linux-kernel - Re: [PATCH] kprobes: Call check_ftrace_location() on CONFIG_KPROBES_ON

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Message-Id: <2DC4EBDA-F668-4DB8-9F0C-F31FAFCF0721@bytedance.com>
Date: Mon, 8 Dec 2025 16:59:24 +0800
From: qingwei hu <huqingwei.kernel@...il.com>
To: "Masami Hiramatsu (Google)" <mhiramat@...nel.org>
Cc: davem@...emloft.net,
 linux-kernel@...r.kernel.org,
 linux-trace-kernel@...r.kernel.org,
 naveen@...nel.org,
 qingwei.hu@...edance.com,
 rostedt@...dmis.org
Subject: Re: [PATCH] kprobes: Call check_ftrace_location() on
 CONFIG_KPROBES_ON_FTRACE


> On Dec 8, 2025, at 15:19, Masami Hiramatsu (Google) <mhiramat@...nel.org> wrote:
> 
> On Mon, 8 Dec 2025 14:54:33 +0800
> qingwei hu <huqingwei.kernel@...il.com> wrote:
> 
>> 
>> 
>>> 2025年12月5日 23:08，Steven Rostedt <rostedt@...dmis.org> 写道：
>>> 
>>> On Fri,  5 Dec 2025 17:29:33 +0800
>>> "qingwei.hu" <qingwei.hu@...edance.com> wrote:
>>> 
>>>> From: Qingwei Hu <qingwei.hu@...edance.com>
>>>> 
>>>> There is a possible configuration dependency:
>>>> 
>>>> KPROBES_ON_FTRACE [=n]
>>>>      ^----- KPROBES [=y]
>>>>        |--- HAVE_KPROBES_ON_FTRACE [=n]
>>>>        |--- DYNAMIC_FTRACE_WITH_REGS [=n]
>>>>               ^----- FTRACE [=y]
>>>>                 |--- DYNAMIC_FTRACE [=y]
>>>>                 |--- HAVE_DYNAMIC_FTRACE_WITH_REGS [=n]
>>>> 
>>>> With DYNAMIC_FTRACE=y, ftrace_location() is meaningful and may
>>>> return the same address as the probe target.
>>>> 
>>>> However, when KPROBES_ON_FTRACE=n, the current implementation
>>>> returns -EINVAL after calling check_ftrace_location(), causing
>>>> the validation to fail.
>>> 
>>> This is a feature not a bug.
>>> 
>>> The reason is if you put a kprobe on a ftrace location, it can cause ftrace
>>> to trigger a bug, as kprobes will modify the location and ftrace will see
>>> something it doesn't expect and think the system is corrupted. We don't want
>>> that either.
>>> 
>>> If you say "KPROBES_ON_FTRACE=n" and place a kprobe on a location that is
>>> controlled by ftrace, it had better fail!
>>> 
>>> NAK
>>> 
>>> -- Steve
>> 
>> Thanks for your clear explanation. I will look into other approaches
>> that work with this configuration.
>> 
> 
> Can you check the code under arch/<your machine>/ implements the
> kprobe_ftrace_handler() correctly? if so, it should be enabled automatically.
> 
> Thank you,
> 
> 
> -- 
> Masami Hiramatsu (Google) <mhiramat@...nel.org>

I am working on the risc-v that have removed support for KPROBE_ON_FTRACE and
probe_ftrace_hander() since 7caa9765465f60 and 3308172276db5d.

As 7caa9765465f60 commit say KPROBES_ON_FTRACE will be supplanted by FPROBES,
I have tested and verified this feature's usability. In this situation, may
be we should use fprobe instead of kprobe.

But in the latest version, risc-v adjusts ftrace location to an offset 4 byte
from the function symbol. So it will kprobe successfully in the beginning of
the symbol while not support KPROBE_ON_FTRACE. I am not sure whether this
arrangement is consistent with Steve’s explanation. If the adjustment is valid,
I can temporarily backport the adjust function in order to support kprobe.