linux-kernel - Re: [PATCH] x86/ibt: make is

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Message-ID: <20250918130543.GM3245006@noisy.programming.kicks-ass.net>
Date: Thu, 18 Sep 2025 15:05:43 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Menglong Dong <menglong8.dong@...il.com>
Cc: jolsa@...nel.org, tglx@...utronix.de, mingo@...hat.com, bp@...en8.de,
	dave.hansen@...ux.intel.com, x86@...nel.org, hpa@...or.com,
	kees@...nel.org, samitolvanen@...gle.com, rppt@...nel.org,
	luto@...nel.org, mhiramat@...nel.org, ast@...nel.org,
	andrii@...nel.org, linux-kernel@...r.kernel.org,
	bpf@...r.kernel.org
Subject: Re: [PATCH] x86/ibt: make is_endbr() notrace

On Thu, Sep 18, 2025 at 08:09:39PM +0800, Menglong Dong wrote:
> is_endbr() is called in __ftrace_return_to_handler -> fprobe_return ->
> kprobe_multi_link_exit_handler -> is_endbr.
> 
> It is not protected by the "bpf_prog_active", so it can't be traced by
> kprobe-multi, which can cause recurring and panic the kernel. Fix it by
> make it notrace.

This is very much a riddle wrapped in an enigma. Notably
kprobe_multi_link_exit_handler() does not call is_endbr(). Nor is that
cryptic next line sufficient to explain why its a problem.

I suspect the is_endbr() you did mean is the one in
arch_ftrace_get_symaddr(), but who knows.

Also, depending on compiler insanity, it is possible the thing
out-of-lines things like __is_endbr(), getting you yet another
__fentry__ site.

Please try again.