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

[Uros Bizjak]

On Wed, Apr 9, 2025 at 5:09 PM H. Peter Anvin <hpa@xxxxxxxxx> wrote:
>
> On April 9, 2025 4:43:27 AM PDT, Uros Bizjak <ubizjak@xxxxxxxxx> wrote:
> >On Wed, Apr 9, 2025 at 1:07 PM Jiri Slaby <jirislaby@xxxxxxxxxx> 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@xxxxxxxxx/

> 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.