linux-kernel - Re: [PATCH v3 1/2] uapi: fix __DECLARE_FLEX

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Message-ID: <202309151208.C99747375@keescook>
Date:   Fri, 15 Sep 2023 12:14:06 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Alexey Dobriyan <adobriyan@...il.com>
Cc:     akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
        linux-api@...r.kernel.org, linux-hardening@...r.kernel.org,
        David.Laight@...lab.com
Subject: Re: [PATCH v3 1/2] uapi: fix __DECLARE_FLEX_ARRAY for C++

On Tue, Sep 12, 2023 at 07:22:24PM +0300, Alexey Dobriyan wrote:
> __DECLARE_FLEX_ARRAY(T, member) macro expands to
> 
> 	struct {
> 		struct {} __empty_member;
> 		T member[];
> 	};
> 
> which is subtly wrong in C++ because sizeof(struct{}) is 1 not 0,
> changing UAPI structures layouts.

Looking at this again just now, what about using a 0-length array
instead of an anonymous struct?

https://godbolt.org/z/rGaxPWjef

Then we don't need an #ifdef at all...

 	struct {
 		int __empty_member[0];
 		T member[];
 	};

-Kees

-- 
Kees Cook