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Date:   Wed, 2 Feb 2022 12:53:23 -0800
From:   Kees Cook <keescook@chromium.org>
To:     Rasmus Villemoes <linux@rasmusvillemoes.dk>
Cc:     Nick Desaulniers <ndesaulniers@google.com>,
        Miguel Ojeda <ojeda@kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Nathan Chancellor <nathan@kernel.org>,
        linux-kernel@vger.kernel.org, llvm@lists.linux.dev
Subject: Re: [PATCH] include: drop pointless __compiler_offsetof indirection
Message-ID: <202202021253.B62C357@keescook>
References: <20220202102147.326672-1-linux@rasmusvillemoes.dk>
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On Wed, Feb 02, 2022 at 11:21:47AM +0100, Rasmus Villemoes wrote:
> (1) compiler_types.h is unconditionally included via an -include
> flag (see scripts/Makefile.lib), and it defines __compiler_offsetof
> unconditionally. So testing for definedness of __compiler_offsetof is
> mostly pointless.
> 
> (2) Every relevant compiler provides __builtin_offsetof (even sparse
> has had that for 14 years), and if for whatever reason one would end
> up picking up the poor man's fallback definition (C file compiler with
> completely custom CFLAGS?), newer clang versions won't treat the
> result as an Integer Constant Expression, so if used in place where
> such is required (static initializer or static_assert), one would
> get errors like
> 
> t.c:11:16: error: static_assert expression is not an integral constant expression
> t.c:11:16: note: cast that performs the conversions of a reinterpret_cast is not allowed in a constant expression
> t.c:4:33: note: expanded from macro 'offsetof'
> #define offsetof(TYPE, MEMBER)  ((size_t)&((TYPE *)0)->MEMBER)
> 
> So just define offsetof unconditionally and directly in terms of
> __builtin_offsetof.
> 
> Signed-off-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>

Yes please. :)

Reviewed-by: Kees Cook <keescook@chromium.org>

-- 
Kees Cook