Re: [PATCH] Do not drop 'nocast' modifier when taking the address.

[Luc Van Oostenryck <luc.vanoostenryck@gmail.com>]

On Wed, Feb 03, 2016 at 04:25:44AM +0800, Christopher Li wrote:
> On Wed, Jan 6, 2016 at 12:25 AM, Luc Van Oostenryck
> <luc.vanoostenryck@gmail.com> wrote:
> > With the following code:
> >         typedef unsigned long __nocast cputime_t;
> >
> >         void task_cputime_adjusted(cputime_t *);
> >
> >         void current_task_runtime_100ns(void)
> >         {
> >                 cputime_t utime;
> >
> >                 task_cputime_adjusted(&utime);
> >         }
> >
> > sparse emits the following message:
> >         x.c:16:32: warning: incorrect type in argument 1 (different modifiers)
> >         x.c:16:32:    expected unsigned long [nocast] [usertype] *<noident>
> >         x.c:16:32:    got unsigned long *<noident>
> >         x.c:16:32: warning: implicit cast to nocast type
> >
> > In other words, when taking the address of 'utime', sparse drops the 'nocast'
> > modifier and then complains that task_cputime_adjusted() is not given a
> > 'nocast' pointer as expected ...
> 
> I think there is a bug some where else. In the above example,
> "cputime_t *" and "&utime" should have the same type regardless
> pointer inherent the nocast attribute or not. I haven't fully understand
> where the nocast attribute get dropped.

The nocast mod is dropped and lost in the function create_pointer().
In the example above, "cputime_t *" has type :
	unsigned long [nocast] [usertype] *
while &utime is just:
	unsigned long *

So, for sparse and its extended notion of type, the type we get when
taking the address of a [variable of some] type X is not the same as
directly using a pointer to the type X.
Which is very fine, just that MOD_NOCAST is dropped while the example
shows that it should not.
OTOH, MOD_STORAGE is kept but I think should be dropped; but that's
another story.


Regards,
Luc