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 10:44:31 -0600
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        the arch/x86 maintainers <x86@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.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@...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 05:39:23PM +0100, Peter Zijlstra wrote:
> On Mon, Nov 26, 2018 at 05:11:05PM +0100, Ard Biesheuvel wrote:
> > On Mon, 26 Nov 2018 at 17:08, Peter Zijlstra <peterz@...radead.org> wrote:
> > >
> > > 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.
> > 
> > That is my fault.
> > 
> > The original name was 'poison' iirc, but on arm64, we need to retain
> > the trampoline for cases where the direct branch is out of range, and
> > so poisoning is semantically inaccurate.
> > 
> > But since you opened your dictionary anyway, any better suggestions? :-)
> 
> I was leaning towards: "prime", but I'm not entirely sure that works
> with your case.

Maybe we should just go back to "poison", along with a comment that it
will not necessarily be poisoned for all arches.  I think "poison" at
least describes the intent, if not always the implementation.

-- 
Josh