linux-kernel - Re: [RFC 1/3] compiler.h: enable builtin overflow checkers and add fallback code

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Date:	Tue, 21 Jul 2015 09:26:09 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Rasmus Villemoes <linux@...musvillemoes.dk>
Cc:	Sasha Levin <sasha.levin@...cle.com>, akpm@...ux-foundation.org,
	linux-kernel@...r.kernel.org,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [RFC 1/3] compiler.h: enable builtin overflow checkers and add
 fallback code


Linus Cc:-ed so he can chime in if he wants to.

Thanks,

	Ingo

* Rasmus Villemoes <linux@...musvillemoes.dk> wrote:

> On Mon, Jul 20 2015, Sasha Levin <sasha.levin@...cle.com> wrote:
> 
> > On 07/19/2015 07:17 PM, Rasmus Villemoes wrote:
> >> Last year, Sasha Levin suggested adding wrappers for the
> >> __builtin_*_overflow functions introduced with gcc 5.1 (based on
> >> similar, but type-specific, functions in clang). This is another
> >> attempt at providing such wrappers and fallback code for older compilers.
> >
> > What's the difference between this version and the one Linus essentially
> > rejected?
> 
> Assuming you're referring to
> http://thread.gmane.org/gmane.linux.kernel/1838832 (the latest I could
> find, and the one Linus "[didn't] like"):
> 
> I've tried to ensure that the fallback code has the same semantics as
> the gcc builtins [1] (in particular, to handle all kinds of overflow) -
> I think it would be rather dangerous if the types of overflow detected
> depended on the gcc version.
> 
> The fallback code in the version referred to above had a number of
> problems:
> 
> * relies on UB for signed types
> 
> * both false positives and false negatives (because it more or less
>   implicitly assumed that all values are positive)
> 
> * even for unsigned types, plain a+b<a is broken for types narrower than
>   int
> 
> It's also inconvenient for the user to have to pass the appropriate
> type_max value to the mul_overflow checker. 
> 
> Rasmus
> 
> [1] though with the extra requirement of all three arguments having the
> same type.
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