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Message-Id: <20180312155524.b421f07d7f08f24c57bd1887@linux-foundation.org>
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
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"David S. Miller" <davem@...emloft.net>, Alexey Kuznetsov
<kuznet@....inr.ac.ru>, Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>, Ingo
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Shevchenko <andriy.shevchenko@...ux.intel.com>, Pantelis Antoniou
<pantelis.antoniou@...sulko.com>, Linux Btrfs
<linux-btrfs@...r.kernel.org>, Network Development
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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
_
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