Re: [PATCH] x86: vdso: Use $LD instead of $CC to link

[Andy Lutomirski]

On Wed, Jul 18, 2018 at 3:41 PM, Alistair Strachan <astrachan@xxxxxxxxxx> wrote:
> The vdso{32,64}.so can fail to link with CC=clang when clang tries to
> find a suitable GCC toolchain to link these libraries with.
>
> /usr/bin/ld: arch/x86/entry/vdso/vclock_gettime.o:
>   access beyond end of merged section (782)
>
> This happens because the host environment leaked into the cross
> compiler environment due to the way clang searches for suitable GCC
> toolchains.
>
> Clang is a retargetable compiler, and each invocation of it must
> provide --target=<something> --gcc-toolchain=<something> to allow it
> to find the correct binutils for cross compilation. These flags had
> been added to KBUILD_CFLAGS, but the vdso code uses CC and not
> KBUILD_CFLAGS (for various reasons) which breaks clang's ability to
> find the correct linker when cross compiling.
>
> Most of the time this goes unnoticed because the host linker is new
> enough to work anyway, or is incompatible and skipped, but this cannot
> be reliably assumed.
>
> This change alters the vdso makefile to just use LD directly, which
> bypasses clang and thus the searching problem. The makefile will just
> use ${CROSS_COMPILE}ld instead, which is always what we want. This
> matches the method used to link vmlinux.

Acked-by: Andy Lutomirski <luto@xxxxxxxxxx>



>
> Cc: Andy Lutomirski <luto@xxxxxxxxxx>
> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> Cc: Ingo Molnar <mingo@xxxxxxxxxx>
> Cc: "H. Peter Anvin" <hpa@xxxxxxxxx>
> Cc: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
> Cc: x86@xxxxxxxxxx
> Cc: kernel-team@xxxxxxxxxxx
> Cc: joel@xxxxxxxxxxxxxxxxx
> Signed-off-by: Alistair Strachan <astrachan@xxxxxxxxxx>
> ---
> Supersedes "x86: vdso: Fix leaky vdso link with CC=clang"
>  arch/x86/entry/vdso/Makefile | 22 +++++++++-------------
>  1 file changed, 9 insertions(+), 13 deletions(-)
>
> diff --git a/arch/x86/entry/vdso/Makefile b/arch/x86/entry/vdso/Makefile
> index 1943aebadede..896a55204386 100644
> --- a/arch/x86/entry/vdso/Makefile
> +++ b/arch/x86/entry/vdso/Makefile
> @@ -48,10 +48,8 @@ targets += $(vdso_img_sodbg)
>
>  export CPPFLAGS_vdso.lds += -P -C
>
> -VDSO_LDFLAGS_vdso.lds = -m64 -Wl,-soname=linux-vdso.so.1 \
> -                       -Wl,--no-undefined \
> -                       -Wl,-z,max-page-size=4096 -Wl,-z,common-page-size=4096 \
> -                       $(DISABLE_LTO)
> +VDSO_LDFLAGS_vdso.lds = -m elf_x86_64 -soname linux-vdso.so.1 --no-undefined \
> +                       -z max-page-size=4096 -z common-page-size=4096
>
>  $(obj)/vdso64.so.dbg: $(src)/vdso.lds $(vobjs) FORCE
>         $(call if_changed,vdso)
> @@ -97,10 +95,8 @@ CFLAGS_REMOVE_vvar.o = -pg
>  #
>
>  CPPFLAGS_vdsox32.lds = $(CPPFLAGS_vdso.lds)
> -VDSO_LDFLAGS_vdsox32.lds = -Wl,-m,elf32_x86_64 \
> -                          -Wl,-soname=linux-vdso.so.1 \
> -                          -Wl,-z,max-page-size=4096 \
> -                          -Wl,-z,common-page-size=4096
> +VDSO_LDFLAGS_vdsox32.lds = -m elf32_x86_64 -soname linux-vdso.so.1 \
> +                          -z max-page-size=4096 -z common-page-size=4096
>
>  # 64-bit objects to re-brand as x32
>  vobjs64-for-x32 := $(filter-out $(vobjs-nox32),$(vobjs-y))
> @@ -128,7 +124,7 @@ $(obj)/vdsox32.so.dbg: $(src)/vdsox32.lds $(vobjx32s) FORCE
>         $(call if_changed,vdso)
>
>  CPPFLAGS_vdso32.lds = $(CPPFLAGS_vdso.lds)
> -VDSO_LDFLAGS_vdso32.lds = -m32 -Wl,-m,elf_i386 -Wl,-soname=linux-gate.so.1
> +VDSO_LDFLAGS_vdso32.lds = -m elf_i386 -soname linux-gate.so.1
>
>  targets += vdso32/vdso32.lds
>  targets += vdso32/note.o vdso32/system_call.o vdso32/sigreturn.o
> @@ -162,13 +158,13 @@ $(obj)/vdso32.so.dbg: FORCE \
>  # The DSO images are built using a special linker script.
>  #
>  quiet_cmd_vdso = VDSO    $@
> -      cmd_vdso = $(CC) -nostdlib -o $@ \
> +      cmd_vdso = $(LD) -nostdlib -o $@ \
>                        $(VDSO_LDFLAGS) $(VDSO_LDFLAGS_$(filter %.lds,$(^F))) \
> -                      -Wl,-T,$(filter %.lds,$^) $(filter %.o,$^) && \
> +                      -T $(filter %.lds,$^) $(filter %.o,$^) && \
>                  sh $(srctree)/$(src)/checkundef.sh '$(NM)' '$@'
>
> -VDSO_LDFLAGS = -fPIC -shared $(call cc-ldoption, -Wl$(comma)--hash-style=both) \
> -       $(call cc-ldoption, -Wl$(comma)--build-id) -Wl,-Bsymbolic $(LTO_CFLAGS)
> +VDSO_LDFLAGS = -shared $(call ld-option, --hash-style=both) \
> +       $(call ld-option, --build-id) -Bsymbolic
>  GCOV_PROFILE := n
>
>  #