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

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Message-ID: <20150915052202.GB14215@gmail.com>
Date:	Tue, 15 Sep 2015 07:22:02 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	John Stultz <john.stultz@...aro.org>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.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>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: Re: [RFC][PATCH 5/5] abs(): Provide build error on passing 64bit
 value to abs()


* John Stultz <john.stultz@...aro.org> wrote:

> 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;		\

I think this should be a compiletime_warning() - that will be visible enough.

Thanks,

	Ingo
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