[Bug c/108896] provide "element_count" attribute to give more context to __builtin_dynamic_object_size() and -fsanitize=bounds

["isanbard at gmail dot com" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108896

--- Comment #14 from Bill Wendling <isanbard at gmail dot com> ---
(In reply to Martin Uecker from comment #9)
> > > Considering that the GNU extensions is rarely used, one could consider
> > > redefining the meaning of
> > > 
> > > int n = 1;
> > > struct {
> > >   int n;
> > >   char buf[n];
> > > };
> > > 
> > > so that the 'n' refers to the member. Or we add a new syntax similar to
> > > designators (which intuitively makes sense to me).
> > designator might be better IMO.
> > 
> > a question here is:
> > 
> > for the following nested structure: 
> > 
> > struct object {
> >         ...
> >         char items;
> >         ...
> >         struct inner {
> >                 ...
> >                 int flex[];
> >         };
> > } *ptr;
> > 
> > what kind of syntax is good to represent the upper bound of "flex" in the inner
> > struct with "items" in the outer structure? any suggestion?
> 
> I would disallow it. At least at first. It also raises some
> questions: For example, one could form a pointer to the inner
> struct, and then it is not clear how 'items' could be accessed
> anymore.
> 

That would be limiting its use in the Linux kernel. It seems that there are
ways to refer to struct members already using something like "offsetof":

struct object {
    ...
    char items;
    ...
    struct inner {
        ...
        int flex[] __attribute__((__element_count__(offsetof(struct object,
items))));
    };
} *ptr;

The object referenced by "offsetof" would be from the containing structure (see
"container_of" macro in Linux).

It has the benefit of not needing to extend C's syntax to allow for designated
initializers outside of initialization lists. It also has the benefit of
allowing one to reference a variable not in the structure:

const int items;
struct object {
    ...
    char items;
    ...
    struct inner {
        ...
        int flex[] __attribute__((__element_count__(items))); /* References
global "items" */
    };
} *ptr;

-bw