Re: [tip: x86/percpu] x86/percpu: Declare const_pcpu_hot as extern const variable

[Uros Bizjak]

On Fri, Dec 1, 2023 at 5:25 PM Borislav Petkov <bp@xxxxxxxxx> wrote:
>
> On Thu, Nov 30, 2023 at 09:16:30PM -0000, tip-bot2 for Uros Bizjak wrote:
> > The following commit has been merged into the x86/percpu branch of tip:
> >
> > Commit-ID:     4604c052b84d66407f5e68045a1939685eac401e
> > Gitweb:        https://git.kernel.org/tip/4604c052b84d66407f5e68045a1939685eac401e
> > Author:        Uros Bizjak <ubizjak@xxxxxxxxx>
> > AuthorDate:    Thu, 30 Nov 2023 17:27:35 +01:00
> > Committer:     Ingo Molnar <mingo@xxxxxxxxxx>
> > CommitterDate: Thu, 30 Nov 2023 20:19:33 +01:00
> >
> > x86/percpu: Declare const_pcpu_hot as extern const variable
> >
> > const_pcpu_hot is aliased by linker to pcpu_hot, so there is no need
> > to use the DECLARE_PER_CPU_ALIGNED() macro. Also, declare const_pcpu_hot
> > as extern to avoid allocating storage space for the aliased structure.
> >
> > Fixes: ed2f752e0e0a ("x86/percpu: Introduce const-qualified const_pcpu_hot to micro-optimize code generation")
> > Reported-by: kernel test robot <lkp@xxxxxxxxx>
> > Signed-off-by: Uros Bizjak <ubizjak@xxxxxxxxx>
> > Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
> > Link: https://lore.kernel.org/r/20231130162949.83518-1-ubizjak@xxxxxxxxx
> > Closes: https://lore.kernel.org/oe-kbuild-all/202311302257.tSFtZnly-lkp@xxxxxxxxx/
> > ---
> >  arch/x86/include/asm/current.h | 3 +--
> >  1 file changed, 1 insertion(+), 2 deletions(-)
> >
> > diff --git a/arch/x86/include/asm/current.h b/arch/x86/include/asm/current.h
> > index 0538d24..9fbd7cb 100644
> > --- a/arch/x86/include/asm/current.h
> > +++ b/arch/x86/include/asm/current.h
> > @@ -37,8 +37,7 @@ static_assert(sizeof(struct pcpu_hot) == 64);
> >  DECLARE_PER_CPU_ALIGNED(struct pcpu_hot, pcpu_hot);
> >
> >  /* const-qualified alias to pcpu_hot, aliased by linker. */
> > -DECLARE_PER_CPU_ALIGNED(const struct pcpu_hot __percpu_seg_override,
> > -                     const_pcpu_hot);
> > +extern const struct pcpu_hot __percpu_seg_override const_pcpu_hot;
> >
> >  static __always_inline struct task_struct *get_current(void)
> >  {
>
> WARNING: modpost: EXPORT symbol "const_pcpu_hot" [vmlinux] version generation failed, symbol will not be versioned.
> Is "const_pcpu_hot" prototyped in <asm/asm-prototypes.h>?
> WARNING: modpost: "const_pcpu_hot" [arch/x86/kvm/kvm.ko] has no CRC!
> WARNING: modpost: "const_pcpu_hot" [arch/x86/kvm/kvm-intel.ko] has no CRC!
> WARNING: modpost: "const_pcpu_hot" [kernel/trace/preemptirq_delay_test.ko] has no CRC!
> WARNING: modpost: "const_pcpu_hot" [fs/exfat/exfat.ko] has no CRC!
> WARNING: modpost: "const_pcpu_hot" [fs/ntfs/ntfs.ko] has no CRC!
> WARNING: modpost: "const_pcpu_hot" [fs/xfs/xfs.ko] has no CRC!
> WARNING: modpost: "const_pcpu_hot" [fs/btrfs/btrfs.ko] has no CRC!
> WARNING: modpost: "const_pcpu_hot" [drivers/acpi/acpi_pad.ko] has no CRC!
> WARNING: modpost: "const_pcpu_hot" [drivers/gpu/drm/display/drm_display_helper.ko] has no CRC!
> WARNING: modpost: "const_pcpu_hot" [drivers/gpu/drm/drm.ko] has no CRC!
> WARNING: modpost: "const_pcpu_hot" [drivers/gpu/drm/ttm/ttm.ko] has no CRC!
> WARNING: modpost: "const_pcpu_hot" [drivers/gpu/drm/i915/i915.ko] has no CRC!
> WARNING: modpost: "const_pcpu_hot" [drivers/misc/mei/mei.ko] has no CRC!
> WARNING: modpost: "const_pcpu_hot" [drivers/usb/core/usbcore.ko] has no CRC!
> WARNING: modpost: "const_pcpu_hot" [drivers/usb/class/cdc-acm.ko] has no CRC!
> WARNING: modpost: "const_pcpu_hot" [drivers/usb/class/usblp.ko] has no CRC!
> WARNING: modpost: "const_pcpu_hot" [drivers/usb/storage/usb-storage.ko] has no CRC!
>
> And the usual fix for those things:
>
> diff --git a/arch/x86/include/asm/asm-prototypes.h b/arch/x86/include/asm/asm-prototypes.h
> index b1a98fa38828..7e6440bae63d 100644
> --- a/arch/x86/include/asm/asm-prototypes.h
> +++ b/arch/x86/include/asm/asm-prototypes.h
> @@ -13,6 +13,7 @@
>  #include <asm/preempt.h>
>  #include <asm/asm.h>
>  #include <asm/gsseg.h>
> +#include <asm/current.h>
>
>  #ifndef CONFIG_X86_CMPXCHG64
>  extern void cmpxchg8b_emu(void);
>
> does NOT fix it.

I looked into this problem a bit, it is triggered by CONFIG_MODVERSIONS=y

The problem is, that genksyms does not know the __seg_gs keyword and
simply ignores the symbol.

Running:

make V=1 arch/x86/kernel/cpu/common.symtypes

There is NOTHING regarding const_pcpu_hot in the resulting common.symtypes file.

The unpatched source should also fail here due to __seg_gs keyword,
but it magically didn't.

I don't know what to do here.

Uros.