linux-kernel - Re: [PATCH v2 4/4] x86/static_call: Add inline static call implementation for x86-64

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Date:   Mon, 26 Nov 2018 17:08:37 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
Cc:     x86@...nel.org, linux-kernel@...r.kernel.org,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Andy Lutomirski <luto@...nel.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ingo Molnar <mingo@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Jason Baron <jbaron@...mai.com>, Jiri Kosina <jkosina@...e.cz>,
        David Laight <David.Laight@...LAB.COM>,
        Borislav Petkov <bp@...en8.de>,
        Julia Cartwright <julia@...com>, Jessica Yu <jeyu@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH v2 4/4] x86/static_call: Add inline static call
 implementation for x86-64

On Mon, Nov 26, 2018 at 07:55:00AM -0600, Josh Poimboeuf wrote:
> +#ifdef CONFIG_HAVE_STATIC_CALL_INLINE
> +void arch_static_call_defuse_tramp(void *site, void *tramp)
> +{
> +	unsigned short opcode = INSN_UD2;
> +
> +	mutex_lock(&text_mutex);
> +	text_poke((void *)tramp, &opcode, 2);
> +	mutex_unlock(&text_mutex);
> +}
> +#endif

I would rather think that makes the trampoline _more_ dangerous, rather
than less so.

My dictionary sayeth:

  defuse: verb

    - remove the fuse from (an explosive device) in order to prevent it
      from exploding.

    - make (a situation) less tense or dangerous

patching in an UD2 seems to do the exact opposite.