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

From: Nick Desaulniers
Date: Tue Jan 08 2019 - 15:46:32 EST

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> On Mon, Jan 7, 2019 at 3:11 PM Tri Vo <trong@xxxxxxxxxxx> wrote:
> >
> > Hello, hope everyone's had great holidays.
> > Could someone review at this patch?

Boris,
I assume Tri meant to post this to you in the To: line? I'm happy
with this patch, and am happy to add my reviewed by tag (in addition
to my tested-by below), but I think Tri meant to ask you. WDYT?

> >
> > On Tue, Dec 18, 2018 at 3:39 AM George Rimar <grimar@xxxxxxxxxxxxxxxx> 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
> >>
> >> ________________________________________
> >> ÐÑ: Tri Vo <trong@xxxxxxxxxxx>
> >> ÐÑÐÑÐÐÐÐÐÐ: 12 ÐÐÐÐÐÑÑ 2018 Ð. 22:48
> >> ÐÐÐÑ: Nick Desaulniers
> >> ÐÐÐÐÑ: bp@xxxxxxxxx; x86@xxxxxxxxxx; tglx@xxxxxxxxxxxxx; mingo@xxxxxxxxxx; George Rimar; Dmitry Golovin; Bill Wendling; hpa@xxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx
> >> ÐÐÐÐ: 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@xxxxxxxxxx> wrote:
> >> >
> >> > On Mon, Dec 10, 2018 at 2:50 PM Nick Desaulniers
> >> > <ndesaulniers@xxxxxxxxxx> wrote:
> >> > >
> >> > > On Mon, Dec 10, 2018 at 2:26 PM Tri Vo <trong@xxxxxxxxxxx> wrote:
> >> > > >
> >> > > > From: George Rimar <grimar@xxxxxxxxxxxxxxxx>
> >> > > >
> >> > > > 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@xxxxxxxxxx>
> >> > > > Signed-off-by: Tri Vo <trong@xxxxxxxxxxx>
> >> > > > Tested-by: Tri Vo <trong@xxxxxxxxxxx>
> >> > >
> >> > > 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@xxxxxxxxxx>
> >> > >
> >> > > 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

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