linux-kernel - Re: [PATCH v5 2/9] tracing/probes: Add fprobe events for tracing function entry and exit.

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Message-Id: <20230424132459.2f373669e178c3be74e71b0e@kernel.org>
Date:   Mon, 24 Apr 2023 13:24:59 +0900
From:   Masami Hiramatsu (Google) <mhiramat@...nel.org>
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc:     linux-trace-kernel@...r.kernel.org,
        LKML <linux-kernel@...r.kernel.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Florent Revest <revest@...omium.org>,
        Mark Rutland <mark.rutland@....com>,
        Will Deacon <will@...nel.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Martin KaFai Lau <martin.lau@...ux.dev>,
        bpf <bpf@...r.kernel.org>
Subject: Re: [PATCH v5 2/9] tracing/probes: Add fprobe events for tracing
 function entry and exit.

On Fri, 21 Apr 2023 09:31:12 -0700
Alexei Starovoitov <alexei.starovoitov@...il.com> wrote:

> On Thu, Apr 20, 2023 at 10:38 PM Masami Hiramatsu <mhiramat@...nel.org> wrote:
> >
> > On Thu, 20 Apr 2023 16:46:08 -0700
> > Alexei Starovoitov <alexei.starovoitov@...il.com> wrote:
> >
> > > On Thu, Apr 20, 2023 at 4:41 PM Masami Hiramatsu <mhiramat@...nel.org> wrote:
> > > >
> > > > On Thu, 20 Apr 2023 11:49:32 -0700
> > > > Alexei Starovoitov <alexei.starovoitov@...il.com> wrote:
> > > >
> > > > > On Thu, Apr 20, 2023 at 08:25:50PM +0900, Masami Hiramatsu (Google) wrote:
> > > > > > +static int fentry_perf_func(struct trace_fprobe *tf, unsigned long entry_ip,
> > > > > > +                       struct pt_regs *regs)
> > > > > > +{
> > > > > > +   struct trace_event_call *call = trace_probe_event_call(&tf->tp);
> > > > > > +   struct fentry_trace_entry_head *entry;
> > > > > > +   struct hlist_head *head;
> > > > > > +   int size, __size, dsize;
> > > > > > +   int rctx;
> > > > > > +
> > > > > > +   if (bpf_prog_array_valid(call)) {
> > > > > > +           unsigned long orig_ip = instruction_pointer(regs);
> > > > > > +           int ret;
> > > > > > +
> > > > > > +           ret = trace_call_bpf(call, regs);
> > > > >
> > > > > Please do not call bpf from fprobe.
> > > > > There is no use case for it.
> > > >
> > > > OK.
> > > >
> > > > >
> > > > > > +
> > > > > > +           /*
> > > > > > +            * We need to check and see if we modified the pc of the
> > > > > > +            * pt_regs, and if so return 1 so that we don't do the
> > > > > > +            * single stepping.
> > > > > > +            */
> > > > > > +           if (orig_ip != instruction_pointer(regs))
> > > > > > +                   return 1;
> > > > > > +           if (!ret)
> > > > > > +                   return 0;
> > > > > > +   }
> > > > > > +
> > > > > > +   head = this_cpu_ptr(call->perf_events);
> > > > > > +   if (hlist_empty(head))
> > > > > > +           return 0;
> > > > > > +
> > > > > > +   dsize = __get_data_size(&tf->tp, regs);
> > > > > > +   __size = sizeof(*entry) + tf->tp.size + dsize;
> > > > > > +   size = ALIGN(__size + sizeof(u32), sizeof(u64));
> > > > > > +   size -= sizeof(u32);
> > > > > > +
> > > > > > +   entry = perf_trace_buf_alloc(size, NULL, &rctx);
> > > > > > +   if (!entry)
> > > > > > +           return 0;
> > > > > > +
> > > > > > +   entry->ip = entry_ip;
> > > > > > +   memset(&entry[1], 0, dsize);
> > > > > > +   store_trace_args(&entry[1], &tf->tp, regs, sizeof(*entry), dsize);
> > > > > > +   perf_trace_buf_submit(entry, size, rctx, call->event.type, 1, regs,
> > > > > > +                         head, NULL);
> > > > > > +   return 0;
> > > > > > +}
> > > > > > +NOKPROBE_SYMBOL(fentry_perf_func);
> > > > > > +
> > > > > > +static void
> > > > > > +fexit_perf_func(struct trace_fprobe *tf, unsigned long entry_ip,
> > > > > > +           unsigned long ret_ip, struct pt_regs *regs)
> > > > > > +{
> > > > > > +   struct trace_event_call *call = trace_probe_event_call(&tf->tp);
> > > > > > +   struct fexit_trace_entry_head *entry;
> > > > > > +   struct hlist_head *head;
> > > > > > +   int size, __size, dsize;
> > > > > > +   int rctx;
> > > > > > +
> > > > > > +   if (bpf_prog_array_valid(call) && !trace_call_bpf(call, regs))
> > > > > > +           return;
> > > > >
> > > > > Same here.
> > > > > These two parts look like copy-paste from kprobes.
> > > > > I suspect this code wasn't tested at all.
> > > >
> > > > OK, I missed to test that bpf part. I thought bpf could be appended to
> > > > any "trace-event" (looks like trace-event), isn't it?
> > >
> > > No. We're not applying bpf filtering to any random event
> > > that gets introduced in a tracing subsystem.
> > > fprobe falls into that category.
> > > Every hook where bpf can be invoked has to be thought through.
> > > That mental exercise didn't happen here.
> >
> > OK. Just out of curiousity, where is the "tracepoint" filter applied?
> > In the kernel (verifier?) or the userspace?
> 
> Sorry. I don't understand the question.
> Are you talking about BPF_PROG_TYPE_TRACEPOINT or BPF_PROG_TYPE_RAW_TRACEPOINT ?

I thought that you filtered the available events by name, but I found
that perf_event_set_bpf_prog() checks TRACE_EVENT_FL_* flags and
its combinations. Yeah, in that case this new fprobe event introduced
TRACE_EVENT_FL_FPROBE and bpf will reject to use it.

OK, let me remove the BPF support from this series. I think the fprobe
event can be used as same as kprobe events, but I have a plan to change
it for supporting fprobe wider architectures. Thus it will require a bit
different way to get the register values.

Thank you,


-- 
Masami Hiramatsu (Google) <mhiramat@...nel.org>