Hi Alexei, 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. 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.) Thank you, -- Masami Hiramatsu <mhiramat@xxxxxxxxxx>
