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

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Message-ID: <20250918165935.GB3409427@noisy.programming.kicks-ass.net>
Date: Thu, 18 Sep 2025 18:59:35 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc: Menglong Dong <menglong8.dong@...il.com>, Jiri Olsa <jolsa@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
	Dave Hansen <dave.hansen@...ux.intel.com>, X86 ML <x86@...nel.org>,
	"H. Peter Anvin" <hpa@...or.com>, Kees Cook <kees@...nel.org>,
	Sami Tolvanen <samitolvanen@...gle.com>,
	Mike Rapoport <rppt@...nel.org>, Andy Lutomirski <luto@...nel.org>,
	Masami Hiramatsu <mhiramat@...nel.org>,
	Alexei Starovoitov <ast@...nel.org>,
	Andrii Nakryiko <andrii@...nel.org>,
	LKML <linux-kernel@...r.kernel.org>, bpf <bpf@...r.kernel.org>
Subject: Re: [PATCH] x86/ibt: make is_endbr() notrace

On Thu, Sep 18, 2025 at 09:02:31AM -0700, Alexei Starovoitov wrote:
> On Thu, Sep 18, 2025 at 6:32???AM Menglong Dong <menglong8.dong@...il.com> wrote:
> >
> > On Thu, Sep 18, 2025 at 9:05???PM Peter Zijlstra <peterz@...radead.org> wrote:
> > >
> > > 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.
> >
> > Yeah, I mean
> > kprobe_multi_link_exit_handler -> ftrace_get_entry_ip ->
> > arch_ftrace_get_symaddr -> is_endbr
> > actually. And CONFIG_X86_KERNEL_IBT is enabled of course.
> 
> All this makes sense to me.

As written down, I'm still clueless.

> __noendbr bool is_endbr(u32 *val) needs "notrace",
> since it's in alternative.c and won't get inlined (unless LTO+luck).

notrace don't help with kprobes in general, only with __fentry__ sites.

I've still not clue why there is a panic, or why notrace would be
sufficient.