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

[Kees Cook]

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@xxxxxxxxxx>
> > Signed-off-by: Keith Packard <keithpac@xxxxxxxxxx>
> > Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
> > ---
> >  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