Re: [PATCH] x86: work around clang IAS bug referencing __force_order

[Sedat Dilek]

On Tue, Aug 4, 2020 at 2:09 AM 'Nick Desaulniers' via Clang Built
Linux <clang-built-linux@xxxxxxxxxxxxxxxx> wrote:
>
> On Wed, May 27, 2020 at 6:53 AM Arnd Bergmann <arnd@xxxxxxxx> 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@xxxxxxxx>
>
> Hi Arnd,
> Looks like
> $ ARCH=i386 make CC=clang LLVM_IAS=1 -j71
> defconfig+CONFIG_X86_POWERNOW_K6=m
> is the simplest reproducer.
>
> If I run
> $ llvm-readelf -s drivers/cpufreq/powernow-k6.o | grep __force_order
>     39: 00000000     0 NOTYPE  GLOBAL DEFAULT   UND __force_order
> we can indeed see an undefined reference to __force_order.
>
> If I build the .s file via
> $ ARCH=i386 make CC=clang LLVM_IAS=1 -j71 drivers/cpufreq/powernow-k6.s
> the only reference I see to __force_order is:
> 979   .addrsig_sym __force_order
>
> which is created by Clang's implicit -faddr-sig.  If I disable that
> for this file via:
>
> ```diff
> diff --git a/drivers/cpufreq/Makefile b/drivers/cpufreq/Makefile
> index f1b7e3dd6e5d..87d655d5af49 100644
> --- a/drivers/cpufreq/Makefile
> +++ b/drivers/cpufreq/Makefile
> @@ -28,6 +28,9 @@ obj-$(CONFIG_X86_ACPI_CPUFREQ)                +=
> acpi-cpufreq.o
>  obj-$(CONFIG_X86_POWERNOW_K8)          += powernow-k8.o
>  obj-$(CONFIG_X86_PCC_CPUFREQ)          += pcc-cpufreq.o
>  obj-$(CONFIG_X86_POWERNOW_K6)          += powernow-k6.o
> +ifdef CONFIG_CC_IS_CLANG
> +CFLAGS_powernow-k6.o                   += -fno-addrsig
> +endif
>  obj-$(CONFIG_X86_POWERNOW_K7)          += powernow-k7.o
>  obj-$(CONFIG_X86_LONGHAUL)             += longhaul.o
>  obj-$(CONFIG_X86_E_POWERSAVER)         += e_powersaver.o
> ```
> then the module links without error, and we get no hits for
> __force_order from llvm-readelf -s.  This makes me think there may be
> a bug in Clang generating address significance tables for global
> variables that are otherwise unused, resulting in such linkage
> failures. +pcc@ for that.
>
> I ran a creduce job on drivers/cpufreq/powernow-k6.i where I'd compile
> twice, one with the implicit default value of -faddr-sig and look for
> the undefined __force_order, and again with -fno-addrsig and ensure
> there was no undefined __force_order, which coughed up:
> extern int __force_order;
> int a(void) { asm("" : "=m"(__force_order)); return 0; }
> as the bare minimum for an address significant table.
> https://godbolt.org/z/cjfaqM
>
> I'll bet this is coming from the call to read_cr0() in
> powernow_k6_set_cpu_multiplier().  If __force_order is defined in
> arch/x86/boot/compressed/pgtable_64.c, then I'm not sure it's a good
> idea to build drivers/cpufreq/powernow-k6.c as a kernel module
> (CONFIG_X86_POWERNOW_K6=m) vs statically compiled in.  Wouldn't
> __force_order need to be EXPORT'ed for kernel modules to use it
> safely?
>
> > ---
> >  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
> >

Thanks for the proposal.

I have adapted it to fit my patchset against Linux v5.8.

Both Debian's GCC-10 and a snapshot version of LLVM toolchain
v11.0.0-rc1+ seems to be OK.

MAKE_OPTS="V=1 -j3 CC=gcc-10 LD=ld.bfd"
make $MAKE_OPTS arch/x86/kernel/cpu/common.o

MAKE_OPTS="V=1 -j3 HOSTCC=clang-11 HOSTCXX=clang++-11 HOSTLD=ld.lld-11
HOSTAR=llvm-ar-11 CC=clang-11 LD=ld.lld-11 AR=llvm-ar-11 NM=llvm-nm-11
OBJCOPY=llvm-objcopy-11 OBJDUMP=llvm-objdump-11 OBJSIZE=llvm-size-11
READELF=llvm-readelf-11 STRIP=llvm-strip-11 LLVM_IAS=1"
make $MAKE_OPTS arch/x86/kernel/cpu/common.o

I can send both object files if desired.

I will do a full kernel-build to see if I am able to build the
VirtualBox out-of-tree kernel-modules.

- Sedat -