Re: [PATCH v3 bpf-next] libbpf: add bpf object kern_version attribute setter

[Andrii Nakryiko <andrii.nakryiko@gmail.com>]

On Thu, Mar 25, 2021 at 1:01 PM Rafael David Tinoco
<rafaeldtinoco@ubuntu.com> wrote:
>
> >> Unfortunately some distros don't have their kernel version defined
> >> accurately in <linux/version.h> due to different long term support
> >> reasons.
> >>
> >> It is important to have a way to override the bpf kern_version
> >> attribute during runtime: some old kernels might still check for
> >> kern_version attribute during bpf_prog_load().
> >>
> >> Signed-off-by: Rafael David Tinoco <rafaeldtinoco@ubuntu.com>
> >> ---
> >>   tools/lib/bpf/libbpf.c   | 10 ++++++++++
> >>   tools/lib/bpf/libbpf.h   |  1 +
> >>   tools/lib/bpf/libbpf.map |  1 +
> >>   3 files changed, 12 insertions(+)
> >>
> >
> > Hi Andrii and Rafael,
> >
> > Did you consider making kernel version an attribute of the load
> > API, bpf_prog_load_xattr()? This feels slightly more natural
> > to me, to tell the API the kernel you need at load time.
>
> Hi John,
>
> This is how I'm using:
> https://github.com/rafaeldtinoco/portablebpf/blob/master/mine.c#L285
>
> > Although, I don't use the skeleton pieces so maybe it would be
> > awkward for that usage.
>
> having a xxx_bpf object:
>
> xxx_bpf__open_and_load() -> xxx_bpf__load() ->
> bpf_object__load_skeleton() -> bpf_object_load() -> bpf_object__loadxattr()
>
> We would have to add kern_version to 'bpf_object_skeleton' struct, to be
> passed to 'bpf_object__load_skeleton()' to have it passed on.
>
> I'll let Andrii to see what he prefers.

See my reply to John. What he asked for already exists. Having a test
would be nice, but selftests/bpf don't have an infrastructure to even
perform this test, so I don't think it's worth it.

So in summary, LGTM. I'll apply it once bpf-next is ready to accept new patches.

Acked-by: Andrii Nakryiko <andrii@kernel.org>

>
> Note:
>
> Reason for all this (including the legacy kprobe logic, in other commit)
> is to continue the
> https://github.com/rafaeldtinoco/conntracker/tree/poc-cmd project,
> making sure it supports 4.x kernels. Still adding bpf support to it
> (identify task/pid per flow) and will replace the libnf* usage with bpf
> after that.
>
> Thanks
> -rafaeldtinoco