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

[Ard Biesheuvel]

On 9 November 2018 at 18:52, Ard Biesheuvel <ard.biesheuvel@xxxxxxxxxx> wrote:
> On 9 November 2018 at 18:46, Josh Poimboeuf <jpoimboe@xxxxxxxxxx> wrote:
>> On Fri, Nov 09, 2018 at 06:33:03PM +0100, Ard Biesheuvel wrote:
>>> 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.
>>
>> Makes sense.  Do you also account for the possibility that the original
>> call emitted by GCC was far away and thus used the PLT?
>>
>
> That doesn't really happen. vmlinux is never over 128 MB, and the
> modules are only partially linked, so the linker never gets to the
> stage where it needs to emit veneers.
>
> It's a bit fiddly since inline and out-of-line both use
> arch_static_call_transform(), but what I need to do is basically:
>
> - for out-of-line, the trampoline needs to be patched into a
> movn/movk/movk/br sequence if the target is too far
> - for inline, the trampoline itself needs to be patched from
> adrp/ldr/br (which does a load and a indirect branch) to
> movn/movk/movk/br (which uses immediates), and the call sites need to
> be patched into calls to the veneer if the target is out of range.
>
> So arch_static_call_transform() needs to know where the trampoline is
> for this use case, so could we perhaps add a 'void *orig' in the key
> struct that keeps track of the original value of 'addr' ?

... and pass it to arch_static_call_transform