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

[Steven Rostedt <rostedt@goodmis.org>]

On Fri,  5 Dec 2025 17:29:33 +0800
"qingwei.hu" <qingwei.hu@bytedance.com> wrote:

> From: Qingwei Hu <qingwei.hu@bytedance.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