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

Andrii Nakryiko
2022-02-18T19:46:19Z

On Thu, Feb 17, 2022 at 8:07 PM Masami Hiramatsu <mhiramat@kernel.org> wrote:
>
> On Thu, 17 Feb 2022 14:01:30 -0800
> Andrii Nakryiko <andrii.nakryiko@gmail.com> wrote:
>
>
> > > > Is there any chance to support this fast multi-attach for uprobe? If
> > > > yes, we might want to reuse the same link for both (so should we name
> > > > it more generically?
> > >
> > > There is no interface to do that but also there is no limitation to
> > > expand uprobes. For the kprobes, there are some limitations for the
> > > function entry because it needs to share the space with ftrace. So
> > > I introduced fprobe for easier to use.
> > >
> > > > on the other hand BPF program type for uprobe is
> > > > BPF_PROG_TYPE_KPROBE anyway, so keeping it as "kprobe" also would be
> > > > consistent with what we have today).
> > >
> > > Hmm, I'm not sure why BPF made such design choice... (Uprobe needs
> > > the target program.)
> > >
> >
> > We've been talking about sleepable uprobe programs, so we might need
> > to add uprobe-specific program type, probably. But historically, from
> > BPF point of view there was no difference between kprobe and uprobe
> > programs (in terms of how they are run and what's available to them).
> > From BPF point of view, it was just attaching BPF program to a
> > perf_event.
>
> Got it, so that will reuse the uprobe_events in ftrace. But I think
> the uprobe requires a "path" to the attached binary, how is it
> specified?

It's passed as a string to perf subsystem during perf_event_open() syscall.

>
> > > > But yeah, the main question is whether there is something preventing
> > > > us from supporting multi-attach uprobe as well? It would be really
> > > > great for USDT use case.
> > >
> > > Ah, for the USDT, it will be useful. But since now we will have "user-event"
> > > which is faster than uprobes, we may be better to consider to use it.
> >
> > Any pointers? I'm not sure what "user-event" refers to.
>
> Here is the user-events series, which allows user program to define
> raw dynamic events and it can write raw event data directly from
> user space.
>
> https://lore.kernel.org/all/20220118204326.2169-1-beaub@linux.microsoft.com/
>

Thanks for the link! I'll check it out.

> Thank you,
>
> --
> Masami Hiramatsu <mhiramat@kernel.org>