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

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On Tue, 10 Sep 2019 20:15:19 +0200
Thomas Huth <thuth@xxxxxxxxxx> 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@xxxxxxx>
> > 
> > [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.

Cheers,
Andre.



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