Re: [PATCH kvm-unit-tests] arm: prevent compiler from using unaligned accesses

[Thomas Huth <thuth@redhat.com>]

On 05/09/2019 19.15, Andre Przywara wrote:
> The ARM architecture requires all accesses to device memory to be
> naturally aligned[1][2]. Normal memory does not have this strict
> requirement, and in fact many systems do ignore unaligned accesses
> (by the means of clearing the A bit in SCTLR and accessing normal
> memory). So the default behaviour of GCC assumes that unaligned accesses
> are fine, at least if happening on the stack.
> 
> Now kvm-unit-tests runs some C code with the MMU off, which degrades the
> whole system memory to device memory. Now every unaligned access will
> fault, regardless of the A bit.
> In fact there is at least one place in lib/printf.c where GCC merges
> two consecutive char* accesses into one "strh" instruction, writing to
> a potentially unaligned address.
> This can be reproduced by configuring kvm-unit-tests for kvmtool, but
> running it on QEMU, which triggers an early printf that exercises this
> particular code path.
> 
> Add the -mstrict-align compiler option to the arm64 CFLAGS to fix this
> problem. Also add the respective -mno-unaligned-access flag for arm.
> 
> Thanks to Alexandru for helping debugging this.
> 
> Signed-off-by: Andre Przywara <andre.przywara@arm.com>
> 
> [1] ARMv8 ARM DDI 0487E.a, B2.5.2
> [2] ARMv7 ARM DDI 0406C.d, A3.2.1
> ---
>  arm/Makefile.arm   | 1 +
>  arm/Makefile.arm64 | 1 +
>  2 files changed, 2 insertions(+)
> 
> diff --git a/arm/Makefile.arm b/arm/Makefile.arm
> index a625267..43b4be1 100644
> --- a/arm/Makefile.arm
> +++ b/arm/Makefile.arm
> @@ -12,6 +12,7 @@ KEEP_FRAME_POINTER := y
>  
>  CFLAGS += $(machine)
>  CFLAGS += -mcpu=$(PROCESSOR)
> +CFLAGS += -mno-unaligned-access
>  
>  arch_LDFLAGS = -Ttext=40010000
>  
> diff --git a/arm/Makefile.arm64 b/arm/Makefile.arm64
> index 02c24e8..35de5ea 100644
> --- a/arm/Makefile.arm64
> +++ b/arm/Makefile.arm64
> @@ -7,6 +7,7 @@ bits = 64
>  ldarch = elf64-littleaarch64
>  
>  arch_LDFLAGS = -pie -n
> +CFLAGS += -mstrict-align
>  
>  define arch_elf_check =
>  	$(if $(shell ! $(OBJDUMP) -R $(1) >&/dev/null && echo "nok"),
> 

FWIW (after finally reading the patch properly ;-)) :
Reviewed-by: Thomas Huth <thuth@redhat.com>
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