On Tue, Mar 29, 2022 at 8:03 PM Hengqi Chen <hengqi.chen@xxxxxxxxx> wrote: > > > > On 2022/3/30 10:50 AM, Andrii Nakryiko wrote: > > On Tue, Mar 29, 2022 at 7:30 PM Hengqi Chen <hengqi.chen@xxxxxxxxx> 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@xxxxxxxxx> 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@xxxxxxxxx> > >>>> --- > >>> > >>> 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
