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Message-ID: <202308170957.F511E69@keescook> Date: Thu, 17 Aug 2023 10:00:17 -0700 From: Kees Cook <keescook@...omium.org> To: David Laight <David.Laight@...lab.com> Cc: 'Przemek Kitszel' <przemyslaw.kitszel@...el.com>, "netdev@...r.kernel.org" <netdev@...r.kernel.org>, 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 On Thu, Aug 17, 2023 at 02:35:23PM +0000, David Laight wrote: > 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. It works only on flex array structs, and defines a specific instance. I think naming is okay here. > > > +#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. In the sense that since "type" is in the union, it's okay? > I'm not sure you should be forcing the memset() either. This already got discussed: better to fail safe. > > > + type *name = (type *)&name##_u > > How about? > type *const name = &name_##_u.obj; This is by design (see earlier threads) so that __builtin_object_size(name, 1) will get the correct size. Otherwise it doesn't include the FAM elements in the size. > > You might want to add: > Static_assert(is_constexpr(count), "DEFINE_FLEX: non-constant count " #count); That would be nice, though can Static_assert()s live in the middle of variable definitions? -Kees -- Kees Cook
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