Re: [PATCH v3 08/19] x86/head64: Replace pointer fixups with PIE codegen

Borislav Petkov
2024-02-12T10:29:31Z

On Mon, Jan 29, 2024 at 07:05:11PM +0100, Ard Biesheuvel wrote:
> From: Ard Biesheuvel <ardb@kernel.org>
>
> Some of the C code in head64.c may be called from a different virtual
> address than it was linked at. Currently, we deal with this by using

Yeah, make passive pls: "Currently, this is done by using... "

> ordinary, position dependent codegen, and fixing up all symbol
> references on the fly. This is fragile and tricky to maintain. It is
> also unnecessary: we can use position independent codegen (with hidden
^^^
Ditto: "use ..."

In the comments below too, pls, where it says "we".

> visibility) to ensure that all compiler generated symbol references are
> RIP-relative, removing the need for fixups entirely.
>
> It does mean we need explicit references to kernel virtual addresses to
> be generated by hand, so generate those using a movabs instruction in
> inline asm in the handful places where we actually need this.
>
> Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
> ---
> arch/x86/Makefile | 8 ++
> arch/x86/boot/compressed/Makefile | 2 +-
> arch/x86/include/asm/desc.h | 3 +-
> arch/x86/include/asm/setup.h | 4 +-
> arch/x86/kernel/Makefile | 5 ++
> arch/x86/kernel/head64.c | 88 +++++++-------------
> arch/x86/kernel/head_64.S | 5 +-
> 7 files changed, 51 insertions(+), 64 deletions(-)
>
> diff --git a/arch/x86/Makefile b/arch/x86/Makefile
> index 1a068de12a56..2b5954e75318 100644
> --- a/arch/x86/Makefile
> +++ b/arch/x86/Makefile
> @@ -168,6 +168,14 @@ else
> KBUILD_CFLAGS += -mcmodel=kernel
> KBUILD_RUSTFLAGS += -Cno-redzone=y
> KBUILD_RUSTFLAGS += -Ccode-model=kernel
> +
> + PIE_CFLAGS-$(CONFIG_STACKPROTECTOR) += -fno-stack-protector

Main Makefile has

KBUILD_CFLAGS += -fno-PIE

and this ends up being:

gcc -Wp,-MMD,arch/x86/kernel/.head64.s.d -nostdinc ... -fno-PIE ... -fpie ... -fverbose-asm -S -o arch/x86/kernel/head64.s arch/x86/kernel/head64.c

Can you pls remove -fno-PIE from those TUs which use PIE_CFLAGS so that
there's no confusion when staring at V=1 output?

> + PIE_CFLAGS-$(CONFIG_LTO) += -fno-lto
> +
> + PIE_CFLAGS := -fpie -mcmodel=small $(PIE_CFLAGS-y) \
> + -include $(srctree)/include/linux/hidden.h
> +
> + export PIE_CFLAGS
> endif
>
> #

Other than that, that code becomes much more readable, cool!

Thx.

--
Regards/Gruss,
Boris.