nospec: Move array_index_nospec() parameter checking into separate macro

For architectures providing their own implementation of
array_index_mask_nospec() in asm/barrier.h, attempting to use WARN_ONCE() to
complain about out-of-range parameters using WARN_ON() results in a mess
of mutually-dependent include files.

Rather than unpick the dependencies, simply have the core code in nospec.h
perform the checking for us.

Signed-off-by: Will Deacon <will.deacon@arm.com>
Acked-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Dan Williams <dan.j.williams@intel.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/1517840166-15399-1-git-send-email-will.deacon@arm.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

include/linux/nospec.h

@@ -20,6 +20,16 @@ static inline unsigned long array_index_mask_nospec(unsigned long index,
 						    unsigned long size)
 {
 	/*
+	 * Always calculate and emit the mask even if the compiler
+	 * thinks the mask is not needed. The compiler does not take
+	 * into account the value of @index under speculation.
+	 */
+	OPTIMIZER_HIDE_VAR(index);
+	return ~(long)(index | (size - 1UL - index)) >> (BITS_PER_LONG - 1);
+}
+#endif
+
+/*
  * Warn developers about inappropriate array_index_nospec() usage.
  *
  * Even if the CPU speculates past the WARN_ONCE branch, the
@@ -29,19 +39,15 @@ static inline unsigned long array_index_mask_nospec(unsigned long index,
  * This warning is compiled out when the compiler can infer that
  * @index and @size are less than LONG_MAX.
  */
-	if (WARN_ONCE(index > LONG_MAX || size > LONG_MAX,
-			"array_index_nospec() limited to range of [0, LONG_MAX]\n"))
-		return 0;
-
-	/*
-	 * Always calculate and emit the mask even if the compiler
-	 * thinks the mask is not needed. The compiler does not take
-	 * into account the value of @index under speculation.
-	 */
-	OPTIMIZER_HIDE_VAR(index);
-	return ~(long)(index | (size - 1UL - index)) >> (BITS_PER_LONG - 1);
-}
-#endif
+#define array_index_mask_nospec_check(index, size)				\
+({										\
+	if (WARN_ONCE(index > LONG_MAX || size > LONG_MAX,			\
+	    "array_index_nospec() limited to range of [0, LONG_MAX]\n"))	\
+		_mask = 0;							\
+	else									\
+		_mask = array_index_mask_nospec(index, size);			\
+	_mask;									\
+})
 
 /*
  * array_index_nospec - sanitize an array index after a bounds check
@@ -61,7 +67,7 @@ static inline unsigned long array_index_mask_nospec(unsigned long index,
 ({									\
 	typeof(index) _i = (index);					\
 	typeof(size) _s = (size);					\
-	unsigned long _mask = array_index_mask_nospec(_i, _s);		\
+	unsigned long _mask = array_index_mask_nospec_check(_i, _s);	\
 									\
 	BUILD_BUG_ON(sizeof(_i) > sizeof(long));			\
 	BUILD_BUG_ON(sizeof(_s) > sizeof(long));			\