Re: [PATCH] kprobes: Call check_ftrace_location() on CONFIG_KPROBES_ON_FTRACE

[qingwei hu]

> On Dec 8, 2025, at 15:19, Masami Hiramatsu (Google) <mhiramat@xxxxxxxxxx> wrote:
> 
> On Mon, 8 Dec 2025 14:54:33 +0800
> qingwei hu <huqingwei.kernel@xxxxxxxxx> wrote:
> 
>> 
>> 
>>> 2025年12月5日 23:08，Steven Rostedt <rostedt@xxxxxxxxxxx> 写道：
>>> 
>>> On Fri,  5 Dec 2025 17:29:33 +0800
>>> "qingwei.hu" <qingwei.hu@xxxxxxxxxxxxx> wrote:
>>> 
>>>> From: Qingwei Hu <qingwei.hu@xxxxxxxxxxxxx>
>>>> 
>>>> 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@xxxxxxxxxx>

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.