[tip:x86/build] x86/build: Specify elf_i386 linker emulation explicitly for i386 objects

[tip-bot for George Rimar]

Commit-ID:  927185c124d62a9a4d35878d7f6d432a166b74e3
Gitweb:     https://git.kernel.org/tip/927185c124d62a9a4d35878d7f6d432a166b74e3
Author:     George Rimar <grimar@xxxxxxxxxxxxxxxx>
AuthorDate: Fri, 11 Jan 2019 12:10:12 -0800
Committer:  Borislav Petkov <bp@xxxxxxx>
CommitDate: Sat, 12 Jan 2019 00:11:39 +0100

x86/build: Specify elf_i386 linker emulation explicitly for i386 objects

The kernel uses the OUTPUT_FORMAT linker script command in it's linker
scripts. Most of the time, the -m option is passed to the linker with
correct architecture, but sometimes (at least for x86_64) the -m option
contradicts the OUTPUT_FORMAT directive.

Specifically, arch/x86/boot and arch/x86/realmode/rm produce i386 object
files, but are linked with the -m elf_x86_64 linker flag when building
for x86_64.

The GNU linker manpage doesn't explicitly state any tie-breakers between
-m and OUTPUT_FORMAT. But with BFD and Gold linkers, OUTPUT_FORMAT
overrides the emulation value specified with the -m option.

LLVM lld has a different behavior, however. When supplied with
contradicting -m and OUTPUT_FORMAT values it fails with the following
error message:

  ld.lld: error: arch/x86/realmode/rm/header.o is incompatible with elf_x86_64

Therefore, just add the correct -m after the incorrect one (it overrides
it), so the linker invocation looks like this:

  ld -m elf_x86_64 -z max-page-size=0x200000 -m elf_i386 --emit-relocs -T \
    realmode.lds header.o trampoline_64.o stack.o reboot.o -o realmode.elf

This is not a functional change for GNU ld, because (although not
explicitly documented) OUTPUT_FORMAT overrides -m EMULATION.

Tested by building x86_64 kernel with GNU gcc/ld toolchain and booting
it in QEMU.

 [ bp: massage and clarify text. ]

Suggested-by: Dmitry Golovin <dima@xxxxxxxxxx>
Signed-off-by: George Rimar <grimar@xxxxxxxxxxxxxxxx>
Signed-off-by: Tri Vo <trong@xxxxxxxxxxx>
Signed-off-by: Borislav Petkov <bp@xxxxxxx>
Tested-by: Tri Vo <trong@xxxxxxxxxxx>
Tested-by: Nick Desaulniers <ndesaulniers@xxxxxxxxxx>
Cc: "H. Peter Anvin" <hpa@xxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: Michael Matz <matz@xxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: morbo@xxxxxxxxxx
Cc: ndesaulniers@xxxxxxxxxx
Cc: ruiu@xxxxxxxxxx
Cc: x86-ml <x86@xxxxxxxxxx>
Link: https://lkml.kernel.org/r/20190111201012.71210-1-trong@xxxxxxxxxxx
---
 arch/x86/boot/Makefile        | 2 +-
 arch/x86/realmode/rm/Makefile | 2 +-
 2 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/arch/x86/boot/Makefile b/arch/x86/boot/Makefile
index 9b5adae9cc40..e2839b5c246c 100644
--- a/arch/x86/boot/Makefile
+++ b/arch/x86/boot/Makefile
@@ -100,7 +100,7 @@ $(obj)/zoffset.h: $(obj)/compressed/vmlinux FORCE
 AFLAGS_header.o += -I$(objtree)/$(obj)
 $(obj)/header.o: $(obj)/zoffset.h
 
-LDFLAGS_setup.elf	:= -T
+LDFLAGS_setup.elf	:= -m elf_i386 -T
 $(obj)/setup.elf: $(src)/setup.ld $(SETUP_OBJS) FORCE
 	$(call if_changed,ld)
 
diff --git a/arch/x86/realmode/rm/Makefile b/arch/x86/realmode/rm/Makefile
index 4463fa72db94..96cb20de08af 100644
--- a/arch/x86/realmode/rm/Makefile
+++ b/arch/x86/realmode/rm/Makefile
@@ -47,7 +47,7 @@ $(obj)/pasyms.h: $(REALMODE_OBJS) FORCE
 targets += realmode.lds
 $(obj)/realmode.lds: $(obj)/pasyms.h
 
-LDFLAGS_realmode.elf := --emit-relocs -T
+LDFLAGS_realmode.elf := -m elf_i386 --emit-relocs -T
 CPPFLAGS_realmode.lds += -P -C -I$(objtree)/$(obj)
 
 targets += realmode.elf