linux-hardening - RE: [PATCH net-next v3 1/7] overflow: add DEFINE

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Date:   Thu, 17 Aug 2023 14:35:23 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Przemek Kitszel' <przemyslaw.kitszel@...el.com>,
        Kees Cook <keescook@...omium.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>
CC:     Jacob Keller <jacob.e.keller@...el.com>,
        "intel-wired-lan@...ts.osuosl.org" <intel-wired-lan@...ts.osuosl.org>,
        Alexander Lobakin <aleksander.lobakin@...el.com>,
        "linux-hardening@...r.kernel.org" <linux-hardening@...r.kernel.org>,
        "Steven Zou" <steven.zou@...el.com>
Subject: RE: [PATCH net-next v3 1/7] overflow: add DEFINE_FLEX() for on-stack
 allocs

From: Przemek Kitszel
> Sent: Wednesday, August 16, 2023 3:06 PM
> 
> Using underlying array for on-stack storage lets us to declare
> known-at-compile-time structures without kzalloc().

Isn't DEFINE_FLEX() a bit misleading?
One thing it isn't is 'flexible' since it has a fixed size.

> +#define DEFINE_FLEX(type, name, member, count)					\
> +	union {									\
> +		u8 bytes[struct_size_t(type, member, count)];			\
> +		type obj;							\
> +	} name##_u __aligned(_Alignof(type)) = {};				\

You shouldn't need the _Alignof() it is the default.
I'm not sure you should be forcing the memset() either.

> +	type *name = (type *)&name##_u

How about?
	type *const name = &name_##_u.obj;

You might want to add:
	Static_assert(is_constexpr(count), "DEFINE_FLEX: non-constant count " #count);

	David

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