Re: [PATCHv2 perf/core 2/3] perf tools: Register fallback libbpf section handler

[Jiri Olsa <olsajiri@gmail.com>]

On Tue, May 10, 2022 at 04:45:01PM -0700, Andrii Nakryiko wrote:
> On Tue, May 10, 2022 at 12:47 AM Jiri Olsa <jolsa@kernel.org> 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 perf support to register 'fallback' section handler to take
> > care of perf kprobe programs. The fallback means that it handles
> > any section definition besides the ones that libbpf handles.
> >
> > The handler serves 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 <jolsa@kernel.org>
> > ---
> >  tools/perf/util/bpf-loader.c | 47 ++++++++++++++++++++++++++++++++++++
> >  1 file changed, 47 insertions(+)
> >
> > diff --git a/tools/perf/util/bpf-loader.c b/tools/perf/util/bpf-loader.c
> > index f8ad581ea247..2a2c9512c4e8 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 +100,58 @@ 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),
> 
> R0 should be initialized before exit anyway. R1 contains context, so
> doesn't need initialization, so I think you only need R2-R5?

ah right, I'll remove that

thanks,
jirka

> 
> > +       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),
> > +};
> > +
> > +static int libbpf_prog_prepare_load_fn(struct bpf_program *prog,
> > +                                      struct bpf_prog_load_opts *opts __maybe_unused,
> > +                                      long cookie __maybe_unused)
> > +{
> > +       size_t init_size_cnt = ARRAY_SIZE(prologue_init_insn);
> > +       size_t orig_insn_cnt, insn_cnt, init_size, orig_size;
> > +       const struct bpf_insn *orig_insn;
> > +       struct bpf_insn *insn;
> > +
> > +       /* prepend initialization code to program instructions */
> > +       orig_insn = bpf_program__insns(prog);
> > +       orig_insn_cnt = bpf_program__insn_cnt(prog);
> > +       init_size = init_size_cnt * sizeof(*insn);
> > +       orig_size = orig_insn_cnt * sizeof(*insn);
> > +
> > +       insn_cnt = orig_insn_cnt + init_size_cnt;
> > +       insn = malloc(insn_cnt * sizeof(*insn));
> > +       if (!insn)
> > +               return -ENOMEM;
> > +
> > +       memcpy(insn, prologue_init_insn, init_size);
> > +       memcpy((char *) insn + init_size, orig_insn, orig_size);
> > +       bpf_program__set_insns(prog, insn, insn_cnt);
> > +       return 0;
> > +}
> > +
> >  static int libbpf_init(void)
> >  {
> > +       LIBBPF_OPTS(libbpf_prog_handler_opts, handler_opts,
> > +               .prog_prepare_load_fn = libbpf_prog_prepare_load_fn,
> > +       );
> > +
> >         if (libbpf_initialized)
> >                 return 0;
> >
> >         libbpf_set_print(libbpf_perf_print);
> > +       libbpf_sec_handler = libbpf_register_prog_handler(NULL, BPF_PROG_TYPE_KPROBE,
> > +                                                         0, &handler_opts);
> > +       if (libbpf_sec_handler < 0) {
> > +               pr_debug("bpf: failed to register libbpf section handler: %d\n",
> > +                        libbpf_sec_handler);
> > +               return -BPF_LOADER_ERRNO__INTERNAL;
> > +       }
> >         libbpf_initialized = true;
> >         return 0;
> >  }
> > --
> > 2.35.3
> >