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

[Thomas Huth <thuth@redhat.com>]

On 11/09/2019 10.16, Andre Przywara wrote:
> On Tue, 10 Sep 2019 20:15:19 +0200
> Thomas Huth <thuth@redhat.com> wrote:
> 
> Hi,
> 
>> 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  
>>
>> Instead of adding it to both, Makefile.arm and Makefile.arm64, you could
>> also simply add it to Makefile.common instead.
> 
> But the arguments are not the same (admittedly against intuition)?
> I thought about defining arch_CFLAGS in both files, then adding that to Makefile.common, but didn't see the advantage over this straightforward approach here.

D'oh, never mind, I didn't read the patch properly. I somehow thought
that the arguments are the same. It's quite weird that the compiler
developers chose different names here...

 Thomas
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