s390: current_stack_pointer shouldn't be a function

s390 defines current_stack_pointer as function while all other
architectures use 'register unsigned long asm("<stackptr reg>").

This make codes like the following from check_stack_object() fail:

	if (IS_ENABLED(CONFIG_STACK_GROWSUP)) {
		if ((void *)current_stack_pointer < obj + len)
			return BAD_STACK;
	} else {
		if (obj < (void *)current_stack_pointer)
			return BAD_STACK;
	}

because this would compare the address of current_stack_pointer() and
not the stackpointer value.
Reported-by: Karsten Graul <kgraul@linux.ibm.com>
Fixes: 2792d84e ("usercopy: Check valid lifetime via stack depth")
Cc: Kees Cook <keescook@chromium.org>
Cc: Vasily Gorbik <gor@linux.ibm.com>
Cc: Alexander Gordeev <agordeev@linux.ibm.com>
Signed-off-by: Sven Schnelle <svens@linux.ibm.com>
Reviewed-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>

arch/s390/include/asm/entry-common.h

@@ -58,7 +58,7 @@ static inline void arch_exit_to_user_mode_prepare(struct pt_regs *regs,
 
 static inline bool on_thread_stack(void)
 {
-	return !(((unsigned long)(current->stack) ^ current_stack_pointer()) & ~(THREAD_SIZE - 1));
+	return !(((unsigned long)(current->stack) ^ current_stack_pointer) & ~(THREAD_SIZE - 1));
 }
 
 #endif

arch/s390/include/asm/processor.h

@@ -200,13 +200,7 @@ unsigned long __get_wchan(struct task_struct *p);
 /* Has task runtime instrumentation enabled ? */
 #define is_ri_task(tsk) (!!(tsk)->thread.ri_cb)
 
-static __always_inline unsigned long current_stack_pointer(void)
-{
-	unsigned long sp;
-
-	asm volatile("la %0,0(15)" : "=a" (sp));
-	return sp;
-}
+register unsigned long current_stack_pointer asm("r15");
 
 static __always_inline unsigned short stap(void)
 {

arch/s390/include/asm/stacktrace.h

@@ -46,7 +46,7 @@ struct stack_frame {
 };
 
 /*
- * Unlike current_stack_pointer() which simply returns current value of %r15
+ * Unlike current_stack_pointer which simply contains the current value of %r15
  * current_frame_address() returns function stack frame address, which matches
  * %r15 upon function invocation. It may differ from %r15 later if function
  * allocates stack for local variables or new stack frame to call other

arch/s390/lib/test_unwind.c

@@ -147,7 +147,7 @@ static __always_inline struct pt_regs fake_pt_regs(void)
 	struct pt_regs regs;
 
 	memset(&regs, 0, sizeof(regs));
-	regs.gprs[15] = current_stack_pointer();
+	regs.gprs[15] = current_stack_pointer;
 
 	asm volatile(
 		"basr	%[psw_addr],0\n"