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

Uros Bizjak
2024-03-03T20:21:35Z

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().

Uros.

> Right now this simply does not work and __verify_pcp_ptr(ptr) is not
> doing anything except when sparse looks at it.
>
> Sigh.
>
> tglx
>