Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
From:   YingChi Long <me@inclyc.cn>
To:     david.laight@aculab.com
Cc:     bp@alien8.de, chang.seok.bae@intel.com,
        dave.hansen@linux.intel.com, hpa@zytor.com,
        linux-kernel@vger.kernel.org, me@inclyc.cn, mingo@redhat.com,
        ndesaulniers@google.com, pbonzini@redhat.com, tglx@linutronix.de,
        x86@kernel.org
Subject: RE: [PATCH v2] x86/fpu: use _Alignof to avoid UB in TYPE_ALIGN
Date:   Wed, 28 Sep 2022 00:44:11 +0800
Message-Id: <20220927164411.99297-1-me@inclyc.cn>
In-Reply-To: <38114d1b752c497eba1640360daf5b9e@AcuMS.aculab.com>
References: <38114d1b752c497eba1640360daf5b9e@AcuMS.aculab.com>
MIME-Version: 1.0
Content-Transfer-Encoding: 8bit
Precedence: bulk

> Interesting - what justification do they give?
> Linux kernel requires that the compiler add no unnecessary padding
> so that structure definitions are well defined.

Yes, that's a clarification given in 2019.

> So using a type definition inside offsetof() won't give a
> useful value - but it still isn't really UB.

WG14 may worry about commas and the scope of new definitions. So they provide
new words into the standard and said:

> If the specified type defines a new type or if the specified member is a
> bit-field, the behavior is undefined.

https://www.open-std.org/jtc1/sc22/wg14/www/docs/n2350.htm

I've provided this link in the patch.

> Has that ever worked?
> Given:
> 	struct foo {
> 		int a;
> 		char b;
> 		char c;
> 	};

TYPE_ALIGN(struct foo) evaluates to 4 in the previous approach (based on
offsetof). _Align(struct foo) evaluates to the same value.

See https://godbolt.org/z/sqebhEnsq

> I think CHECK_MEMBER_AT_END_OF_TYPE(struct foo, b) is true.

Hmm, both the previous version and after this patch the macro gives me
false. (See the godbolt link).