minmax: sanity check constant bounds when clamping

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.

[akpm@linux-foundation.org: coding-style cleanups]
[akpm@linux-foundation.org: s@&&\@&& \@]
Link: https://lkml.kernel.org/r/20220926133435.1333846-1-Jason@zx2c4.com
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Cc: Kees Cook <keescook@chromium.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

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