RE: [PATCH net-next v3 1/7] overflow: add DEFINE_FLEX() for on-stack allocs

[David Laight <David.Laight@ACULAB.COM>]

From: Kees Cook
> Sent: Thursday, August 17, 2023 6:00 PM
> 
> On Thu, Aug 17, 2023 at 02:35:23PM +0000, David Laight wrote:
> > From: Przemek Kitszel
> > > Sent: Wednesday, August 16, 2023 3:06 PM
...
> > > +#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?

The alignment of the union is the larger of the alignments
of all its members.
Which is what you want.

> > I'm not sure you should be forcing the memset() either.
> 
> This already got discussed: better to fail safe.

Perhaps call it DEFINE_FLEX_Z() to make this clear and
give the option for a non-zeroing version later.
Not everyone wants the expense of zeroing everything.

..
> > 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?

I checked and it is fine.
(I double-checked by adding a statement and getting an error.)

	David

-
Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
Registration No: 1397386 (Wales)