linux-kernel - [PATCH v2] x86: vdso: Use $LD instead of $CC to link

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Date:   Fri,  3 Aug 2018 10:39:31 -0700
From:   Alistair Strachan <astrachan@...gle.com>
To:     linux-kernel@...r.kernel.org
Cc:     Andy Lutomirski <luto@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        x86@...nel.org, kernel-team@...roid.com, joel@...lfernandes.org
Subject: [PATCH v2] x86: vdso: Use $LD instead of $CC to link

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.

This change drops references to DISABLE_LTO; this option doesn't seem to
be set anywhere, and not knowing what its possible values are, it's not
clear how to convert it from CC to LD flag.

Cc: Andy Lutomirski <luto@...nel.org>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: Ingo Molnar <mingo@...hat.com>
Cc: "H. Peter Anvin" <hpa@...or.com>
Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc: x86@...nel.org
Cc: kernel-team@...roid.com
Cc: joel@...lfernandes.org
Signed-off-by: Alistair Strachan <astrachan@...gle.com>
Acked-by: Andy Lutomirski <luto@...nel.org>
---
v2: Updated changelog and rediffed
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 261802b1cc50..42c6c1bea4f4 100644
--- a/arch/x86/entry/vdso/Makefile
+++ b/arch/x86/entry/vdso/Makefile
@@ -46,10 +46,8 @@ targets += $(vdso_img_sodbg) $(vdso_img-y:%=vdso%.so)
 
 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: $(obj)/vdso.lds $(vobjs) FORCE
 	$(call if_changed,vdso)
@@ -95,10 +93,8 @@ 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
 
 # x32-rebranded versions
 vobjx32s-y := $(vobjs-y:.o=-x32.o)
@@ -123,7 +119,7 @@ $(obj)/vdsox32.so.dbg: $(obj)/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
@@ -157,13 +153,13 @@ $(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
 
 #