Re: [PATCH 0/8] bpf: Add fprobe link

[Jiri Olsa <olsajiri@gmail.com>]

On Fri, Feb 04, 2022 at 12:59:42PM +0900, Masami Hiramatsu wrote:
> Hi Alexei,
> 
> On Thu, 3 Feb 2022 18:42:22 -0800
> Alexei Starovoitov <alexei.starovoitov@gmail.com> wrote:
> 
> > On Thu, Feb 3, 2022 at 6:19 PM Steven Rostedt <rostedt@goodmis.org> wrote:
> > >
> > > On Thu, 3 Feb 2022 18:12:11 -0800
> > > Alexei Starovoitov <alexei.starovoitov@gmail.com> wrote:
> > >
> > > > > No, fprobe is NOT kprobe on ftrace, kprobe on ftrace is already implemented
> > > > > transparently.
> > > >
> > > > Not true.
> > > > fprobe is nothing but _explicit_ kprobe on ftrace.
> > > > There was an implicit optimization for kprobe when ftrace
> > > > could be used.
> > > > All this new interface is doing is making it explicit.
> > > > So a new name is not warranted here.
> > > >
> > > > > from that viewpoint, fprobe and kprobe interface are similar but different.
> > > >
> > > > What is the difference?
> > > > I don't see it.
> > >
> > > IIUC, a kprobe on a function (or ftrace, aka fprobe) gives some extra
> > > abilities that a normal kprobe does not. Namely, "what is the function
> > > parameters?"
> > >
> > > You can only reliably get the parameters at function entry. Hence, by
> > > having a probe that is unique to functions as supposed to the middle of a
> > > function, makes sense to me.
> > >
> > > That is, the API can change. "Give me parameter X". That along with some
> > > BTF reading, could figure out how to get parameter X, and record that.
> > 
> > This is more or less a description of kprobe on ftrace :)
> > The bpf+kprobe users were relying on that for a long time.
> > See PT_REGS_PARM1() macros in bpf_tracing.h
> > They're meaningful only with kprobe on ftrace.
> > So, no, fprobe is not inventing anything new here.
> 
> Hmm, you may be misleading why PT_REGS_PARAM1() macro works. You can use
> it even if CONFIG_FUNCITON_TRACER=n if your kernel is built with
> CONFIG_KPROBES=y. It is valid unless you put a probe out of function
> entry.
> 
> > No one is using kprobe in the middle of the function.
> > It's too difficult to make anything useful out of it,
> > so no one bothers.
> > When people say "kprobe" 99 out of 100 they mean
> > kprobe on ftrace/fentry.
> 
> I see. But the kprobe is kprobe. It is not designed to support multiple
> probe points. If I'm forced to say, I can rename the struct fprobe to
> struct multi_kprobe, but that doesn't change the essence. You may need
> to use both of kprobes and so-called multi_kprobe properly. (Someone
> need to do that.)

hi,
tying to kick things further ;-) I was thinking about bpf side of this
and we could use following interface:

  enum bpf_attach_type {
    ...
    BPF_TRACE_KPROBE_MULTI
  };

  enum bpf_link_type {
    ...
    BPF_LINK_TYPE_KPROBE_MULTI
  };

  union bpf_attr {

    struct {
      ...
      struct {
        __aligned_u64   syms;
        __aligned_u64   addrs;
        __aligned_u64   cookies;
        __u32           cnt;
        __u32           flags;
      } kprobe_multi;
    } link_create;
  }

because from bpf user POV it's new link for attaching multiple kprobes
and I agree new 'fprobe' type name in here brings more confusion, using
kprobe_multi is straightforward

thoguhts?

thanks,
jirka