linux-kernel - Re: [PATCH] x86: work around clang IAS bug referencing __force

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Date:   Sat, 1 Aug 2020 13:50:39 +0200
From:   Sedat Dilek <sedat.dilek@...il.com>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        x86@...nel.org, "H. Peter Anvin" <hpa@...or.com>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Zhenzhong Duan <zhenzhong.duan@...cle.com>,
        Kees Cook <keescook@...omium.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Juergen Gross <jgross@...e.com>,
        Andy Lutomirski <luto@...nel.org>,
        Andrew Cooper <andrew.cooper3@...rix.com>,
        linux-kernel@...r.kernel.org,
        Clang-Built-Linux ML <clang-built-linux@...glegroups.com>
Subject: Re: [PATCH] x86: work around clang IAS bug referencing __force_order

On Wed, May 27, 2020 at 3:53 PM Arnd Bergmann <arnd@...db.de> wrote:
>
> When using the clang integrated assembler, we get a reference
> to __force_order that should normally get ignored in a few
> rare cases:
>
> ERROR: modpost: "__force_order" [drivers/cpufreq/powernow-k6.ko] undefined!
>
> Add a 'static' definition so any file in which this happens can
> have a local copy.
>
> Signed-off-by: Arnd Bergmann <arnd@...db.de>

Hi,

what is the status of this patch?

I needed this one to be able to build VirtualBox via DKMS as an
out-of-tree kernel-module.
Package: virtualbox-dkms version 6.1.12-dfsg-8 (Debian/unstable)

To quote myself (see [1]):
Passing LLVM_IAS=1 results in:

  AR      /var/lib/dkms/virtualbox/6.1.12/build/built-in.a
  MODPOST /var/lib/dkms/virtualbox/6.1.12/build/Module.symvers
ERROR: modpost: "__force_order"
[/var/lib/dkms/virtualbox/6.1.12/build/vboxdrv/vboxdrv.ko] undefined!

Arnd's patch is mandatory to build with Clang's Integrated Assembler
(make LLVM_IAS=1).
Here: LLVM toolchain version 11.0.0-rc1

Feel free to add:
Reported-by: Sedat Dilek <sedat.dilek@...il.com>
Tested-by: Sedat Dilek <sedat.dilek@...il.com>

Can one of the tip maintainers pick this up, please?

Thanks.

Regards,
- Sedat -

[1] https://github.com/ClangBuiltLinux/linux/issues/1104#issuecomment-667470053


> ---
>  arch/x86/boot/compressed/pgtable_64.c | 2 ++
>  arch/x86/include/asm/special_insns.h  | 7 +++++++
>  2 files changed, 9 insertions(+)
>
> diff --git a/arch/x86/boot/compressed/pgtable_64.c b/arch/x86/boot/compressed/pgtable_64.c
> index c8862696a47b..8595194cea41 100644
> --- a/arch/x86/boot/compressed/pgtable_64.c
> +++ b/arch/x86/boot/compressed/pgtable_64.c
> @@ -12,7 +12,9 @@
>   * It is not referenced from the code, but GCC < 5 with -fPIE would fail
>   * due to an undefined symbol. Define it to make these ancient GCCs work.
>   */
> +#ifndef CONFIG_CC_IS_CLANG
>  unsigned long __force_order;
> +#endif
>
>  #define BIOS_START_MIN         0x20000U        /* 128K, less than this is insane */
>  #define BIOS_START_MAX         0x9f000U        /* 640K, absolute maximum */
> diff --git a/arch/x86/include/asm/special_insns.h b/arch/x86/include/asm/special_insns.h
> index 82436cb04ccf..7081e587c1ea 100644
> --- a/arch/x86/include/asm/special_insns.h
> +++ b/arch/x86/include/asm/special_insns.h
> @@ -16,8 +16,15 @@
>   * A memory clobber would solve the problem, but would prevent reordering of
>   * all loads stores around it, which can hurt performance. Solution is to
>   * use a variable and mimic reads and writes to it to enforce serialization
> + *
> + * Clang sometimes fails to kill the reference to the dummy variable, so
> + * provide an actual copy.
>   */
> +#ifdef CONFIG_CC_IS_CLANG
> +static unsigned long __force_order;
> +#else
>  extern unsigned long __force_order;
> +#endif
>
>  void native_write_cr0(unsigned long val);
>
> --
> 2.26.2
>
> --
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