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

[Ahmad Fatoum <a.fatoum@pengutronix.de>]

Hello Sascha,


On 24.10.22 10:55, Sascha Hauer wrote:
> 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?

Perhaps we decide to place something there in future.
For example, `ccmp    x18, #0, #0xd, pl` is a fancy nop
whose first 2 bytes are the ELF/PE magic 'MZ' when read in
little-endian.

For now, I left it as normal nops. The .org 20 below is
not (only) for alignment, but as a sanity check, if we
add too much code 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.

I want to keep at the same location as ARM32.

Cheers,
Ahmad


> 
> Sascha
> 


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