On Thu, 31 Oct 2019 at 08:17, Magnus Karlsson [off-list ref] wrote:

On Wed, Oct 30, 2019 at 2:36 PM Toke Høiland-Jørgensen [off-list ref] wrote:

quoted

Magnus Karlsson [off-list ref] writes:

quoted

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?