netdev - Re: [PATCH net-next v3 1/7] overflow: add DEFINE

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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