linux-kernel - Re: [RFC PATCH] x86_64: Add "-m elf_i386" when linking i386 object files.

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Message-ID: <CANA+-vCq6-RBAxqZO0yFXm5Vq7qRRzsZbX=OsgcaxasEbZcV6w@mail.gmail.com>
Date:   Mon, 7 Jan 2019 15:44:15 -0800
From:   Tri Vo <trong@...roid.com>
To:     George Rimar <grimar@...esssoftek.com>
Cc:     Nick Desaulniers <ndesaulniers@...gle.com>,
        "bp@...en8.de" <bp@...en8.de>, "x86@...nel.org" <x86@...nel.org>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "mingo@...hat.com" <mingo@...hat.com>,
        Dmitry Golovin <dima@...ovin.in>,
        Bill Wendling <morbo@...gle.com>,
        "hpa@...or.com" <hpa@...or.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "ruiu@...gle.com" <ruiu@...gle.com>
Subject: Re: [RFC PATCH] x86_64: Add "-m elf_i386" when linking i386 object files.

Re-sending as plain text. Sorry.

On Mon, Jan 7, 2019 at 3:11 PM Tri Vo <trong@...roid.com> wrote:
>
> Hello, hope everyone's had great holidays.
> Could someone review at this patch?
>
> On Tue, Dec 18, 2018 at 3:39 AM George Rimar <grimar@...esssoftek.com> wrote:
>>
>> Added Rui, an LLD code owner.
>>
>> This patch contains the approach we discussed earlier during LLD development when
>> faced this issue first time (and as a result of the discussion,
>> the same fix was suggested: https://bugzilla.kernel.org/show_bug.cgi?id=194091#c0),
>> so I think it is fine.
>>
>> Best regards,
>> George | Developer | Access Softek, Inc
>>
>> ________________________________________
>> От: Tri Vo <trong@...roid.com>
>> Отправлено: 12 декабря 2018 г. 22:48
>> Кому: Nick Desaulniers
>> Копия: bp@...en8.de; x86@...nel.org; tglx@...utronix.de; mingo@...hat.com; George Rimar; Dmitry Golovin; Bill Wendling; hpa@...or.com; linux-kernel@...r.kernel.org
>> Тема: Re: [RFC PATCH] x86_64: Add "-m elf_i386" when linking i386 object files.
>>
>> CAUTION: This email originated from outside of the organization. Do not click links or open attachments unless you recognize the sender and know the content is safe.
>>
>> Adding appropriate people, lists.
>> On Tue, Dec 11, 2018 at 1:07 PM Nick Desaulniers
>> <ndesaulniers@...gle.com> wrote:
>> >
>> > On Mon, Dec 10, 2018 at 2:50 PM Nick Desaulniers
>> > <ndesaulniers@...gle.com> wrote:
>> > >
>> > > On Mon, Dec 10, 2018 at 2:26 PM Tri Vo <trong@...roid.com> wrote:
>> > > >
>> > > > From: George Rimar <grimar@...esssoftek.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.
>> > > >
>> > > > BFD and Gold linkers are OK with this, but lld fails:
>> > > > 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 (it will also
>> > > > work with GNU ld, because it supports OUTPUT_FORMAT and just ignores -m
>> > > > options if this directive is in the linker script).
>> > > >
>> > > > Tested by building x86_64 kernel with GNU gcc/ld toolchain and booting
>> > > > it in QEMU.
>> > > >
>> > > > Suggested-by: Dmitry Golovin <dima@...ovin.in>
>> > > > Signed-off-by: Tri Vo <trong@...roid.com>
>> > > > Tested-by: Tri Vo <trong@...roid.com>
>> > >
>> > > This fixes the following linkage error I observe when linking an x86
>> > > kernel with LLD:
>> > >
>> > > ```
>> > >   LD      arch/x86/realmode/rm/realmode.elf
>> > > ld.lld: error: arch/x86/realmode/rm/header.o is incompatible with elf_x86_64
>> > > ld.lld: error: arch/x86/realmode/rm/trampoline_64.o is incompatible
>> > > with elf_x86_64
>> > > ld.lld: error: arch/x86/realmode/rm/stack.o is incompatible with elf_x86_64
>> > > ld.lld: error: arch/x86/realmode/rm/reboot.o is incompatible with elf_x86_64
>> > > ld.lld: error: arch/x86/realmode/rm/wakeup_asm.o is incompatible with elf_x86_64
>> > > ld.lld: error: arch/x86/realmode/rm/wakemain.o is incompatible with elf_x86_64
>> > > ld.lld: error: arch/x86/realmode/rm/video-mode.o is incompatible with elf_x86_64
>> > > ld.lld: error: arch/x86/realmode/rm/copy.o is incompatible with elf_x86_64
>> > > ld.lld: error: arch/x86/realmode/rm/bioscall.o is incompatible with elf_x86_64
>> > > ld.lld: error: arch/x86/realmode/rm/regs.o is incompatible with elf_x86_64
>> > > ld.lld: error: arch/x86/realmode/rm/video-vga.o is incompatible with elf_x86_64
>> > > ld.lld: error: arch/x86/realmode/rm/video-vesa.o is incompatible with elf_x86_64
>> > > ld.lld: error: arch/x86/realmode/rm/video-bios.o is incompatible with elf_x86_64
>> > > arch/x86/realmode/rm/Makefile:55: recipe for target
>> > > 'arch/x86/realmode/rm/realmode.elf' failed
>> > > ```
>> > > Tested-by: Nick Desaulniers <ndesaulniers@...gle.com>
>> > >
>> > > Looks like we still have a few other (unrelated) issues to track down
>> > > with LLD, but this gets us one step closer.
>> > >
>> > > Thanks for sending this, Tri!
>> >
>> > Just some additional thoughts on this:
>> >
>> > The kernel is adding -m elf_x86_64 to the linker command line
>> > parameters, but then relying on binutils tie-breaking-behavior by
>> > specifying a different architecture via linker script. So it's
>> > ambiguous which arch the kernel is trying to link for, and the kernel
>> > gets lucky/relies on the way ld.bfd happens to resolve this ambiguity
>> > without warning. An alternative fix may be to just filter out/remove
>> > the -m elf_x86_64 for this target's linker flags.
>> >
>> > >
>> > > > ---
>> > > >  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
>> > > > --
>> > > > 2.20.0.rc2.403.gdbc3b29805-goog
>> > > >
>> > >
>> > >
>> > > --
>> > > Thanks,
>> > > ~Nick Desaulniers
>> >
>> >
>> >
>> > --
>> > Thanks,
>> > ~Nick Desaulniers