[ARM] 3102/1: ARM EABI: stack pointer must be 64-bit aligned after a CPU exception

Patch from Nicolas Pitre

The ARM EABI says that the stack pointer has to be 64-bit aligned for
reasons already mentioned in patch #3101 when calling C functions.

We therefore must verify and adjust sp accordingly when taking an
exception from kernel mode since sp might not necessarily be 64-bit
aligned if the exception occurs in the middle of a kernel function.

If the exception occurs while in user mode then no sp fixup is needed as
long as sizeof(struct pt_regs) as well as any additional syscall data
stack space remain multiples of 8.

Signed-off-by: Nicolas Pitre <nico@cam.org>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>

arch/arm/kernel/entry-header.S

@@ -19,6 +19,7 @@
 @
 @ Most of the stack format comes from struct pt_regs, but with
 @ the addition of 8 bytes for storing syscall args 5 and 6.
+@ This _must_ remain a multiple of 8 for EABI.
 @
 #define S_OFF		8