Date:   Thu, 31 Oct 2019 08:17:43 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Toke Høiland-Jørgensen <toke@...hat.com>
Cc:     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 7:52 AM Toke Høiland-Jørgensen <toke@...hat.com> wrote:
>
> Alexei Starovoitov <alexei.starovoitov@...il.com> writes:
>
> > On Thu, Oct 31, 2019 at 7:26 AM Toke Høiland-Jørgensen <toke@...hat.com> wrote:
> >>
> >> Alexei Starovoitov <alexei.starovoitov@...il.com> writes:
> >>
> >> > On Thu, Oct 31, 2019 at 7:13 AM Toke Høiland-Jørgensen <toke@...hat.com> wrote:
> >> >>
> >> >> Alexei Starovoitov <alexei.starovoitov@...il.com> writes:
> >> >>
> >> >> > On Thu, Oct 31, 2019 at 1:03 AM Björn Töpel <bjorn.topel@...il.com> wrote:
> >> >> >>
> >> >> >> 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?
> >> >> >
> >> >> > libbpf has to be backward and forward compatible.
> >> >> > Once compiled it has to run on older and newer kernels.
> >> >> > Conditional compilation is not an option obviously.
> >> >>
> >> >> So what do we do, then? Redefine the constants in libbpf/xsh.h if
> >> >> they're not in the kernel header file?
> >> >
> >> > why? How and whom it will help?
> >> > To libbpf.rpm creating person or to end user?
> >>
> >> Anyone who tries to compile a new libbpf against an older kernel. You're
> >> saying yourself that "libbpf has to be backward and forward compatible".
> >> Surely that extends to compile time as well as runtime?
> >
> > how old that older kernel?
> > Does it have up-to-date bpf.h in /usr/include ?
> > Also consider that running kernel is often not the same
> > thing as installed in /usr/include
> > vmlinux and /usr/include are different packages.
>
> In this case, it's a constant introduced in the kernel in the current
> (5.4) cycle; so currently, you can't compile libbpf with
> kernel-headers-5.3. And we're discussing how to handle this in a
> backwards compatible way in libbpf...

you simply don't.
It's not a problem to begin with.

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