linux-hardening - Re: [PATCH] linux/const.h: Explain how __is

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Date:   Wed, 2 Feb 2022 15:43:02 +0000
From:   "Uecker, Martin" <Martin.Uecker@....uni-goettingen.de>
To:     "keescook@...omium.org" <keescook@...omium.org>,
        "linux@...musvillemoes.dk" <linux@...musvillemoes.dk>,
        "corbet@....net" <corbet@....net>,
        "David.Laight@...LAB.COM" <David.Laight@...LAB.COM>
CC:     "mingo@...nel.org" <mingo@...nel.org>,
        "rikard.falkeborn@...il.com" <rikard.falkeborn@...il.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-hardening@...r.kernel.org" <linux-hardening@...r.kernel.org>,
        "torvalds@...ux-foundation.org" <torvalds@...ux-foundation.org>,
        "miguel.ojeda.sandonis@...il.com" <miguel.ojeda.sandonis@...il.com>,
        "gustavoars@...nel.org" <gustavoars@...nel.org>,
        "penguin-kernel@...ove.SAKURA.ne.jp" 
        <penguin-kernel@...ove.SAKURA.ne.jp>,
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        "linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>,
        "ndesaulniers@...gle.com" <ndesaulniers@...gle.com>
Subject: Re: [PATCH] linux/const.h: Explain how __is_constexpr() works

Am Mittwoch, den 02.02.2022, 08:49 +0000 schrieb David Laight:
> From: Rasmus Villemoes
> > Sent: 01 February 2022 13:06
> ...
> > > + * - The C standard defines an "integer constant expression" as different
> > > + *   from a "null pointer constant" (an integer constant 0 pointer).
> > 
> > I don't see the point of this bullet. Yes, an ICE is a distinct concept
> > from a null pointer constant, obviously. One is defined in terms of the
> > other - and your parenthesis is not an accurate paraphrase of the
> > definition of a null pointer constant.
> 
> From what I remember a "null pointer constant" is in "integer constant
> expression with value 0 cast to a pointer type".
> So (void *)(1-1) is just as valid as (void *)0.
> 
> Not sure any of it is relevant here.

The C standard (at least public drafts essentially
identical to the actual standards) can be found here:

http://www.open-std.org/jtc1/sc22/wg14/www/projects#9899

"An integer constant expression with the value 0, or such
an expression cast to type void *, is called a null pointer
constant."

BTW: I think it would be very valuable if WG14 would get
proposals and/or comments from the kernel community.

Martin



> 	David
> 
> -
> Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
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