[Bug libc/31383] _FORTIFY_SOURCE=3 and __fortified_attr_access vs size of 0 and zero size types

["siddhesh at sourceware dot org" <sourceware-bugzilla@sourceware.org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=31383

--- Comment #3 from Siddhesh Poyarekar <siddhesh at sourceware dot org> ---
(In reply to Siddhesh Poyarekar from comment #1)
> ```
> When no size-index argument is specified, the pointer argument must be
> either null or point to a space that is suitably aligned and large for __at
> least one object__ of the referenced type (this implies that a past-the-end
> pointer is not a valid argument).
> ```
> 
> Well technically, the pointer argument *is* suitably aligned and large for
> 16 objects of 0 size, but the implication that it is hence not a
> past-the-end pointer is invalid.  I'll drop the access attribute (since the
> additional value from having it is not really significant enough) but IMO
> -Wstringop-overflow needs to be fixed to handle pointers to zero-sized
> structs, i.e. it needs to ignore them and not conjure up an access size of 1
> out of nowhere.

Actually, no, I was wrong. The referenced type is void*, which is why the
warning is 'correct'.  Maybe there's scope for better wording, but it does make
sense to warn in such cases:

extern void f2 (void *) __attribute__ ((__access__ (__write_only__, 1)));

void
f1 (void)
{
  struct A {} a[16];

  f2 (a);
}

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