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

[Ard Biesheuvel]

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.