[PATCH v3 1/2] minmax: sanity check constant bounds when clamping

["Jason A. Donenfeld" <Jason@zx2c4.com>]

The clamp family of functions only makes sense if hi>=lo. If hi and lo
are compile-time constants, then raise a build error. Doing so has
already caught buggy code. This also introduces the infrastructure to
improve the clamping function in subsequent commits.

Cc: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Kees Cook <keescook@chromium.org>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
---
 include/linux/minmax.h | 26 ++++++++++++++++++++++++--
 1 file changed, 24 insertions(+), 2 deletions(-)

diff --git a/include/linux/minmax.h b/include/linux/minmax.h
index 5433c08fcc68..293a66ad7830 100644
--- a/include/linux/minmax.h
+++ b/include/linux/minmax.h
@@ -37,6 +37,28 @@
 		__cmp(x, y, op), \
 		__cmp_once(x, y, __UNIQUE_ID(__x), __UNIQUE_ID(__y), op))
 
+#define __clamp(val, lo, hi)							\
+	__cmp(__cmp(val, lo, >), hi, <)
+
+#define __clamp_once(val, lo, hi, unique_val, unique_lo, unique_hi) ({		\
+		typeof(val) unique_val = (val);					\
+		typeof(lo) unique_lo = (lo);					\
+		typeof(hi) unique_hi = (hi);					\
+		__clamp(unique_val, unique_lo, unique_hi); })
+
+#define __clamp_input_check(lo, hi)						\
+        (BUILD_BUG_ON_ZERO(__builtin_choose_expr(				\
+                __is_constexpr((lo) > (hi)), (lo) > (hi), false)))
+
+#define __careful_clamp(val, lo, hi) ({						\
+	__clamp_input_check(lo, hi) + 						\
+	__builtin_choose_expr(__typecheck(val, lo) && __typecheck(val, hi) &&	\
+			      __typecheck(hi, lo) && __is_constexpr(val) && 	\
+			      __is_constexpr(lo) && __is_constexpr(hi),		\
+		__clamp(val, lo, hi),						\
+		__clamp_once(val, lo, hi, __UNIQUE_ID(__val),			\
+			     __UNIQUE_ID(__lo), __UNIQUE_ID(__hi))); })
+
 /**
  * min - return minimum of two values of the same or compatible types
  * @x: first value
@@ -86,7 +108,7 @@
  * This macro does strict typechecking of @lo/@hi to make sure they are of the
  * same type as @val.  See the unnecessary pointer comparisons.
  */
-#define clamp(val, lo, hi) min((typeof(val))max(val, lo), hi)
+#define clamp(val, lo, hi) __careful_clamp(val, lo, hi)
 
 /*
  * ..and if you can't take the strict
@@ -121,7 +143,7 @@
  * This macro does no typechecking and uses temporary variables of type
  * @type to make all the comparisons.
  */
-#define clamp_t(type, val, lo, hi) min_t(type, max_t(type, val, lo), hi)
+#define clamp_t(type, val, lo, hi) __careful_clamp((type)(val), (type)(lo), (type)(hi))
 
 /**
  * clamp_val - return a value clamped to a given range using val's type
-- 
2.37.3