Re: [PATCH bpf-next] libbpf: Allow kprobe attach using legacy debugfs interface

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

On Tue, Mar 29, 2022 at 8:03 PM Hengqi Chen <hengqi.chen@gmail.com> wrote:
>
>
>
> On 2022/3/30 10:50 AM, Andrii Nakryiko wrote:
> > On Tue, Mar 29, 2022 at 7:30 PM Hengqi Chen <hengqi.chen@gmail.com> wrote:
> >>
> >> Hello, Andrii
> >>
> >> On 2022/3/30 7:18 AM, Andrii Nakryiko wrote:
> >>> On Sat, Mar 26, 2022 at 7:43 AM Hengqi Chen <hengqi.chen@gmail.com> wrote:
> >>>>
> >>>> On some old kernels, kprobe auto-attach may fail when attach to symbols
> >>>> like udp_send_skb.isra.52 . This is because the kernel has kprobe PMU
> >>>> but don't allow attach to a symbol with '.' ([0]). Add a new option to
> >>>> bpf_kprobe_opts to allow using the legacy kprobe attach directly.
> >>>> This way, users can use bpf_program__attach_kprobe_opts in a dedicated
> >>>> custom sec handler to handle such case.
> >>>>
> >>>>   [0]: https://github.com/torvalds/linux/blob/v4.18/kernel/trace/trace_kprobe.c#L340-L343
> >>>>
> >>>> Signed-off-by: Hengqi Chen <hengqi.chen@gmail.com>
> >>>> ---
> >>>
> >>> It's sad, but it makes sense. But, let's have a selftests that
> >>> validates uses legacy option explicitly (e.g., in
> >>> prog_tests/attach_probe.c). Also, let's fix this limitation in the
> >>
> >> OK, will add a selftest to exercise the new option.
> >>
> >>> kernel? It makes no sense to limit attaching to a proper kallsym
> >>> symbol.
> >>
> >> This limitation is lifted in newer kernel. Kernel v5.4 don't have this issue.
> >
> > Oh, ok. So how about another plan of attack then: if kprobe target
> > function has '.' *and* we are on the kernel that doesn't support that,
> > switch to legacy kprobe automatically? No need for a new option,
> > libbpf handles this transparently.
> >
>
> That's better, and also eliminate the need for custom SEC() handler.
>
> > Still need a test for kprobe with '.' in it, though not sure how
> > reliable that will be... We can use kallsyms cache to check if
> > expected xxx.isra.0 (or whatever) is present, and if not - skip
> > subtest?
> >
>
> Not sure how to do that. Even if such symbol exists, how to reliably
> trigger it is another problem.

In addition to what Alan proposed, which relies on compiler to do this
whole isra thingy. I wonder if we can just create an alias symbol with
dots in its name? I haven't tried, but would be curious to see if that
works in bpf_testmod.

>
> >>
> >>>
> >>>>  tools/lib/bpf/libbpf.c | 9 ++++++++-
> >>>>  tools/lib/bpf/libbpf.h | 4 +++-
> >>>>  2 files changed, 11 insertions(+), 2 deletions(-)
> >>>>
> >>>
> >>> [...]
> >>
> >> --
> >> Hengqi