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

[Josh Poimboeuf]

On Fri, Nov 09, 2018 at 11:31:06AM -0600, Josh Poimboeuf 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?

Or are you only going to use the trampoline for out-of-range calls,
otherwise just do direct calls?

-- 
Josh