[PATCH] x86: vdso: Fix leaky vdso link with CC=clang

[Alistair Strachan <astrachan@google.com>]

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.

Most of the time this goes unnoticed because the host linker is new
enough to work anyway, but this cannot be reliably assumed.

Extract the needed --target and --gcc-toolchain flags added in the top
level Makefile from KBUILD_CFLAGS. These flags direct clang to the
appropriate toolchain to link the vdsos.

Cc: Andy Lutomirski <luto@kernel.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: x86@kernel.org
Cc: kernel-team@android.com
Cc: joel@joelfernandes.com
Signed-off-by: Alistair Strachan <astrachan@google.com>
---
 arch/x86/entry/vdso/Makefile | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/arch/x86/entry/vdso/Makefile b/arch/x86/entry/vdso/Makefile
index 261802b1cc50..062ee94d2d26 100644
--- a/arch/x86/entry/vdso/Makefile
+++ b/arch/x86/entry/vdso/Makefile
@@ -163,7 +163,8 @@ quiet_cmd_vdso = VDSO    $@
 		 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)
+	$(call cc-ldoption, -Wl$(comma)--build-id) -Wl,-Bsymbolic $(LTO_CFLAGS) \
+	$(filter --target=% --gcc-toolchain=%,$(KBUILD_CFLAGS))
 GCOV_PROFILE := n
 
 #