arm: userspace: fix initial user sp location

The initial user stack pointer gets rounded to STACK_ALIGN_SIZE,
which can be much finer granularity than MPU regions. With
certain stack size values passed to k_thread_create(), the stack
pointer can be set past the defined region for the user thread's
stack, causing an immediate MPU fault.

Signed-off-by: Andrew Boie <andrew.p.boie@intel.com>

arch/arm/core/thread.c

@@ -134,6 +134,12 @@ FUNC_NORETURN void _arch_user_mode_enter(k_thread_entry_t user_entry,
 	_current->arch.priv_stack_size =
 		(u32_t)CONFIG_PRIVILEGED_STACK_SIZE;
 
+	/* FIXME: Need a general API for aligning stacks so thet the initial
+	 * user thread stack pointer doesn't overshoot the granularity of MPU
+	 * regions, that works for ARM/NXP/QEMU.
+	 */
+	_current->stack_info.size &= ~0x1f;
+
 	_arm_userspace_enter(user_entry, p1, p2, p3,
 			     (u32_t)_current->stack_info.start,
 			     _current->stack_info.size);