Re: [PATCH 5/8] ARM64: asm: rewrite ENTRY_FUNCTION(_WITHSTACK) fully in assembly

[Sascha Hauer <sha@pengutronix.de>]

On Mon, Oct 24, 2022 at 08:57:13AM +0200, Ahmad Fatoum wrote:
> Recent episode with pointer authentication showed again that for
> platforms without __attribute__((naked)), we are better off writing
> the early header in assembly. We still want to keep the board specific
> entry points in C for ease of use, so we have ENTRY_FUNCTION_WITHSTACK
> generate two symbols:
> 
>   - A 32-bit stack top value that's placed in .rodata
> 
>   - An entry point with the normal C code, including stack-using
>     prologues
> 
> The new common assembly head code will access the stack pointer in a
> position-independent manner and set it up, before continuing with the
> C code. The barebox header is part of the common assembly head code
> ensuring it's not moved around due to compiler code generation.
> 
> The common code will need access to board-specific entry point and stack
> top. The former is readily available as the alias __pbl_board_entry.
> The latter is a bit more complicated, as the symbol may not exist for
> boards not using the common header in a multi-image build.
> 
> Signed-off-by: Ahmad Fatoum <a.fatoum@pengutronix.de>
> ---
>  Makefile                           |  3 +--
>  arch/arm/cpu/Makefile              |  2 ++
>  arch/arm/cpu/head_64.S             | 36 ++++++++++++++++++++++++++++++
>  arch/arm/include/asm/barebox-arm.h | 28 +++++++----------------
>  arch/arm/lib/pbl.lds.S             |  9 ++++++++
>  5 files changed, 56 insertions(+), 22 deletions(-)
>  create mode 100644 arch/arm/cpu/head_64.S
> 
> diff --git a/Makefile b/Makefile
> index 81ef44122367..a4b9f3c021d7 100644
> --- a/Makefile
> +++ b/Makefile
> @@ -660,8 +660,7 @@ KBUILD_CFLAGS  += $(call cc-option,-fno-stack-check)
>  KBUILD_CFLAGS += $(call cc-option,-fcf-protection=none)
>  
>  # We don't have the necessary infrastructure to benefit from ARMv8.3+ pointer
> -# authentication. On older CPUs, they are interpreted as NOPs and blot the
> -# code and break less portable code that expects a very specific code layout
> +# authentication. On older CPUs, they are interpreted as NOPs bloating the code
>  KBUILD_CFLAGS += $(call cc-option,-mbranch-protection=none)
>  
>  KBUILD_CFLAGS   += $(call cc-disable-warning, address-of-packed-member)
> diff --git a/arch/arm/cpu/Makefile b/arch/arm/cpu/Makefile
> index c0993c1abe5d..7674c1464c1f 100644
> --- a/arch/arm/cpu/Makefile
> +++ b/arch/arm/cpu/Makefile
> @@ -13,6 +13,8 @@ AFLAGS_hyp.pbl.o :=-Wa,-march=armv7-a -Wa,-mcpu=all
>  obj-y += start.o entry.o entry_ll$(S64).o
>  KASAN_SANITIZE_start.o := n
>  
> +pbl-$(CONFIG_CPU_64) += head_64.o
> +
>  pbl-$(CONFIG_BOARD_ARM_GENERIC_DT) += board-dt-2nd.o
>  pbl-$(CONFIG_BOARD_ARM_GENERIC_DT_AARCH64) += board-dt-2nd-aarch64.o
>  
> diff --git a/arch/arm/cpu/head_64.S b/arch/arm/cpu/head_64.S
> new file mode 100644
> index 000000000000..4ed4ffb05250
> --- /dev/null
> +++ b/arch/arm/cpu/head_64.S
> @@ -0,0 +1,36 @@
> +/* SPDX-License-Identifier: GPL-2.0-only */
> +#include <linux/linkage.h>
> +#include <asm/barebox-arm64.h>
> +#include <asm/image.h>
> +
> +/* Linker will point these at board-specific symbols */
> +.globl __pbl_board_stack_top
> +.globl __pbl_board_entry
> +
> +.section .text_head_prologue_common, "x"
> +ENTRY(__barebox_arm64_head)
> +	nop
> +	nop
> +	nop

Why these nops here?

> +	adr x9, __pbl_board_stack_top
> +	ldr w9, [x9]
> +	cbz x9, 1f
> +	mov sp, x9
> +	b 1f
> +	.org 0x20
> +	.asciz "barebox"
> +	.word 0xffffffff
> +	.word _barebox_image_size	/* image size to copy */
> +	.rept 8
> +	.word 0x55555555
> +	.endr
> +1:
> +#ifdef CONFIG_PBL_BREAK
> +	brk #17
> +	nop
> +#else
> +	nop
> +	nop
> +#endif

Why do you jump over the barebox header? You could put all the code
before it.

Sascha

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