Re: [RFC PATCH 1/3] static_call: Add static call infrastructure

[Ard Biesheuvel]

On 9 November 2018 at 18:31, Josh Poimboeuf <jpoimboe@xxxxxxxxxx> 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@xxxxxxxxxx> wrote:
>> > On 9 November 2018 at 16:10, Josh Poimboeuf <jpoimboe@xxxxxxxxxx> 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.