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

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Message-ID: <20250922063616.GN3245006@noisy.programming.kicks-ass.net>
Date: Mon, 22 Sep 2025 08:36:16 +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 10:53:51AM -0700, Alexei Starovoitov wrote:

> > notrace don't help with kprobes in general, only with __fentry__ sites.
> 
> Are you sure ? Last time I checked "notrace" prevents kprobing
> anywhere in that function.

#define notrace                 __attribute__((__no_instrument_function__))

AFAICT that only inhibits the compiler from generating __fentry__ calls.

There is NOKPROBE_SYMBOL() to explicitly exclude kprobes from a symbol.

And we have noinstr, which along with a gazillion code-gen attributes
puts things in a .noinstr.text section and that section is also excluded
from kprobe placement.