RE: [PATCH v3] tracing: precise log info for kretprobe addr err

[Jianlin Lv]

> -----Original Message-----
> From: Masami Hiramatsu <mhiramat@xxxxxxxxxx>
> Sent: Wednesday, January 27, 2021 10:02 AM
> To: Oleg Nesterov <oleg@xxxxxxxxxx>
> Cc: Steven Rostedt <rostedt@xxxxxxxxxxx>; Jianlin Lv <Jianlin.Lv@xxxxxxx>;
> mingo@xxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx
> Subject: Re: [PATCH v3] tracing: precise log info for kretprobe addr err
>
> On Tue, 26 Jan 2021 21:20:59 +0100
> Oleg Nesterov <oleg@xxxxxxxxxx> wrote:
>
> > On 01/26, Masami Hiramatsu wrote:
> > >
> > > > >
> > > > > IOW, the "offset != 0" check removed by this patch is obviously wrong,
> right?
> > > > >
> > >
> > > No, not wrong. Even offset != 0, if the symbol exists in the kernel,
> > > kprobe_on_func_entry() will check it.
> >
> > Yes, but unless I am totally confused... if kprobe_on_func_entry()
> > returns false, then trace_kprobe_create() should fail with BAD_RETPROBE
> even if offset == 0 ?
>
> Yes, if kprobe_on_func_entry() returns false, register_kretprobe() also
> returns an error.
>
> -----
> int register_kretprobe(struct kretprobe *rp) {
>         int ret = 0;
>         struct kretprobe_instance *inst;
>         int i;
>         void *addr;
>
>         if (!kprobe_on_func_entry(rp->kp.addr, rp->kp.symbol_name, rp-
> >kp.offset))
>                 return -EINVAL;
>
> -----
>
> Thank you,
>
> --
> Masami Hiramatsu <mhiramat@xxxxxxxxxx>


If register_kretprobe()returns an error -EINVAL.
This means that __register_trace_kprobe return -EINVAL,

---
ret = __register_trace_kprobe(tk);
if (ret == -ENOENT && !trace_kprobe_module_exist(tk)) {
pr_warn("This probe might be able to register after target module is loaded. Continue.\n");
ret = 0;
}
---
As code show, cannot enable kretprobe for an unloaded module.

This is consistent with my test results (no VXLAN module is loaded).

# perf probe -m /lib/modules/5.11.0-rc2+/kernel/drivers/net/vxlan.ko  \
'vxlan_xmit%return $retval'
Failed to write event: Invalid argument
  Error: Failed to add events.

Is this a bug?

Jianlin

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