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From:   George Rimar <grimar@accesssoftek.com>
To:     Tri Vo <trong@android.com>, "x86@kernel.org" <x86@kernel.org>,
        "tglx@linutronix.de" <tglx@linutronix.de>,
        "mingo@redhat.com" <mingo@redhat.com>,
        "bp@alien8.de" <bp@alien8.de>, "hpa@zytor.com" <hpa@zytor.com>
CC:     "dima@golovin.in" <dima@golovin.in>,
        "morbo@google.com" <morbo@google.com>,
        "ndesaulniers@google.com" <ndesaulniers@google.com>,
        "matz@suse.de" <matz@suse.de>, "ruiu@google.com" <ruiu@google.com>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH v3] x86_64: Add "-m elf_i386" when linking i386 object
 files.
Thread-Topic: [PATCH v3] x86_64: Add "-m elf_i386" when linking i386 object
 files.
Thread-Index: AQHUqemzjPdoQZ+FoUOJC0WuWoOLa6WrZ3sx
Date:   Sat, 12 Jan 2019 10:00:36 +0000
Message-ID: <1547286899727.75112@accesssoftek.com>
References: <CAKwvOdkR=1AzePbuFBcCW32shAK_cvR1iPiTGqHF=7MRH_oAjA@mail.gmail.com>,<20190111201012.71210-1-trong@android.com>
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Signed-off-by: George Rimar <grimar@accesssoftek.com>

Best regards,
George | Developer | Access Softek, Inc

________________________________________
??: Tri Vo <trong@android.com>
??????????: 11 ?????? 2019 ?. 23:10
????: x86@kernel.org; tglx@linutronix.de; mingo@redhat.com; bp@alien8.de; hpa@zytor.com
?????: George Rimar; dima@golovin.in; morbo@google.com; ndesaulniers@google.com; matz@suse.de; ruiu@google.com; linux-kernel@vger.kernel.org; Tri Vo
????: [PATCH v3] x86_64: Add "-m elf_i386" when linking i386 object files.

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From: George Rimar <grimar@accesssoftek.com>

Linux kernel uses OUTPUT_FORMAT in it's linker scripts. Most of the time
-m option is passed to the linker with correct architecture, but
sometimes (at least for x86_64) the -m option contradicts OUTPUT_FORMAT
directive. Specifically, arch/x86/boot and arch/x86/realmode/rm modules
have i386 object files, but are linked with -m elf_x86_64 linker flag
when building for x86_64.

"man ld" doesn't explicitly state any tie-breakers between -m and
OUTPUT_FORMAT. BFD and Gold linkers override -m value with
OUTPUT_FORMAT. But LLVM lld has a different behavior. 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

Suggested fix: just add correct -m after 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) it already overrides -m EMULATION with
OUTPUT_FORMAT.

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

Suggested-by: Dmitry Golovin <dima@golovin.in>
Signed-off-by: George Rimar <grimar@accesssoftek.com>
Signed-off-by: Tri Vo <trong@android.com>
Tested-by: Tri Vo <trong@android.com>
Tested-by: Nick Desaulniers <ndesaulniers@google.com>
---
v2: updated commit message to clarify that ld documentation is ambiguous w.r.t
    -m vs OUTPUT_FORMAT behavior.
v3: fixed/added SOB and "Tested-by" fields.

 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
--
2.20.1.97.g81188d93c3-goog