Re: [PATCH v3] kernel.h: Skip single-eval logic on literals in min()/max()
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From: Andrew Morton <akpm@linux-foundation.org>
Date: 2018-03-12 22:55:24
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On Fri, 9 Mar 2018 17:30:15 -0800 Kees Cook [off-list ref] wrote:
quoted
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
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