Am Montag, dem 26.08.2024 um 13:30 -0700 schrieb Kees Cook:
> On Mon, Aug 26, 2024 at 07:30:15PM +0000, Qing Zhao wrote:
> > Hi, Martin,
> >
> > Looks like that there is some issue when I tried to use the _Generic for
> > the testing cases, and then I narrowed down to a
> > small testing case that shows the problem without any change to GCC.
> >
> > [opc@qinzhao-ol8u3-x86 gcc]$ cat t1.c
> > struct annotated {
> > char b;
> > int c[];
> > } *array_annotated;
> > extern void * counted_by_ref (int *);
> >
> > int main(int argc, char *argv[])
> > {
> > typeof(counted_by_ref (array_annotated->c)) ret
> > = counted_by_ref (array_annotated->c);
> > _Generic (ret, void* : (void)0, default: *ret = 10);
> >
> > return 0;
> > }
> > [opc@qinzhao-ol8u3-x86 gcc]$ /home/opc/Install/latest/bin/gcc t1.c
> > t1.c: In function ‘main’:
> > t1.c:12:44: warning: dereferencing ‘void *’ pointer
> > 12 | _Generic (ret, void* : (void)0, default: *ret = 10);
> > | ^~~~
> > t1.c:12:49: error: invalid use of void expression
> > 12 | _Generic (ret, void* : (void)0, default: *ret = 10);
> > | ^
>
> I implemented it like this[1] in the Linux kernel. So yours could be:
>
> struct annotated {
> char b;
> int c[] __attribute__((counted_by(b));
> };
> extern struct annotated *array_annotated;
>
> int main(int argc, char *argv[])
> {
> typeof(_Generic(__builtin_get_counted_by(array_annotated->c),
> void *: (size_t *)NULL,
> default: __builtin_get_counted_by(array_annotated->c)))
> ret = __builtin_get_counted_by(array_annotated->c);
> if (ret)
> *ret = 10;
>
> return 0;
> }
>
> It's a bit cumbersome, but it does what's needed.
>
> This is, however, just doing exactly what Bill has suggested: it is
> converting the (void *)NULL into (size_t *)NULL when there is no
> counted_by annotation...
>
> -Kees
>
> [1]
> https://lore.kernel.org/linux-hardening/20240822231324.make.666-k...@kernel.org/
Interesting. Will __builtin_get_counted_by(array_annotated->c) give
a null pointer (or an invalid pointer) of the correct type if
array_annotated is a null pointer of an annotated struct type?
I also wonder a bit about the multiple macro evaluations of the arguments
for P and SIZE.
Martin
>
