On Fri, Nov 09, 2018 at 06:33:03PM +0100, Ard Biesheuvel wrote:
> On 9 November 2018 at 18:31, Josh Poimboeuf <jpoim...@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.biesheu...@linaro.org>
> >> wrote:
> >> > On 9 November 2018 at 16:10, Josh Poimboeuf <jpoim...@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
