Re: [PATCH 04/64] stddef: Introduce struct_group() helper macro

Kees Cook
2021-07-28T21:59:22Z

On Wed, Jul 28, 2021 at 12:54:18PM +0200, Rasmus Villemoes wrote:
> On 27/07/2021 22.57, Kees Cook wrote:
>
> > In order to have a regular programmatic way to describe a struct
> > region that can be used for references and sizing, can be examined for
> > bounds checking, avoids forcing the use of intermediate identifiers,
> > and avoids polluting the global namespace, introduce the struct_group()
> > macro. This macro wraps the member declarations to create an anonymous
> > union of an anonymous struct (no intermediate name) and a named struct
> > (for references and sizing):
> >
> > struct foo {
> > int one;
> > struct_group(thing,
> > int two,
> > int three,
> > );
> > int four;
> > };
>
> That example won't compile, the commas after two and three should be
> semicolons.

Oops, yes, thanks. This is why I shouldn't write code that doesn't first
go through a compiler. ;)

> And your implementation relies on MEMBERS not containing any comma
> tokens, but as
>
> int a, b, c, d;
>
> is a valid way to declare multiple members, consider making MEMBERS
> variadic
>
> #define struct_group(NAME, MEMBERS...)
>
> to have it slurp up every subsequent argument and make that work.

Ah! Perfect, thank you. I totally forgot I could do it that way.

>
> >
> > Co-developed-by: Keith Packard <keithpac@amazon.com>
> > Signed-off-by: Keith Packard <keithpac@amazon.com>
> > Signed-off-by: Kees Cook <keescook@chromium.org>
> > ---
> > include/linux/stddef.h | 34 ++++++++++++++++++++++++++++++++++
>
> Bikeshedding a bit, but do we need to add 34 lines that need to be
> preprocessed to virtually each and every translation unit [as opposed to
> adding a struct_group.h header]? Oh well, you need it for struct
> skbuff.h, so it would be pulled in by a lot regardless :(

My instinct is to make these kinds of helpers "always available" (like
sizeof_field(), etc), but I have no strong opinion on where it should
live. If the consensus is to move it, I certainly can! :)

-Kees

--
Kees Cook