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

From: Toke Høiland-Jørgensen <[email-masked]>
Date: Thu, 31 Oct 2019 15:13:40 +0100

Alexei Starovoitov <[email-masked]> writes:

> On Thu, Oct 31, 2019 at 1:03 AM Björn Töpel <[email-masked]> wrote:
>>
>> On Thu, 31 Oct 2019 at 08:17, Magnus Karlsson <[email-masked]> wrote:
>> >
>> > On Wed, Oct 30, 2019 at 2:36 PM Toke Høiland-Jørgensen <[email-masked]> wrote:
>> > >
>> > > Magnus Karlsson <[email-masked]> 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?

-Toke