linux-kernel - Re: [PATCH v3] tracing: precise log info for kretprobe addr err

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Message-ID: <20210126211722.GA23645@redhat.com>
Date:   Tue, 26 Jan 2021 22:17:23 +0100
From:   Oleg Nesterov <oleg@...hat.com>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     Masami Hiramatsu <mhiramat@...nel.org>,
        Jianlin Lv <Jianlin.Lv@....com>, mingo@...hat.com,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3] tracing: precise log info for kretprobe addr err

On 01/26, Steven Rostedt wrote:
>
> On Tue, 26 Jan 2021 21:20:59 +0100
> Oleg Nesterov <oleg@...hat.com> wrote:
>
> > > 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 ?
>
> From what I understand. kprobe_on_func_entry() can return false if you pass
> in: "MOD:not_yet_loaded_module_func", but this is OK, because when the
> module is loaded, and the "not_yet_loaded_module_func" exists, the
> kretprobe will then be added.
>
> The strchr(symbol,":") check is to see if "MOD:" (or some other ":" command)
> is in the name, and we don't want it to fail if it is. Which is why we
> should have that commented.

Agreed, this matches my understanding.

But just in case... not sure I read this code correctly, but I think that
module_kallsyms_lookup_name("not_yet_loaded_module_func") should work even
without the "MOD:" prefix.

IOW, kprobe_on_func_entry("not_yet_loaded_module_func") can fail, and then
later succeed if you load the module which provides this symbol.

But even if I am right, I agree with the strchr(symbol,":") check.

Oleg.