linux-kernel - [RFC][PATCH 5/5] abs(): Provide build error on passing 64bit value to abs()

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Date:	Mon, 14 Sep 2015 18:05:24 -0700
From:	John Stultz <john.stultz@...aro.org>
To:	LKML <linux-kernel@...r.kernel.org>
Cc:	John Stultz <john.stultz@...aro.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Ingo Molnar <mingo@...nel.org>,
	"Steven Rostedt (Red Hat)" <rostedt@...dmis.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	Michal Nazarewicz <mina86@...a86.com>
Subject: [RFC][PATCH 5/5] abs(): Provide build error on passing 64bit value to abs()

As noted in the comment above abs():
 "abs() should not be used for 64-bit types (s64, u64, long long)
  - use abs64()  for those."

Unfortunately, its quite easy to pass 64-bit values to abs()
accidentally, and the compiler provides no warning when the
returned value is erroniously capped at 32-bits.

So this patch tries to make it easier to detect when 64-bit
values are passed to abs() by generating a build error.

Obviously, since this causes build errors, this patch is last
in the series, and I tried to fix up all of the issues I ran
into in my build testing. But there are likely still some out
there.

Cc: Andrew Morton <akpm@...ux-foundation.org>
Cc: Ingo Molnar <mingo@...nel.org>
Cc: "Steven Rostedt (Red Hat)" <rostedt@...dmis.org>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>
Cc: Michal Nazarewicz <mina86@...a86.com>
Signed-off-by: John Stultz <john.stultz@...aro.org>
---
 include/linux/kernel.h | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/include/linux/kernel.h b/include/linux/kernel.h
index 5582410..6f01151 100644
--- a/include/linux/kernel.h
+++ b/include/linux/kernel.h
@@ -208,6 +208,9 @@ extern int _cond_resched(void);
  */
 #define abs(x) ({						\
 		long ret;					\
+		compiletime_assert(				\
+			!(sizeof(typeof(x)) > sizeof(long)),	\
+			"abs() should not be used for 64-bit types - use abs64()");\
 		if (sizeof(x) == sizeof(long)) {		\
 			long __x = (x);				\
 			ret = (__x < 0) ? -__x : __x;		\
-- 
1.9.1

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