Re: [PATCH 3/3] perf tools: Rework prologue generation code

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

On Fri, Feb 18, 2022 at 1:01 AM Jiri Olsa <olsajiri@gmail.com> wrote:
>
> On Thu, Feb 17, 2022 at 01:53:16PM -0800, Andrii Nakryiko wrote:
> > On Thu, Feb 17, 2022 at 5:19 AM Jiri Olsa <jolsa@kernel.org> wrote:
> > >
> > > Some functions we use now for bpf prologue generation are
> > > going to be deprecated, so reworking the current code not
> > > to use them.
> > >
> > > We need to replace following functions/struct:
> > >    bpf_program__set_prep
> > >    bpf_program__nth_fd
> > >    struct bpf_prog_prep_result
> > >
> > > Current code uses bpf_program__set_prep to hook perf callback
> > > before the program is loaded and provide new instructions with
> > > the prologue.
> > >
> > > We workaround this by using objects's 'unloaded' programs instructions
> > > for that specific program and load new ebpf programs with prologue
> > > using separate bpf_prog_load calls.
> > >
> > > We keep new ebpf program instances descriptors in bpf programs
> > > private struct.
> > >
> > > Suggested-by: Andrii Nakryiko <andrii@kernel.org>
> > > Signed-off-by: Jiri Olsa <jolsa@kernel.org>
> > > ---
> > >  tools/perf/util/bpf-loader.c | 122 +++++++++++++++++++++++++++++------
> > >  1 file changed, 104 insertions(+), 18 deletions(-)
> > >
> >
> > [...]
> >
> > >  errout:
> > > @@ -696,7 +718,7 @@ static int hook_load_preprocessor(struct bpf_program *prog)
> > >         struct bpf_prog_priv *priv = program_priv(prog);
> > >         struct perf_probe_event *pev;
> > >         bool need_prologue = false;
> > > -       int err, i;
> > > +       int i;
> > >
> > >         if (IS_ERR_OR_NULL(priv)) {
> > >                 pr_debug("Internal error when hook preprocessor\n");
> > > @@ -727,6 +749,12 @@ static int hook_load_preprocessor(struct bpf_program *prog)
> > >                 return 0;
> > >         }
> > >
> > > +       /*
> > > +        * Do not load programs that need prologue, because we need
> > > +        * to add prologue first, check bpf_object__load_prologue.
> > > +        */
> > > +       bpf_program__set_autoload(prog, false);
> >
> > if you set autoload to false, program instructions might be invalid in
> > the end. Libbpf doesn't apply some (all?) relocations to such
> > programs, doesn't resolve CO-RE, etc, etc. You have to let
> > "prototypal" BPF program to be loaded before you can grab final
> > instructions. It's not great, but in your case it should work, right?
>
> hum, do we care? it should all be done when the 'new' program with
> the prologue is loaded, right?

yeah, you should care. If there is any BPF map involved, it is
properly resolved to correct FD (which is put into ldimm64 instruction
in BPF program code) during the load. If program is not autoloaded,
this is skipped. Same for any global variable or subprog call (if it's
not always inlined). So you very much should care for any non-trivial
program.

>
> I switched it off because the verifier failed to load the program
> without the prologue.. because in the original program there's no
> code to grab the arguments that the rest of the code depends on,
> so the verifier sees invalid access

Do you have an example of C code and corresponding BPF instructions
before/after prologue generation? Just curious to see in details how
this is done.

>
> >
> > > +
> > >         priv->need_prologue = true;
> > >         priv->insns_buf = malloc(sizeof(struct bpf_insn) * BPF_MAXINSNS);
> > >         if (!priv->insns_buf) {
> > > @@ -734,6 +762,13 @@ static int hook_load_preprocessor(struct bpf_program *prog)
> > >                 return -ENOMEM;
> > >         }
> > >
> >
> > [...]
> >
> > > +               /*
> > > +                * For each program that needs prologue we do following:
> > > +                *
> > > +                * - take its current instructions and use them
> > > +                *   to generate the new code with prologue
> > > +                *
> > > +                * - load new instructions with bpf_prog_load
> > > +                *   and keep the fd in proglogue_fds
> > > +                *
> > > +                * - new fd will be used bpf__foreach_event
> > > +                *   to connect this program with perf evsel
> > > +                */
> > > +               orig_insns = bpf_program__insns(prog);
> > > +               orig_insns_cnt = bpf_program__insn_cnt(prog);
> > > +
> > > +               pev = &priv->pev;
> > > +               for (i = 0; i < pev->ntevs; i++) {
> > > +                       err = preproc_gen_prologue(prog, i, orig_insns,
> > > +                                                  orig_insns_cnt, &res);
> > > +                       if (err)
> > > +                               return err;
> > > +
> > > +                       fd = bpf_prog_load(bpf_program__get_type(prog),
> >
> > nit: bpf_program__type() is preferred (we are deprecating/discouraging
> > "get_" prefixed getters in libbpf 1.0)
>
> ok, will change

It's been added in v0.7, yeah.

>
> >
> > > +                                          bpf_program__name(prog), "GPL",
> >
> > would it make sense to give each clone a distinct name?
>
> AFAICS the original code uses same prog name for instances,
> so I'd rather keep it that way
>

sure, np

> thanks,
> jirka
>
> >
> > > +                                          res.new_insn_ptr,
> > > +                                          res.new_insn_cnt, NULL);
> > > +                       if (fd < 0) {
> > > +                               char bf[128];
> > > +
> >
> > [...]