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

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Date:   Mon, 12 Mar 2018 15:55:24 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Kees Cook <keescook@...omium.org>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        "Gustavo A. R. Silva" <gustavo@...eddedor.com>,
        "Tobin C. Harding" <me@...in.cc>,
        Steven Rostedt <rostedt@...dmis.org>,
        Jonathan Corbet <corbet@....net>, Chris Mason <clm@...com>,
        Josef Bacik <jbacik@...com>, David Sterba <dsterba@...e.com>,
        "David S. Miller" <davem@...emloft.net>,
        Alexey Kuznetsov <kuznet@....inr.ac.ru>,
        Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>,
        Ingo Molnar <mingo@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Masahiro Yamada <yamada.masahiro@...ionext.com>,
        Borislav Petkov <bp@...e.de>,
        Randy Dunlap <rdunlap@...radead.org>,
        Ian Abbott <abbotti@....co.uk>,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        Petr Mladek <pmladek@...e.com>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Pantelis Antoniou <pantelis.antoniou@...sulko.com>,
        Linux Btrfs <linux-btrfs@...r.kernel.org>,
        Network Development <netdev@...r.kernel.org>,
        Kernel Hardening <kernel-hardening@...ts.openwall.com>
Subject: Re: [PATCH v3] kernel.h: Skip single-eval logic on literals in
 min()/max()

On Fri, 9 Mar 2018 17:30:15 -0800 Kees Cook <keescook@...omium.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
_