[PATCH] arm: kprobes: Align stack to 8-bytes in test code

[tixy@linaro.org (Jon Medhurst (Tixy))]

On Fri, 2017-03-17 at 12:10 +0000, Russell King - ARM Linux wrote:
> On Thu, Mar 16, 2017 at 01:53:59PM +0000, Jon Medhurst wrote:
> > Without this fix, some test cases will generate alignment faults on
> > systems where alignment is enforced. Even if the kernel is configured to
> > handle these faults in software, triggering them is ugly. It also
> > exposes limitations in the fault handling code which doesn't cope with
> > writes to the stack. E.g. when handling this instruction
> > 
> >    strd r6, [sp, #-64]!
> > 
> > the fault handling code will write to a stack location below the SP
> > value at the point the fault occurred, which coincides with where the
> > exception handler has pushed the saved register context. This results in
> > corruption of those registers.
> 
> The general rule today is that the stack must always be 64-bit aligned,
> so an even number of registers must always be pushed to the stack.
> 
> > diff --git a/arch/arm/probes/kprobes/test-core.c b/arch/arm/probes/kprobes/test-core.c
> > index c893726aa52d..1c98a87786ca 100644
> > --- a/arch/arm/probes/kprobes/test-core.c
> > +++ b/arch/arm/probes/kprobes/test-core.c
> > @@ -977,7 +977,10 @@ static void coverage_end(void)
> >  void __naked __kprobes_test_case_start(void)
> >  {
> >  	__asm__ __volatile__ (
> > -		"stmdb	sp!, {r4-r11}				\n\t"
> > +		"mov	r2, sp					\n\t"
> > +		"bic	r3, r2, #7				\n\t"
> > +		"mov	sp, r3					\n\t"
> > +		"stmdb	sp!, {r2-r11}				\n\t"
> 
> I'm not sure these is where the problem is - on entry, the stack
> should be 64-bit aligned

It isn't, because GCC turns code like this

void foo(void)
{
        asm volatile("bl __kprobes_test_case_start"
                     : : : "r0", "r1", "r2", "r3", "ip", "lr", "memory", "cc");
}

into this...

8010e4ac <foo>:
8010e4ac:       e52de004        push    {lr}            ; (str lr, [sp, #-4]!)
8010e4b0:       eb002c99        bl      8011971c <__kprobes_test_case_start>
8010e4b4:       e49df004        pop     {pc}            ; (ldr pc, [sp], #4)

Perhaps we need a way of telling GCC we are using the stack but I've not
managed to spot a way of doing that.

-- 
Tixy