> On Aug 26, 2024, at 16:30, Kees Cook <k...@kernel.org> wrote:
>
> 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...
That’s the reason why I returned a (size_t *) instead of a (void *) for the
NULL pointer when there is no counted_by annotation in the 1st version of the
patch, then the conversion from (void *) NULL to (size_t *) NULL in the source
code level will not be needed.
Then the above can be simplified as:
typeof (__builtin_get_counted_by(array_annotated->c)) ret =
__builtin_get_counted_by(array_annotated->c);
If (ret) *ret = 10;
I am wondering shall I still keep the (size_t *) for the returned NULL pointer
instead of the (void *)?
Qing
>
> -Kees
>
> [1]
> https://lore.kernel.org/linux-hardening/20240822231324.make.666-k...@kernel.org/
>
> --
> Kees Cook
