x86/delay: Fix the wrong asm constraint in delay_loop()

[ Upstream commit b86eb74 ]

The asm constraint does not reflect the fact that the asm statement can
modify the value of the local variable loops. Which it does.

Specifying the wrong constraint may lead to undefined behavior, it may
clobber random stuff (e.g. local variable, important temporary value in
regs, etc.). This is especially dangerous when the compiler decides to
inline the function and since it doesn't know that the value gets
modified, it might decide to use it from a register directly without
reloading it.

Change the constraint to "+a" to denote that the first argument is an
input and an output argument.

  [ bp: Fix typo, massage commit message. ]

Fixes: e01b70e ("x86: fix bug in arch/i386/lib/delay.c file, delay_loop function")
Signed-off-by: Ammar Faizi <ammarfaizi2@gnuweeb.org>
Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lore.kernel.org/r/20220329104705.65256-2-ammarfaizi2@gnuweeb.org
Signed-off-by: Sasha Levin <sashal@kernel.org>

arch/x86/lib/delay.c

@@ -54,8 +54,8 @@ static void delay_loop(u64 __loops)
 		"	jnz 2b		\n"
 		"3:	dec %0		\n"
 
-		: /* we don't need output */
-		:"a" (loops)
+		: "+a" (loops)
+		:
 	);
 }