linux-kernel - Re: kprobe on local function

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Message-ID: <20250122172936.09bc8c9a@gandalf.local.home>
Date: Wed, 22 Jan 2025 17:29:36 -0500
From: Steven Rostedt <rostedt@...dmis.org>
To: Nam Cao <namcao@...utronix.de>
Cc: naveen@...nel.org, anil.s.keshavamurthy@...el.com, davem@...emloft.net,
 mhiramat@...nel.org, linux-kernel@...r.kernel.org,
 linux-trace-kernel@...r.kernel.org, john.ogness@...utronix.de
Subject: Re: kprobe on local function

On Wed, 22 Jan 2025 08:59:39 +0100
Nam Cao <namcao@...utronix.de> wrote:

> Hi,
> 
> I installed a kretprobe on the function "rt_mutex_slowunlock". Although the
> function is called, the probe is never hit.
> 

How did you install the kretprobe?

> The reason is because there are 3 copies of rt_mutex_slowunlock() in
> vmlinux (at least for x86 defconfig + CONFIG_PREEMPT_RT). But probe point
> is only installed in 1 copy.
> 
> Is this considered a bug? Or is kprobe not intended to be used for local
> function? Should we modify kprobe to support such cases?

It's not a bug, but just the way the compiler works.

Have you tried "perf probe" to install the probes? That uses dwarf info to
find all the functions.

-- Steve