > /usr/include/bpf/xsk.h:173:16: error: ‘XSK_UNALIGNED_BUF_ADDR_MASK’ undeclared (first use in this function)
> >   173 |  return addr & XSK_UNALIGNED_BUF_ADDR_MASK;
> >       |                ^~~~~~~~~~~~~~~~~~~~~~~~~~~
> > /usr/include/bpf/xsk.h: In function ‘xsk_umem__extract_offset’:
> > /usr/include/bpf/xsk.h:178:17: error: ‘XSK_UNALIGNED_BUF_OFFSET_SHIFT’ undeclared (first use in this function)
> >   178 |  return addr >> XSK_UNALIGNED_BUF_OFFSET_SHIFT;
> >       |                 ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> >
> >
> >
> > How would you prefer to handle this? A patch like the one below will fix
> > the compile errors, but I'm not sure it makes sense semantically?
>
> Thanks Toke for finding this. Of course it should be possible to
> compile this on an older kernel, but without getting any of the newer
> functionality that is not present in that older kernel.

Is the plan to support source compatibility for the headers only, or
the whole the libbpf itself? Is the usecase here, that you've built
libbpf.so with system headers X, and then would like to use the
library on a system with older system headers X~10? XDP sockets? BTF?