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, 03 Mar 2024 21:10:50 +0100
From: Thomas Gleixner <tglx@...utronix.de>
To: Uros Bizjak <ubizjak@...il.com>
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 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.

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

Sigh.

        tglx