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

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Date: Sun, 3 Mar 2024 21:24:00 +0100
From: Uros Bizjak <ubizjak@...il.com>
To: Thomas Gleixner <tglx@...utronix.de>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>, kernel test robot <lkp@...el.com>, 
	oe-kbuild-all@...ts.linux.dev, linux-kernel@...r.kernel.org, 
	Arjan van de Ven <arjan@...ux.intel.com>, x86@...nel.org, 
	Luc Van Oostenryck <luc.vanoostenryck@...il.com>, 
	Sparse Mailing-list <linux-sparse@...r.kernel.org>
Subject: Re: arch/x86/include/asm/processor.h:698:16: sparse: sparse:
 incorrect type in initializer (different address spaces)

On Sun, Mar 3, 2024 at 9:21 PM Uros Bizjak <ubizjak@...il.com> wrote:
>
> On Sun, Mar 3, 2024 at 9:10 PM Thomas Gleixner <tglx@...utronix.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@...utronix.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

Uros.