Subject: Re: [PATCH perf/core 4/5] perf tools: Register perfkprobe libbpf section handler

From: Jiri Olsa <[email-masked]>
Date: Tue, 26 Apr 2022 08:58:48 +0200

On Mon, Apr 25, 2022 at 11:22:54PM -0700, Andrii Nakryiko wrote:
> On Fri, Apr 22, 2022 at 3:01 AM Jiri Olsa <[email-masked]> wrote:
> >
> > Perf is using section name to declare special kprobe arguments,
> > which no longer works with current libbpf, that either requires
> > certain form of the section name or allows to register custom
> > handler.
> >
> > Adding support for 'perfkprobe/' section name handler to take
> > care of perf kprobe programs.
> >
> > The handler servers two purposes:
> >   - allows perf programs to have special arguments in section name
> >   - allows perf to use pre-load callback where we can attach init
> >     code (zeroing all argument registers) to each perf program
> >
> > The second is essential part of new prologue generation code,
> > that's coming in following patch.
> >
> > Signed-off-by: Jiri Olsa <[email-masked]>
> > ---
> >  tools/perf/util/bpf-loader.c | 50 ++++++++++++++++++++++++++++++++++++
> >  1 file changed, 50 insertions(+)
> >
> > diff --git a/tools/perf/util/bpf-loader.c b/tools/perf/util/bpf-loader.c
> > index f8ad581ea247..92dd8cc18edb 100644
> > --- a/tools/perf/util/bpf-loader.c
> > +++ b/tools/perf/util/bpf-loader.c
> > @@ -86,6 +86,7 @@ bpf_perf_object__next(struct bpf_perf_object *prev)
> >              (perf_obj) = (tmp), (tmp) = bpf_perf_object__next(tmp))
> >
> >  static bool libbpf_initialized;
> > +static int libbpf_sec_handler;
> >
> >  static int bpf_perf_object__add(struct bpf_object *obj)
> >  {
> > @@ -99,12 +100,61 @@ static int bpf_perf_object__add(struct bpf_object *obj)
> >         return perf_obj ? 0 : -ENOMEM;
> >  }
> >
> > +static struct bpf_insn prologue_init_insn[] = {
> > +       BPF_MOV64_IMM(BPF_REG_0, 0),
> > +       BPF_MOV64_IMM(BPF_REG_1, 0),
> > +       BPF_MOV64_IMM(BPF_REG_2, 0),
> > +       BPF_MOV64_IMM(BPF_REG_3, 0),
> > +       BPF_MOV64_IMM(BPF_REG_4, 0),
> > +       BPF_MOV64_IMM(BPF_REG_5, 0),
> > +};
> > +
> > +#define LIBBPF_SEC_PREFIX "perfkprobe/"
> 
> libbpf allows to register fallback handler that will handle any SEC()
> definition besides the ones that libbpf handles. Would that work in
> this case instead of adding a custom prefix handler here? See
> prog_tests/custom_sec_handlers.c for example:
> 
> fallback_id = libbpf_register_prog_handler(NULL,
> BPF_PROG_TYPE_SYSCALL, 0, &opts);

nice, I did not see that.. that should be better, no need for the prefix

thanks,
jirka