Re: [PATCH bpf-next 1/2] libbpf: add support to set kprobe/uprobe attach mode

[Menglong Dong]

On Tue, Feb 7, 2023 at 3:59 AM Andrii Nakryiko
<andrii.nakryiko@xxxxxxxxx> wrote:
>
> On Thu, Feb 2, 2023 at 7:18 PM <menglong8.dong@xxxxxxxxx> wrote:
> >
> > From: Menglong Dong <imagedong@xxxxxxxxxxx>
> >
> > By default, libbpf will attach the kprobe/uprobe eBPF program in the
> > latest mode that supported by kernel. In this patch, we add the support
> > to let users manually attach kprobe/uprobe in legacy or perf mode.
> >
> > There are 3 mode that supported by the kernel to attach kprobe/uprobe:
> >
> >   LEGACY: create perf event in legacy way and don't use bpf_link
> >   PERF: create perf event with perf_event_open() and don't use bpf_link
> >   LINK: create perf event with perf_event_open() and use bpf_link
> >
> > Users now can manually choose the mode with
> > bpf_program__attach_uprobe_opts()/bpf_program__attach_kprobe_opts().
> >
> > Link: https://lore.kernel.org/bpf/20230113093427.1666466-1-imagedong@xxxxxxxxxxx/
> > Signed-off-by: Menglong Dong <imagedong@xxxxxxxxxxx>
> > ---
> >  tools/lib/bpf/libbpf.c | 26 +++++++++++++++++++++++++-
> >  tools/lib/bpf/libbpf.h | 19 ++++++++++++++++---
> >  2 files changed, 41 insertions(+), 4 deletions(-)
> >
> > diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
> > index eed5cec6f510..0d20bf1ee301 100644
> > --- a/tools/lib/bpf/libbpf.c
> > +++ b/tools/lib/bpf/libbpf.c
> > @@ -9784,7 +9784,7 @@ struct bpf_link *bpf_program__attach_perf_event_opts(const struct bpf_program *p
> >         link->link.dealloc = &bpf_link_perf_dealloc;
> >         link->perf_event_fd = pfd;
> >
> > -       if (kernel_supports(prog->obj, FEAT_PERF_LINK)) {
> > +       if (kernel_supports(prog->obj, FEAT_PERF_LINK) && !opts->no_link) {
> >                 DECLARE_LIBBPF_OPTS(bpf_link_create_opts, link_opts,
> >                         .perf_event.bpf_cookie = OPTS_GET(opts, bpf_cookie, 0));
> >
> > @@ -10148,16 +10148,28 @@ bpf_program__attach_kprobe_opts(const struct bpf_program *prog,
> >         struct bpf_link *link;
> >         size_t offset;
> >         bool retprobe, legacy;
> > +       enum probe_mode mode;
> >         int pfd, err;
> >
> >         if (!OPTS_VALID(opts, bpf_kprobe_opts))
> >                 return libbpf_err_ptr(-EINVAL);
> >
> > +       mode = OPTS_GET(opts, mode, PROBE_MODE_DEFAULT);
> >         retprobe = OPTS_GET(opts, retprobe, false);
> >         offset = OPTS_GET(opts, offset, 0);
> >         pe_opts.bpf_cookie = OPTS_GET(opts, bpf_cookie, 0);
> >
> >         legacy = determine_kprobe_perf_type() < 0;
> > +       switch (mode) {
> > +       case PROBE_MODE_LEGACY:
> > +               legacy = true;
> > +       case PROBE_MODE_PERF:
> > +               pe_opts.no_link = true;
> > +               break;
> > +       default:
> > +               break;
> > +       }
> > +
> >         if (!legacy) {
> >                 pfd = perf_event_open_probe(false /* uprobe */, retprobe,
> >                                             func_name, offset,
> > @@ -10817,10 +10829,12 @@ bpf_program__attach_uprobe_opts(const struct bpf_program *prog, pid_t pid,
> >         int pfd, err;
> >         bool retprobe, legacy;
> >         const char *func_name;
> > +       enum probe_mode mode;
> >
> >         if (!OPTS_VALID(opts, bpf_uprobe_opts))
> >                 return libbpf_err_ptr(-EINVAL);
> >
> > +       mode = OPTS_GET(opts, mode, PROBE_MODE_DEFAULT);
> >         retprobe = OPTS_GET(opts, retprobe, false);
> >         ref_ctr_off = OPTS_GET(opts, ref_ctr_offset, 0);
> >         pe_opts.bpf_cookie = OPTS_GET(opts, bpf_cookie, 0);
> > @@ -10849,6 +10863,16 @@ bpf_program__attach_uprobe_opts(const struct bpf_program *prog, pid_t pid,
> >         }
> >
> >         legacy = determine_uprobe_perf_type() < 0;
> > +       switch (mode) {
> > +       case PROBE_MODE_LEGACY:
> > +               legacy = true;
> > +       case PROBE_MODE_PERF:
> > +               pe_opts.no_link = true;
> > +               break;
> > +       default:
> > +               break;
> > +       }
> > +
>
> I think this is a good place to also return errors early if, say, user
> requested LINK mode, but that mode is not supported by kernel. Instead
> of returning some -ENOTSUP generic error, we can error out early?
> Similar for PERF mode, if only legacy is supported, and so on. Similar
> for attach_uprobe, of course.
>

Yes, sounds great.

> >         if (!legacy) {
> >                 pfd = perf_event_open_probe(true /* uprobe */, retprobe, binary_path,
> >                                             func_offset, pid, ref_ctr_off);
> > diff --git a/tools/lib/bpf/libbpf.h b/tools/lib/bpf/libbpf.h
> > index 8777ff21ea1d..7fb474e036a3 100644
> > --- a/tools/lib/bpf/libbpf.h
> > +++ b/tools/lib/bpf/libbpf.h
> > @@ -451,8 +451,10 @@ struct bpf_perf_event_opts {
> >         size_t sz;
> >         /* custom user-provided value fetchable through bpf_get_attach_cookie() */
> >         __u64 bpf_cookie;
> > +       /* don't use bpf_link when attach eBPF pprogram */
>
> typo: pprogram

Ops, get it!

>
> > +       bool no_link;
>
> I've been struggling with this "no_link" name a bit. It is quite
> confusing considering that from libbpf's API side we do return `struct
> bpf_link`. So I'm thinking that maybe "force_ioctl_attach" would be a
> better way to describe it (and will be scary enough for people not
> knowing what this is about to not set it to true?)
>

Yeah, "force_ioctl_attach" sounds more appropriate.

> Also, we'll need size_t: 0 at the end to avoid uninitialized padding
> issues (like we have in kprobe_opts and uprobe_opts)
>
> >  };
> > -#define bpf_perf_event_opts__last_field bpf_cookie
> > +#define bpf_perf_event_opts__last_field no_link
> >
> >  LIBBPF_API struct bpf_link *
> >  bpf_program__attach_perf_event(const struct bpf_program *prog, int pfd);
> > @@ -461,6 +463,13 @@ LIBBPF_API struct bpf_link *
> >  bpf_program__attach_perf_event_opts(const struct bpf_program *prog, int pfd,
> >                                     const struct bpf_perf_event_opts *opts);
> >
> > +enum probe_mode {
>
> shall we call it probe_attach_mode?
>
> also let's elaborate a bit more in doc comment that specifying mode
> will force libbpf to use that, but if kernel doesn't support it --
> then we'll error out.
>

OK.

> > +       PROBE_MODE_DEFAULT = 0, /* latest supported by kernel */
> > +       PROBE_MODE_LEGACY,
> > +       PROBE_MODE_PERF,
> > +       PROBE_MODE_LINK,
> > +};
> > +
> >  struct bpf_kprobe_opts {
> >         /* size of this struct, for forward/backward compatiblity */
> >         size_t sz;
> > @@ -470,9 +479,11 @@ struct bpf_kprobe_opts {
> >         size_t offset;
> >         /* kprobe is return probe */
> >         bool retprobe;
> > +       /* kprobe attach mode */
> > +       enum probe_mode mode;
>
> nit: mode -> attach_mode?
>
> >         size_t :0;
> >  };
> > -#define bpf_kprobe_opts__last_field retprobe
> > +#define bpf_kprobe_opts__last_field mode
> >
> >  LIBBPF_API struct bpf_link *
> >  bpf_program__attach_kprobe(const struct bpf_program *prog, bool retprobe,
> > @@ -570,9 +581,11 @@ struct bpf_uprobe_opts {
> >          * binary_path.
> >          */
> >         const char *func_name;
> > +       /* uprobe attach mode */
> > +       enum probe_mode mode;
> >         size_t :0;
> >  };
> > -#define bpf_uprobe_opts__last_field func_name
> > +#define bpf_uprobe_opts__last_field mode
>
> ditto
>

I'll fix these problems in the V2.

Thanks!
Menglong Dong

> >
> >  /**
> >   * @brief **bpf_program__attach_uprobe()** attaches a BPF program
> > --
> > 2.39.0
> >