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

[Magnus Karlsson <magnus.karlsson@gmail.com>]

On Thu, Oct 31, 2019 at 9:03 AM Björn Töpel <bjorn.topel@gmail.com> wrote:
>
> On Thu, 31 Oct 2019 at 08:17, Magnus Karlsson <magnus.karlsson@gmail.com> wrote:
> >
> > On Wed, Oct 30, 2019 at 2:36 PM Toke Høiland-Jørgensen <toke@redhat.com> wrote:
> > >
> > > Magnus Karlsson <magnus.karlsson@intel.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?

Good question. I let someone with more insight answer this. Providing
the support Toke wants does make the header files less pleasant to
look at for sure. But in any case, I think we should provide an error
when you try to enable a new kernel feature using an old libbpf that
has no support for it. Just in case someone mixes things up.