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

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Message-Id: <2E248E7C-773B-4998-B059-A576163B36B0@bytedance.com>
Date: Mon, 8 Dec 2025 14:54:33 +0800
From: qingwei hu <huqingwei.kernel@...il.com>
To: Steven Rostedt <rostedt@...dmis.org>
Cc: davem@...emloft.net,
 linux-kernel@...r.kernel.org,
 linux-trace-kernel@...r.kernel.org,
 mhiramat@...nel.org,
 naveen@...nel.org,
 qingwei.hu@...edance.com
Subject: Re: [PATCH] kprobes: Call check_ftrace_location() on
 CONFIG_KPROBES_ON_FTRACE



> 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.