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

[Masami Hiramatsu]

On Thu, 3 Feb 2022 18:42:22 -0800
Alexei Starovoitov <alexei.starovoitov@xxxxxxxxx> wrote:

> On Thu, Feb 3, 2022 at 6:19 PM Steven Rostedt <rostedt@xxxxxxxxxxx> wrote:
> >
> > On Thu, 3 Feb 2022 18:12:11 -0800
> > Alexei Starovoitov <alexei.starovoitov@xxxxxxxxx> 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.
> 
> 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.

Perf-probe makes it very easy, as easy as gdb does. :-)

Thank you,

> When people say "kprobe" 99 out of 100 they mean
> kprobe on ftrace/fentry.


-- 
Masami Hiramatsu <mhiramat@xxxxxxxxxx>