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

[Jianlin Lv <Jianlin.Lv@arm.com>]

> -----Original Message-----
> From: Masami Hiramatsu <mhiramat@kernel.org>
> Sent: Wednesday, January 27, 2021 9:28 PM
> To: Jianlin Lv <Jianlin.Lv@arm.com>
> Cc: Oleg Nesterov <oleg@redhat.com>; Steven Rostedt
> <rostedt@goodmis.org>; mingo@redhat.com; linux-kernel@vger.kernel.org
> Subject: Re: [PATCH v3] tracing: precise log info for kretprobe addr err
>
> On Wed, 27 Jan 2021 02:46:10 +0000
> Jianlin Lv <Jianlin.Lv@arm.com> wrote:
>
> >
> >
> > > -----Original Message-----
> > > From: Masami Hiramatsu <mhiramat@kernel.org>
> > > Sent: Wednesday, January 27, 2021 10:02 AM
> > > To: Oleg Nesterov <oleg@redhat.com>
> > > Cc: Steven Rostedt <rostedt@goodmis.org>; Jianlin Lv
> > > <Jianlin.Lv@arm.com>; mingo@redhat.com; linux-
> kernel@vger.kernel.org
> > > Subject: Re: [PATCH v3] tracing: precise log info for kretprobe addr
> > > err
> > >
> > > On Tue, 26 Jan 2021 21:20:59 +0100
> > > Oleg Nesterov <oleg@redhat.com> 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@kernel.org>
> >
> >
> > 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?
>
> Oops, good catch!
> It seems that the bug has been introduced when I added
> kprobe_on_func_entry() to register_Kretprobe.
> Let me fix it.
>
> Thank you!
>
>
> --
> Masami Hiramatsu <mhiramat@kernel.org>

After confirming this problem, my worries are eliminated,
and the current patch will be updated later.

I am also investigating this bug, and I think this process will deepen
my understanding of kernel probes.

Jianlin

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