[PATCH v3 2/2] arm64: ftrace: emit ftrace-mod.o contents through code

[Riku Voipio]

On 28 November 2017 at 20:37, Ard Biesheuvel <ard.biesheuvel at linaro.org> wrote:
> On 28 November 2017 at 18:31, Will Deacon <will.deacon at arm.com> wrote:
>> On Mon, Nov 20, 2017 at 05:41:30PM +0000, Ard Biesheuvel wrote:
>>> When building the arm64 kernel with both CONFIG_ARM64_MODULE_PLTS and
>>> CONFIG_DYNAMIC_FTRACE enabled, the ftrace-mod.o object file is built
>>> with the kernel and contains a trampoline that is linked into each
>>> module, so that modules can be loaded far away from the kernel and
>>> still reach the ftrace entry point in the core kernel with an ordinary
>>> relative branch, as is emitted by the compiler instrumentation code
>>> dynamic ftrace relies on.
>>>
>>> In order to be able to build out of tree modules, this object file
>>> needs to be included into the linux-headers or linux-devel packages,
>>> which is undesirable, as it makes arm64 a special case (although a
>>> precedent does exist for 32-bit PPC).
>>>
>>> Given that the trampoline essentially consists of a PLT entry, let's
>>> not bother with a source or object file for it, and simply patch it
>>> in whenever the trampoline is being populated, using the existing
>>> PLT support routines.
>>
>> I'll pick these two up for 4.15. Do you think they need to go to stable as
>> well?

> It is either that, or the distros need to backport themselves. Not
> sure whether it matters much tbh.

Via stable is definitely preferred. Else each distribution using 4.14
needs to independently find out why out-of-tree modules fail to build
and what are the patches needed to fix it.

Riku