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From:   Andy Lutomirski <luto@amacapital.net>
Date:   Wed, 10 Oct 2018 13:57:05 -0700
Message-ID: <CALCETrXGP_m+Ak8JyWS0aG47tVo7X8+eXMht8JNwQ6V8fvTR1Q@mail.gmail.com>
Subject: Re: [POC][RFC][PATCH 1/2] jump_function: Addition of new feature "jump_function"
To:     Josh Poimboeuf <jpoimboe@redhat.com>
Cc:     Steven Rostedt <rostedt@goodmis.org>,
        Peter Zijlstra <peterz@infradead.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Ingo Molnar <mingo@kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Masami Hiramatsu <mhiramat@kernel.org>,
        Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
        Matthew Helsley <mhelsley@vmware.com>,
        "Rafael J. Wysocki" <rafael.j.wysocki@intel.com>,
        David Woodhouse <dwmw2@infradead.org>,
        Paolo Bonzini <pbonzini@redhat.com>,
        Jason Baron <jbaron@akamai.com>, Jiri Kosina <jkosina@suse.cz>,
        Ard Biesheuvel <ard.biesheuvel@linaro.org>,
        Andrew Lutomirski <luto@kernel.org>
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On Wed, Oct 10, 2018 at 1:16 PM Josh Poimboeuf <jpoimboe@redhat.com> wrote:
>
> On Wed, Oct 10, 2018 at 02:56:08PM -0400, Steven Rostedt wrote:
> > On Wed, 10 Oct 2018 13:33:30 -0500
> > Josh Poimboeuf <jpoimboe@redhat.com> wrote:
> >
> > > Re-reading your suggestion, I may have misunderstood what you're
> > > suggesting here, but I'm thinking about doing something like what you
> > > proposed earlier:
> > >
> > >     GLOBAL(tramp)
> > >       jmp *current_func(%rip)
> > >     ENDPROC(tramp)
> > >
> > > That is, doing an indirect jump instead of the above direct jump, so
> > > that any previous references to the trampoline would still work (and it
> > > would also work during early boot).
> > >
> > > Though it should probably be a retpoline instead of an indirect jump.
> >
> > But do we care, as it only takes place during text_poke_bp() right?
> >
> > I don't think we need to worry about training trampoline branch
> > prediction that can only be hit when something enables the jump.
>
> Yeah, I guess it depends on if we'd expect anybody (or gcc) to get a
> function pointer to the trampoline itself.  I can just create a warning
> for that in objtool.
>

The jmp * in the trampoline itself is harmless even with Spectre
because it won't ever execute -- static_call_init() should just patch
it out even if the actual call target is never updated.  And gcc has
no business generating any unprotected indirect branches to it from
anywhere else, since, as far as gcc is concerned, they're just like
indirect branches to any other function.