[PATCH v2 2/2] arm64: ftrace: add support for far branches to dynamic ftrace

[Ard Biesheuvel]

On 7 June 2017 at 15:46, Steven Rostedt <rostedt at goodmis.org> wrote:
> On Mon, 5 Jun 2017 18:15:35 +0100
> Will Deacon <will.deacon at arm.com> wrote:
>
>
>> > +           tramp = (unsigned long *)mod->arch.ftrace_trampoline->sh_addr;
>> > +           if (tramp[0] != addr) {
>> > +                   if (tramp[0] != 0) {
>> > +                           pr_err("ftrace: far branches to multiple entry points unsupported inside a single module\n");
>> > +                           return AARCH64_BREAK_FAULT;
>> > +                   }
>> > +
>> > +                   /* point the trampoline to our ftrace entry point */
>> > +                   module_disable_ro(mod);
>> > +                   tramp[0] = addr;
>> > +                   module_enable_ro(mod, true);
>>
>> I'm not sure what the barrier semantics are for module_enable_ro, but I'd be
>> inclined to stick in a smp_wmb() here to order the write of the trampoline
>> data before the writing of the branch instruction.
>
> I would assume that module_disable/enable_ro() has proper barriers for
> modifying the page tables with respect to code around it, otherwise it
> would probably be an issues elsewhere in the kernel. Specifically in
> the module code itself.
>
> I don't see how a smp_wmb() would be needed here, especially since this
> is serialized code, and not something done by multiple CPUs.
>

But other cores could be invoking the function we are patching here,
no? So when such a core observes (and executes) the updated
instruction before it observes the updated target field of the
trampoline, it will branch to address 0x0.