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From: Uros Bizjak <ubizjak@gmail.com>
Date: Sun, 3 Mar 2024 22:19:50 +0100
Message-ID: <CAFULd4ZQwhASjvfya6eYv+9hMf=aQmSpW0wLSKPES4DRm0qeWQ@mail.gmail.com>
Subject: Re: arch/x86/include/asm/processor.h:698:16: sparse: sparse:
 incorrect type in initializer (different address spaces)
To: Thomas Gleixner <tglx@linutronix.de>
Cc: Linus Torvalds <torvalds@linux-foundation.org>, kernel test robot <lkp@intel.com>, 
	oe-kbuild-all@lists.linux.dev, linux-kernel@vger.kernel.org, 
	Arjan van de Ven <arjan@linux.intel.com>, x86@kernel.org, 
	Luc Van Oostenryck <luc.vanoostenryck@gmail.com>, 
	Sparse Mailing-list <linux-sparse@vger.kernel.org>
Content-Type: text/plain; charset="UTF-8"
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On Sun, Mar 3, 2024 at 9:24=E2=80=AFPM Uros Bizjak <ubizjak@gmail.com> wrot=
e:
>
> On Sun, Mar 3, 2024 at 9:21=E2=80=AFPM Uros Bizjak <ubizjak@gmail.com> wr=
ote:
> >
> > On Sun, Mar 3, 2024 at 9:10=E2=80=AFPM 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=E2=80=AFPM Thomas Gleixner <tglx@linutr=
onix.de> wrote:
> > > >> I did not follow the __set_gs work closely, so I don't know whethe=
r Uros
> > > >> ever tried to actually mark the per CPU variable __set_gs right aw=
ay,
> > > >> 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.