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

From: Jiri Olsa <[email-masked]>
Date: Thu, 31 Oct 2019 20:18:15 +0100

On Thu, Oct 31, 2019 at 11:19:21AM -0700, Alexei Starovoitov wrote:
> On Thu, Oct 31, 2019 at 10:42 AM Jiri Olsa <[email-masked]> wrote:
> >
> > On Thu, Oct 31, 2019 at 08:17:43AM -0700, Alexei Starovoitov wrote:
> > > On Thu, Oct 31, 2019 at 7:52 AM Toke Høiland-Jørgensen <[email-masked]> wrote:
> > > >
> > > > Alexei Starovoitov <[email-masked]> writes:
> > > >
> > > > > On Thu, Oct 31, 2019 at 7:26 AM Toke Høiland-Jørgensen <[email-masked]> wrote:
> > > > >>
> > > > >> Alexei Starovoitov <[email-masked]> writes:
> > > > >>
> > > > >> > On Thu, Oct 31, 2019 at 7:13 AM Toke Høiland-Jørgensen <[email-masked]> wrote:
> > > > >> >>
> > > > >> >> 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?
> > > > >> >
> > > > >> > 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.
> >
> > hum, that's possible case for distro users.. older kernel, newer libbpf
> 
> yes. older vmlinux and newer installed libbpf.so
> or any version of libbpf.a that is statically linked into apps
> is something that libbpf code has to support.
> The server can be rebooted into older than libbpf kernel and
> into newer than libbpf kernel. libbpf has to recognize all these
> combinations and work appropriately.
> That's what backward and forward compatibility is.
> That's what makes libbpf so difficult to test, develop and code review.
> What that particular server has in /usr/include is irrelevant.

sure, anyway we can't compile following:

	tredaell@aldebaran ~ $ 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;
	...

	XDP_RING_NEED_WAKEUP is defined in kernel v5.4-rc1 (77cd0d7b3f257fd0e3096b4fdcff1a7d38e99e10).
	XSK_UNALIGNED_BUF_ADDR_MASK and XSK_UNALIGNED_BUF_OFFSET_SHIFT are defined in kernel v5.4-rc1 (c05cd3645814724bdeb32a2b4d953b12bdea5f8c).

with:
  kernel-headers-5.3.6-300.fc31.x86_64
  libbpf-0.0.5-1.fc31.x86_64

if you're saying this is not supported, I guess we could be postponing
libbpf rpm releases until we have the related fedora kernel released

or how about inluding uapi headers in libbpf-devel.. but that might
actualy cause more confusion

jirka