[PATCH 2/2] ARM: futex: make futex_detect_cmpxchg more reliable

[Arnd Bergmann <arnd@arndb.de>]

Passing registers containing zero as both the address (NULL pointer)
and data into cmpxchg_futex_value_locked() leads clang to assign
the same register for both inputs on ARM, which triggers a warning
explaining that this instruction has unpredictable behavior on ARMv5.

/tmp/futex-7e740e.s: Assembler messages:
/tmp/futex-7e740e.s:12713: Warning: source register same as write-back base

This patch was suggested by Mikael Pettersson back in 2011 (!) with gcc-4.4,
as Mikael wrote:
 "One way of fixing this is to make uaddr an input/output register, since
 "that prevents it from overlapping any other input or output."

but then withdrawn as the warning was determined to be harmless, and it
apparently never showed up again with later gcc versions.

Now the same problem is back when compiling with clang, and we are trying
to get clang to build the kernel without warnings, as gcc normally does.

Cc: Mikael Pettersson <mikpe@it.uu.se>
Cc: Mikael Pettersson <mikpelinux@gmail.com>
Cc: Dave Martin <Dave.Martin@arm.com>
Link: https://lore.kernel.org/linux-arm-kernel/20009.45690.158286.161591@pilspetsen.it.uu.se/
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
---
 arch/arm/include/asm/futex.h | 10 +++++-----
 1 file changed, 5 insertions(+), 5 deletions(-)

diff --git a/arch/arm/include/asm/futex.h b/arch/arm/include/asm/futex.h
index 0a46676b4245..79790912974e 100644
--- a/arch/arm/include/asm/futex.h
+++ b/arch/arm/include/asm/futex.h
@@ -110,13 +110,13 @@ futex_atomic_cmpxchg_inatomic(u32 *uval, u32 __user *uaddr,
 	preempt_disable();
 	__ua_flags = uaccess_save_and_enable();
 	__asm__ __volatile__("@futex_atomic_cmpxchg_inatomic\n"
-	"1:	" TUSER(ldr) "	%1, [%4]\n"
-	"	teq	%1, %2\n"
+	"1:	" TUSER(ldr) "	%1, [%2]\n"
+	"	teq	%1, %3\n"
 	"	it	eq	@ explicit IT needed for the 2b label\n"
-	"2:	" TUSER(streq) "	%3, [%4]\n"
+	"2:	" TUSER(streq) "	%4, [%2]\n"
 	__futex_atomic_ex_table("%5")
-	: "+r" (ret), "=&r" (val)
-	: "r" (oldval), "r" (newval), "r" (uaddr), "Ir" (-EFAULT)
+	: "+&r" (ret), "=&r" (val), "+&r" (uaddr)
+	: "r" (oldval), "r" (newval), "Ir" (-EFAULT)
 	: "cc", "memory");
 	uaccess_restore(__ua_flags);
 
-- 
2.20.0