mips: 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: Thomas Bogendoerfer <tsbogend@alpha.franken.de>
Cc: Marc Zyngier <maz@kernel.org>
Cc: Guenter Roeck <linux@roeck-us.net>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Yanteng Si <siyanteng01@gmail.com>
Cc: linux-mips@vger.kernel.org
Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>

arch/mips/Kconfig

@@ -4,6 +4,7 @@ config MIPS
 	default y
 	select ARCH_32BIT_OFF_T if !64BIT
 	select ARCH_BINFMT_ELF_STATE if MIPS_FP_SUPPORT
+	select ARCH_HAS_CURRENT_STACK_POINTER
 	select ARCH_HAS_DEBUG_VIRTUAL if !64BIT
 	select ARCH_HAS_FORTIFY_SOURCE
 	select ARCH_HAS_KCOV

arch/mips/include/asm/thread_info.h

@@ -69,6 +69,8 @@ static inline struct thread_info *current_thread_info(void)
 	return __current_thread_info;
 }
 
+register unsigned long current_stack_pointer __asm__("sp");
+
 #endif /* !__ASSEMBLY__ */
 
 /* thread information allocation */

arch/mips/kernel/irq.c

@@ -75,9 +75,8 @@ void __init init_IRQ(void)
 #ifdef CONFIG_DEBUG_STACKOVERFLOW
 static inline void check_stack_overflow(void)
 {
-	unsigned long sp;
+	unsigned long sp = current_stack_pointer;
 
-	__asm__ __volatile__("move %0, $sp" : "=r" (sp));
 	sp &= THREAD_MASK;
 
 	/*

arch/mips/lib/uncached.c

@@ -40,9 +40,7 @@ unsigned long run_uncached(void *func)
 	register long ret __asm__("$2");
 	long lfunc = (long)func, ufunc;
 	long usp;
-	long sp;
+	long sp = current_stack_pointer;
-
-	__asm__("move %0, $sp" : "=r" (sp));
 
 	if (sp >= (long)CKSEG0 && sp < (long)CKSEG2)
 		usp = CKSEG1ADDR(sp);