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

["Toke Høiland-Jørgensen" <toke@redhat.com>]

Alexei Starovoitov <alexei.starovoitov@gmail.com> writes:

> On Thu, Oct 31, 2019 at 7:26 AM Toke Høiland-Jørgensen <toke@redhat.com> wrote:
>>
>> Alexei Starovoitov <alexei.starovoitov@gmail.com> writes:
>>
>> > On Thu, Oct 31, 2019 at 7:13 AM Toke Høiland-Jørgensen <toke@redhat.com> wrote:
>> >>
>> >> Alexei Starovoitov <alexei.starovoitov@gmail.com> writes:
>> >>
>> >> > On Thu, Oct 31, 2019 at 1:03 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?
>> >> >
>> >> > 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...

-Toke