netdev - Re: [PATCH v4 6/6] percpu/x86: Enable strict percpu checks via named AS qualifiers

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Message-ID: <CAFULd4ahm3-r66eBJSHV4NfBpnFLUpK2hRak42whuJJeui+B+Q@mail.gmail.com>
Date: Wed, 9 Apr 2025 17:15:45 +0200
From: Uros Bizjak <ubizjak@...il.com>
To: "H. Peter Anvin" <hpa@...or.com>
Cc: Jiri Slaby <jirislaby@...nel.org>, x86@...nel.org, linux-mm@...ck.org, 
	linux-kernel@...r.kernel.org, linux-bcachefs@...r.kernel.org, 
	linux-arch@...r.kernel.org, netdev@...r.kernel.org, 
	Nadav Amit <nadav.amit@...il.com>, Dennis Zhou <dennis@...nel.org>, Tejun Heo <tj@...nel.org>, 
	Christoph Lameter <cl@...ux.com>, Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...nel.org>, 
	Borislav Petkov <bp@...en8.de>, Dave Hansen <dave.hansen@...ux.intel.com>, 
	Linus Torvalds <torvalds@...ux-foundation.org>, Andy Lutomirski <luto@...nel.org>, 
	Brian Gerst <brgerst@...il.com>, Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH v4 6/6] percpu/x86: Enable strict percpu checks via named
 AS qualifiers

On Wed, Apr 9, 2025 at 5:09 PM H. Peter Anvin <hpa@...or.com> wrote:
>
> On April 9, 2025 4:43:27 AM PDT, Uros Bizjak <ubizjak@...il.com> wrote:
> >On Wed, Apr 9, 2025 at 1:07 PM Jiri Slaby <jirislaby@...nel.org> wrote:
> >>
> >> On 27. 01. 25, 17:05, Uros Bizjak wrote:
> >> > This patch declares percpu variables in __seg_gs/__seg_fs named AS
> >> > and keeps them named AS qualified until they are dereferenced with
> >> > percpu accessor. This approach enables various compiler check
> >> > for cross-namespace variable assignments.
> >>
> >> So this causes modpost to fail to version some symbols:
> >>
> >> > WARNING: modpost: EXPORT symbol "xen_vcpu_id" [vmlinux] version generation failed, symbol will not be versioned.
> >> > Is "xen_vcpu_id" prototyped in <asm/asm-prototypes.h>?
> >> > WARNING: modpost: EXPORT symbol "irq_stat" [vmlinux] version generation failed, symbol will not be versioned.
> >> > Is "irq_stat" prototyped in <asm/asm-prototypes.h>?
> >> > WARNING: modpost: EXPORT symbol "fred_rsp0" [vmlinux] version generation failed, symbol will not be versioned.
> >> > Is "fred_rsp0" prototyped in <asm/asm-prototypes.h>?
> >> > WARNING: modpost: EXPORT symbol "cpu_dr7" [vmlinux] version generation failed, symbol will not be versioned.
> >> > Is "cpu_dr7" prototyped in <asm/asm-prototypes.h>?
> >> > WARNING: modpost: EXPORT symbol "cpu_tss_rw" [vmlinux] version generation failed, symbol will not be versioned.
> >> > Is "cpu_tss_rw" prototyped in <asm/asm-prototypes.h>?
> >> > WARNING: modpost: EXPORT symbol "__tss_limit_invalid" [vmlinux] version generation failed, symbol will not be versioned.
> >> > Is "__tss_limit_invalid" prototyped in <asm/asm-prototypes.h>?
> >> > WARNING: modpost: EXPORT symbol "irq_fpu_usable" [vmlinux] version generation failed, symbol will not be versioned.
> >> > Is "irq_fpu_usable" prototyped in <asm/asm-prototypes.h>?
> >> > WARNING: modpost: EXPORT symbol "cpu_info" [vmlinux] version generation failed, symbol will not be versioned.
> >> > Is "cpu_info" prototyped in <asm/asm-prototypes.h>?
> >> > WARNING: modpost: EXPORT symbol "gdt_page" [vmlinux] version generation failed, symbol will not be versioned.
> >> > Is "gdt_page" prototyped in <asm/asm-prototypes.h>?
> >>  > ...
> >>
> >> That happens both with 6.15-rc1 and today's -next. Ideas?
> >
> >https://lore.kernel.org/lkml/20250404102535.705090-1-ubizjak@gmail.com/

> A lot of those seem to be things that definitely shouldn't be expected...

These symbols are just declared with an extra __seg_gs that genksyms
doesn't understand, e.g.:

extern __seg_gs __attribute__((section(".data..percpu" "..hot.."
"this_cpu_off"))) __typeof__(unsigned long) this_cpu_off;

The workaround falls back when __GENKSYMS__ is defined to:

extern __attribute__((section(".data..percpu" "..hot.."
"this_cpu_off"))) __typeof__(unsigned long) this_cpu_off;

which is what genksyms processes without problems.

Uros.