Re: arch/x86/include/asm/processor.h:698:16: sparse: sparse: incorrect type in initializer (different address spaces)

Uros Bizjak
2024-03-03T21:20:04Z

On Sun, Mar 3, 2024 at 9:24 PM Uros Bizjak <ubizjak@gmail.com> wrote:
>
> On Sun, Mar 3, 2024 at 9:21 PM Uros Bizjak <ubizjak@gmail.com> wrote:
> >
> > On Sun, Mar 3, 2024 at 9:10 PM Thomas Gleixner <tglx@linutronix.de> wrote:
> > >
> > > On Sun, Mar 03 2024 at 20:03, Uros Bizjak wrote:
> > > > On Sun, Mar 3, 2024 at 5:31 PM Thomas Gleixner <tglx@linutronix.de> wrote:
> > > >> I did not follow the __set_gs work closely, so I don't know whether Uros
> > > >> ever tried to actually mark the per CPU variable __set_gs right away,
> > > >> which would obviously catch the above 'foo' nonsense.
> > > >
> > > > No, because [1]:
> > > >
> > > > "gcc does not provide a way to remove segment qualifiers, which is needed
> > > > to use typeof() to create local instances of the per-cpu variable. For
> > > > this reason, do not use the segment qualifier for per-cpu variables, and
> > > > do casting using the segment qualifier instead."
> > >
> > > Right. I just figured that out myself when playing with it in user
> > > space.
> > >
> > > That's so sad because it would provide us compiler based __percpu
> > > validation.
> >
> > Unfortunately, the c compiler can't strip qualifiers, so typeof() is
> > of limited use also when const and volatile qualifiers are used.
> > Perhaps some extension could be introduced to c standard to provide an
> > unqualified type, e.g. typeof_unqual().
>
> Oh, there is one in C23 [1].
>
> [1] https://en.cppreference.com/w/c/language/typeof

FYI: gcc-14 compiles this testcase:

--cut here--
__seg_gs int a;
__typeof_unqual__(a) b;

int foo (void)
{
return a;
}

int bar (void)
{
return b;
}
--cut here--

to (gcc -O2):

foo:
movl %gs:a(%rip), %eax
ret

bar:
movl b(%rip), %eax
ret

So, it *does* strip the __seg_gs qualifier here.

Uros.