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From: Thomas Gleixner <tglx@linutronix.de>
To: Uros Bizjak <ubizjak@gmail.com>
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>
Subject: Re: arch/x86/include/asm/processor.h:698:16: sparse: sparse:
 incorrect type in initializer (different address spaces)
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References: <202403020457.RCJoQ3ts-lkp@intel.com> <87edctwr6y.ffs@tglx>
 <87a5nhwpus.ffs@tglx> <87y1b0vp8m.ffs@tglx> <87sf18vdsq.ffs@tglx>
 <87le70uwf0.ffs@tglx>
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 <87edcruvja.ffs@tglx>
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Date: Sun, 03 Mar 2024 21:10:50 +0100
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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@linutronix.d=
e> 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