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Date:   Sat, 23 Sep 2023 19:53:52 +0300
From:   Alexey Dobriyan <adobriyan@gmail.com>
To:     Linus Torvalds <torvalds@linux-foundation.org>
Cc:     Kees Cook <keescook@chromium.org>, linux-kernel@vger.kernel.org
Subject: Re: [GIT PULL] hardening fixes for v6.6-rc3
Message-ID: <2983cda7-f084-4eb2-811d-158f43e99e47@p183>
References: <202309220957.927ADC0586@keescook>
 <CAHk-=wg0C+eEm0Tegpvc1zZjcqkdG9L0ed10tg_rQ1-WZciMGA@mail.gmail.com>
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On Fri, Sep 22, 2023 at 04:55:45PM -0700, Linus Torvalds wrote:
> On Fri, 22 Sept 2023 at 09:59, Kees Cook <keescook@chromium.org> wrote:
> >
> > - Fix UAPI stddef.h to avoid C++-ism (Alexey Dobriyan)
> 
> Ugh. Did we really have to make two different versions of that define?
> 
> Ok, so C++ did something stupid wrt an empty struct. Fine.
> 
> But I think we could have still shared the same definition by just
> using the same 'zero-sized array' trick, regardless of any 'empty
> struct has a size in C++'.
> 
> IOW, wouldn't this just work universally, without any "two completely
> different versions" hack?
> 
> #define __DECLARE_FLEX_ARRAY(TYPE, NAME)        \
>         struct { \
>                 char __empty_ ## NAME[0]; \
>                 TYPE NAME[]; \
>         }

This doesn't work with g++ :-(

#undef __DECLARE_FLEX_ARRAY
#define __DECLARE_FLEX_ARRAY(TYPE, NAME)        \
	struct { \
		char __empty_ ## NAME[0]; \
		TYPE NAME[]; \
	}

struct S1 {
	__DECLARE_FLEX_ARRAY(int, x);
};

main.cc:79:35: error: flexible array member ‘S1::<unnamed struct>::x’ in an otherwise empty ‘struct S1’
   79 |         __DECLARE_FLEX_ARRAY(int, x);