xtensa: Implement "current_stack_pointer"

To follow the existing per-arch conventions replace open-coded uses
of asm "sp" as "current_stack_pointer". This will let it be used in
non-arch places (like HARDENED_USERCOPY).

Cc: Chris Zankel <chris@zankel.net>
Cc: Marc Zyngier <maz@kernel.org>
Cc: linux-xtensa@linux-xtensa.org
Signed-off-by: Kees Cook <keescook@chromium.org>
Acked-by: Max Filippov <jcmvbkbc@gmail.com>
Link: https://lore.kernel.org/lkml/CAMo8BfJFJE-n3=AF+pb9_6oF3gzxX7a+7aBrASHjjNX5byqDqw@mail.gmail.com

arch/xtensa/Kconfig

@@ -3,6 +3,7 @@ config XTENSA
 	def_bool y
 	select ARCH_32BIT_OFF_T
 	select ARCH_HAS_BINFMT_FLAT if !MMU
+	select ARCH_HAS_CURRENT_STACK_POINTER
 	select ARCH_HAS_DMA_PREP_COHERENT if MMU
 	select ARCH_HAS_SYNC_DMA_FOR_CPU if MMU
 	select ARCH_HAS_SYNC_DMA_FOR_DEVICE if MMU

arch/xtensa/include/asm/current.h

@@ -26,6 +26,8 @@ static inline struct task_struct *get_current(void)
 
 #define current get_current()
 
+register unsigned long current_stack_pointer __asm__("a1");
+
 #else
 
 #define GET_CURRENT(reg,sp)		\

arch/xtensa/include/asm/stacktrace.h

@@ -19,14 +19,14 @@ struct stackframe {
 
 static __always_inline unsigned long *stack_pointer(struct task_struct *task)
 {
-	unsigned long *sp;
+	unsigned long sp;
 
 	if (!task || task == current)
-		__asm__ __volatile__ ("mov %0, a1\n" : "=a"(sp));
+		sp = current_stack_pointer;
 	else
-		sp = (unsigned long *)task->thread.sp;
+		sp = task->thread.sp;
 
-	return sp;
+	return (unsigned long *)sp;
 }
 
 void walk_stackframe(unsigned long *sp,

arch/xtensa/kernel/irq.c

@@ -36,9 +36,8 @@ asmlinkage void do_IRQ(int hwirq, struct pt_regs *regs)
 #ifdef CONFIG_DEBUG_STACKOVERFLOW
 	/* Debugging check for stack overflow: is there less than 1KB free? */
 	{
-		unsigned long sp;
+		unsigned long sp = current_stack_pointer;
 
-		__asm__ __volatile__ ("mov %0, a1\n" : "=a" (sp));
 		sp &= THREAD_SIZE - 1;
 
 		if (unlikely(sp < (sizeof(thread_info) + 1024)))