Re: [RFC PATCH 05/28] x86: Define the stack protector guard symbol explicitly

[Uros Bizjak]

On Wed, Sep 25, 2024 at 5:02 PM Ard Biesheuvel <ardb+git@xxxxxxxxxx> wrote:
>
> From: Ard Biesheuvel <ardb@xxxxxxxxxx>
>
> Specify the guard symbol for the stack cookie explicitly, rather than
> positioning it exactly 40 bytes into the per-CPU area. Doing so removes
> the need for the per-CPU region to be absolute rather than relative to
> the placement of the per-CPU template region in the kernel image, and
> this allows the special handling for absolute per-CPU symbols to be
> removed entirely.
>
> This is a worthwhile cleanup in itself, but it is also a prerequisite
> for PIE codegen and PIE linking, which can replace our bespoke and
> rather clunky runtime relocation handling.
>
> Signed-off-by: Ard Biesheuvel <ardb@xxxxxxxxxx>
> ---
>  arch/x86/Makefile                     |  4 ++++
>  arch/x86/include/asm/init.h           |  2 +-
>  arch/x86/include/asm/processor.h      | 11 +++--------
>  arch/x86/include/asm/stackprotector.h |  4 ----
>  tools/perf/util/annotate.c            |  4 ++--
>  5 files changed, 10 insertions(+), 15 deletions(-)
>
> diff --git a/arch/x86/Makefile b/arch/x86/Makefile
> index 6b3fe6e2aadd..b78b7623a4a9 100644
> --- a/arch/x86/Makefile
> +++ b/arch/x86/Makefile
> @@ -193,6 +193,10 @@ else
>          KBUILD_RUSTFLAGS += -Cno-redzone=y
>          KBUILD_RUSTFLAGS += -Ccode-model=kernel
>
> +        ifeq ($(CONFIG_STACKPROTECTOR),y)
> +                KBUILD_CFLAGS += -mstack-protector-guard-symbol=fixed_percpu_data

Looking at:

> +        * Since the irq_stack is the object at %gs:0, the bottom 8 bytes of
> +        * the irq stack are reserved for the canary.

Please note that %gs:0 can also be achieved with
-mstack-protector-guard-offset=0

Uros.