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

[Josh Poimboeuf <jpoimboe@redhat.com>]

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().

> However, patching it here means we poison it before all users are
> patched. I added this on top
> 
> diff --git a/kernel/static_call.c b/kernel/static_call.c
> index 599ebc6fc4f1..d9562329bec6 100644
> --- a/kernel/static_call.c
> +++ b/kernel/static_call.c
> @@ -248,6 +248,7 @@ static void __init static_call_init(void)
>         struct static_call_site *start = __start_static_call_sites;
>         struct static_call_site *stop  = __stop_static_call_sites;
>         struct static_call_site *site;
> +       struct static_call_key *prev_key = NULL;
> 
>         if (start == stop) {
>                 pr_warn("WARNING: empty static call table\n");
> @@ -279,7 +280,9 @@ static void __init static_call_init(void)
>                          * The trampoline should no longer be used.  Poison it
>                          * it with a BUG() to catch any stray callers.
>                          */
> -                       arch_static_call_poison_tramp(addr);
> +                       if (prev_key)
> +
> arch_static_call_poison_tramp((unsigned long)prev_key->func);
> +                       prev_key = key;
>                 }
> 
>                 arch_static_call_transform(addr, key->func);

While it does indeed poison the trampoline before all users are patched,
I had been thinking that it didn't really matter because this is before
the other CPUs have been booted.

But I believe interrupts are enabled at this point during the boot, so
it would indeed be wise to poison it afterwards, in case an irq handler
makes a static call.

-- 
Josh