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From:   Nick Desaulniers <ndesaulniers@google.com>
Date:   Fri, 15 Mar 2019 14:31:09 -0700
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Subject: Re: [PATCH] x86/vdso: include generic __lshrdi3 in 32-bit vDSO
To:     Matthias Kaehlcke <mka@chromium.org>
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>
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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?

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

>
> 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).

> +
>  $(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?

>  #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
> --
> 2.21.0.360.g471c308f928-goog

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,
~Nick Desaulniers