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Date:   Fri, 9 Nov 2018 11:46:08 -0600
From:   Josh Poimboeuf <jpoimboe@redhat.com>
To:     Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc:     Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        the arch/x86 maintainers <x86@kernel.org>,
        Andy Lutomirski <luto@kernel.org>,
        Steven Rostedt <rostedt@goodmis.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Ingo Molnar <mingo@kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Masami Hiramatsu <mhiramat@kernel.org>,
        Jason Baron <jbaron@akamai.com>, Jiri Kosina <jkosina@suse.cz>,
        David Laight <David.Laight@aculab.com>,
        Borislav Petkov <bp@alien8.de>
Subject: Re: [RFC PATCH 1/3] static_call: Add static call infrastructure
Message-ID: <20181109174608.eahqh4fkyl3k2gvs@treble>
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On Fri, Nov 09, 2018 at 06:33:03PM +0100, Ard Biesheuvel wrote:
> On 9 November 2018 at 18:31, Josh Poimboeuf <jpoimboe@redhat.com> wrote:
> > On Fri, Nov 09, 2018 at 06:25:24PM +0100, Ard Biesheuvel wrote:
> >> On 9 November 2018 at 16:14, Ard Biesheuvel <ard.biesheuvel@linaro.org> wrote:
> >> > On 9 November 2018 at 16:10, Josh Poimboeuf <jpoimboe@redhat.com> wrote:
> >> >> On Fri, Nov 09, 2018 at 02:39:17PM +0100, Ard Biesheuvel wrote:
> >> >>> > +       for (site = start; site < stop; site++) {
> >> >>> > +               struct static_call_key *key = static_call_key(site);
> >> >>> > +               unsigned long addr = static_call_addr(site);
> >> >>> > +
> >> >>> > +               if (list_empty(&key->site_mods)) {
> >> >>> > +                       struct static_call_mod *mod;
> >> >>> > +
> >> >>> > +                       mod = kzalloc(sizeof(*mod), GFP_KERNEL);
> >> >>> > +                       if (!mod) {
> >> >>> > +                               WARN(1, "Failed to allocate memory for static calls");
> >> >>> > +                               return;
> >> >>> > +                       }
> >> >>> > +
> >> >>> > +                       mod->sites = site;
> >> >>> > +                       list_add_tail(&mod->list, &key->site_mods);
> >> >>> > +
> >> >>> > +                       /*
> >> >>> > +                        * The trampoline should no longer be used.  Poison it
> >> >>> > +                        * it with a BUG() to catch any stray callers.
> >> >>> > +                        */
> >> >>> > +                       arch_static_call_poison_tramp(addr);
> >> >>>
> >> >>> This patches the wrong thing: the trampoline is at key->func not addr.
> >> >>
> >> >> If you look at the x86 implementation, it actually does poison the
> >> >> trampoline.
> >> >>
> >> >> The address of the trampoline isn't actually known here.  key->func
> >> >> isn't the trampoline address; it's the destination func address.
> >> >>
> >> >> So instead I passed the address of the call instruction.  The arch code
> >> >> then reads the instruction to find the callee (the trampoline).
> >> >>
> >> >> The code is a bit confusing.  To make it more obvious, maybe we should
> >> >> add another arch function to read the call destination.  Then this code
> >> >> can pass that into arch_static_call_poison_tramp().
> >> >>
> >> >
> >> > Ah right, so I am basically missing a dereference in my
> >> > arch_static_call_poison_tramp() code if this breaks.
> >> >
> >>
> >> Could we call it 'defuse' rather than 'poision'? On arm64, we will
> >> need to keep it around to bounce function calls that are out of range,
> >> and replace it with a PLT sequence.
> >
> > Ok, but doesn't that defeat the purpose of the inline approach?
> >
> 
> It does. But this only occurs when a module is loaded far away, and
> this will only happen if you have 2 GB range KASLR enabled, or your
> 128 MB module region gets exhausted for some reason, so the majority
> of calls should use a single relative branch.

Makes sense.  Do you also account for the possibility that the original
call emitted by GCC was far away and thus used the PLT?

-- 
Josh