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Date:   Fri, 15 Mar 2019 15:29:06 -0700
From:   Matthias Kaehlcke <mka@chromium.org>
To:     Nick Desaulniers <ndesaulniers@google.com>
Cc:     Andy Lutomirski <luto@kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        "H . Peter Anvin" <hpa@zytor.com>, x86@kernel.org,
        LKML <linux-kernel@vger.kernel.org>,
        Manoj Gupta <manojgupta@chromium.org>,
        Tiancong Wang <tcwang@chromium.org>,
        Stephen Hines <srhines@google.com>,
        clang-built-linux@googlegroups.com,
        Masahiro Yamada <yamada.masahiro@socionext.com>
Subject: Re: [PATCH] x86/vdso: include generic __lshrdi3 in 32-bit vDSO
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Hi Nick,

On Fri, Mar 15, 2019 at 02:31:09PM -0700, 'Nick Desaulniers' via Clang Built Linux wrote:
> On Fri, Mar 15, 2019 at 12:54 PM Matthias Kaehlcke <mka@chromium.org> wrote:
> >
> > Building the 32-bit vDSO with a recent clang version fails due
> > to undefined symbols:
> >
> > arch/x86/entry/vdso/vdso32.so.dbg: undefined symbols found
> >
> > The undefined symbol in this case is __lshrdi3, which is part of
> > the compiler runtime library, however the vDSO isn't linked against
> > this library.
> >
> > Include the kernel version of __lshrdi3 in the 32-bit vDSO build.
> 
> __lshrdi3 is used for "logical shift right double-word by int" (best
> guess), so anywhere there's a right shift of a u64.  Looks like
> there's a few of these in arch/x86/entry/vdso/, so it's legal for the
> compiler to emit this libcall.  Do you know which function
> specifically in the .so has a relocation referencing __lshrdi3
> specifically?

It's the right shifts in do_realtime() and do_monotonic().

> Is there a config I can set to reproduce this, in order to help
> test?

I encountered it with a Chrome OS specific configuration, but a
defconfig should do. Note that you probably need a development version
of clang to reproduce this.

> >
> > Signed-off-by: Matthias Kaehlcke <mka@chromium.org>
> > ---
> >  arch/x86/entry/vdso/Makefile | 7 ++++++-
> >  lib/lshrdi3.c                | 4 +++-
> >  2 files changed, 9 insertions(+), 2 deletions(-)
> >
> > diff --git a/arch/x86/entry/vdso/Makefile b/arch/x86/entry/vdso/Makefile
> > index 5bfe2243a08f..7517cd87e10b 100644
> > --- a/arch/x86/entry/vdso/Makefile
> > +++ b/arch/x86/entry/vdso/Makefile
> > @@ -144,6 +144,7 @@ KBUILD_CFLAGS_32 += $(call cc-option, -fno-stack-protector)
> >  KBUILD_CFLAGS_32 += $(call cc-option, -foptimize-sibling-calls)
> >  KBUILD_CFLAGS_32 += -fno-omit-frame-pointer
> >  KBUILD_CFLAGS_32 += -DDISABLE_BRANCH_PROFILING
> > +KBUILD_CFLAGS_32 += -DBUILD_VDSO
> >
> >  ifdef CONFIG_RETPOLINE
> >  ifneq ($(RETPOLINE_VDSO_CFLAGS),)
> > @@ -153,12 +154,16 @@ endif
> >
> >  $(obj)/vdso32.so.dbg: KBUILD_CFLAGS = $(KBUILD_CFLAGS_32)
> >
> > +$(obj)/vdso32/lshrdi3.o: $(srctree)/lib/lshrdi3.c FORCE
> > +       $(call if_changed_rule,cc_o_c)
> 
> + Masahiro to help look at this part (I don't understand this part
> of kbuild).

I bluntly stole that from arch/x86/purgatory/Makefile , which does
something similar.

> 
> > +
> >  $(obj)/vdso32.so.dbg: FORCE \
> >                       $(obj)/vdso32/vdso32.lds \
> >                       $(obj)/vdso32/vclock_gettime.o \
> >                       $(obj)/vdso32/note.o \
> >                       $(obj)/vdso32/system_call.o \
> > -                     $(obj)/vdso32/sigreturn.o
> > +                     $(obj)/vdso32/sigreturn.o \
> > +                     $(obj)/vdso32/lshrdi3.o
> >         $(call if_changed,vdso)
> >
> >  #
> > diff --git a/lib/lshrdi3.c b/lib/lshrdi3.c
> > index 99cfa5721f2d..8a4fc6bcf3a4 100644
> > --- a/lib/lshrdi3.c
> > +++ b/lib/lshrdi3.c
> > @@ -16,7 +16,7 @@
> >   * to the Free Software Foundation, Inc.
> >   */
> >
> > -#include <linux/module.h>
> > +#include <linux/export.h>
> 
> Is this a simple cleanup, or?

The vDSO build is unhappy when modules.h draws in a whole bunch of
other kernel headers and export.h is all that's need. It seemed
reasonable to do the 'cleanup' in this patch since we touch it anyway
to place EXPORT_SYMBOL within an #ifdef.

> >  #include <linux/libgcc.h>
> >
> >  long long notrace __lshrdi3(long long u, word_type b)
> > @@ -42,4 +42,6 @@ long long notrace __lshrdi3(long long u, word_type b)
> >
> >         return w.ll;
> >  }
> > +#ifndef BUILD_VDSO
> >  EXPORT_SYMBOL(__lshrdi3);
> > +#endif
> 
> Compilers (GCC and Clang) will always assume their runtime has these
> helper functions; whether or not they emit libcalls vs inline routines
> is implementation defined.  So I agree with this patch; I just would
> like to help confirm/test it.

Thanks for your help!

Matthias