[RFC PATCH] arm64: ftrace: move ftrace-mod.o contents into linker script

[Will Deacon]

Hi Ard,

Thanks for this. Somebody ran across this internally a while back, but
it slipped through the cracks. I remember thinking at the time that the
Kconfig dependencies here looked a little weird too and perhaps
ARM64_MODULE_PLTS should be selected by RANDOMIZE_MODULE_REGION_FULL
instead of RANDOMIZE_BASE.

Anyhow, comments below.

On Mon, Nov 20, 2017 at 01:15:38PM +0000, Ard Biesheuvel wrote:
> When building the arm64 kernel with bot 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).

Agreed that we should avoid the object file, if possible.

> Given that the trampoline only consists of two instructions, let's
> hack it into the module linker script instead.

Why do we have to do this in the linker script? Couldn't we just generate
the .S file we currently use?

> diff --git a/arch/arm64/kernel/module-ftrace.lds b/arch/arm64/kernel/module-ftrace.lds
> new file mode 100644
> index 000000000000..9df69ddaa7bb
> --- /dev/null
> +++ b/arch/arm64/kernel/module-ftrace.lds
> @@ -0,0 +1,7 @@
> +SECTIONS {
> +	.text.ftrace_trampoline : ALIGN (8) {
> +		QUAD (0x0);		/* 0:	.quad	0x0		*/
> +		LONG (0x58ffffd0);	/*	ldr	x16, 0b		*/
> +		LONG (0xd61f0200);	/*	br	x16		*/
> +	}
> +}

How do we guarantee these are little-endian?

Will