Re: [PATCH v3] kernel.h: Skip single-eval logic on literals in min()/max()

[Andrew Morton <akpm@linux-foundation.org>]

On Fri, 9 Mar 2018 17:30:15 -0800 Kees Cook <keescook@chromium.org> wrote:

> > It's one reason why I wondered if simplifying the expression to have
> > just that single __builtin_constant_p() might not end up working..
> 
> Yeah, it seems like it doesn't bail out as "false" for complex
> expressions given to __builtin_constant_p().
> 
> If no magic solution, then which of these?
> 
> - go back to my original "multi-eval max only for constants" macro (meh)
> - add gcc version checks around this and similarly for -Wvla in the future (eww)
> - raise gcc version (yikes)

Replacing the __builtin_choose_expr() with ?: works of course.  What
will be the runtime effects?

I tried replacing

	__builtin_choose_expr(__builtin_constant_p(x) &&
                              __builtin_constant_p(y),

with

	__builtin_choose_expr(__builtin_constant_p(x + y),

but no success.

I'm not sure what else to try to trick gcc into working.

--- a/include/linux/kernel.h~kernelh-skip-single-eval-logic-on-literals-in-min-max-v3-fix
+++ a/include/linux/kernel.h
@@ -804,13 +804,10 @@ static inline void ftrace_dump(enum ftra
  * accidental VLA.
  */
 #define __min(t1, t2, x, y)						\
-	__builtin_choose_expr(__builtin_constant_p(x) &&		\
-			      __builtin_constant_p(y),			\
-			      (t1)(x) < (t2)(y) ? (t1)(x) : (t2)(y),	\
-			      __single_eval_min(t1, t2,			\
-						__UNIQUE_ID(min1_),	\
-						__UNIQUE_ID(min2_),	\
-						x, y))
+	((__builtin_constant_p(x) && __builtin_constant_p(y)) ?		\
+		((t1)(x) < (t2)(y) ? (t1)(x) : (t2)(y)) :		\
+		(__single_eval_min(t1, t2, __UNIQUE_ID(min1_),		\
+				  __UNIQUE_ID(min2_), x, y)))
 
 /**
  * min - return minimum of two values of the same or compatible types
@@ -826,13 +823,11 @@ static inline void ftrace_dump(enum ftra
 	max1 > max2 ? max1 : max2; })
 
 #define __max(t1, t2, x, y)						\
-	__builtin_choose_expr(__builtin_constant_p(x) &&		\
-			      __builtin_constant_p(y),			\
-			      (t1)(x) > (t2)(y) ? (t1)(x) : (t2)(y),	\
-			      __single_eval_max(t1, t2,			\
-						__UNIQUE_ID(max1_),	\
-						__UNIQUE_ID(max2_),	\
-						x, y))
+	((__builtin_constant_p(x) && __builtin_constant_p(y)) ?		\
+		((t1)(x) > (t2)(y) ? (t1)(x) : (t2)(y)) :		\
+		(__single_eval_max(t1, t2, __UNIQUE_ID(max1_),		\
+				  __UNIQUE_ID(max2_), x, y)))
+
 /**
  * max - return maximum of two values of the same or compatible types
  * @x: first value
_