linux-kernel - Re: [PATCH v2] x86_64: Add "-m elf_i386" when linking i386 object files.

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Message-ID: <CAKwvOdkR=1AzePbuFBcCW32shAK_cvR1iPiTGqHF=7MRH_oAjA@mail.gmail.com>
Date:   Fri, 11 Jan 2019 11:27:31 -0800
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     Tri Vo <trong@...roid.com>
Cc:     x86@...nel.org, Thomas Gleixner <tglx@...utronix.de>,
        mingo@...hat.com, Borislav Petkov <bp@...en8.de>, hpa@...or.com,
        George Rimar <grimar@...esssoftek.com>,
        Dmitry Golovin <dima@...ovin.in>,
        Bill Wendling <morbo@...gle.com>, Michael Matz <matz@...e.de>,
        Rui Ueyama <ruiu@...gle.com>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] x86_64: Add "-m elf_i386" when linking i386 object files.

On Fri, Jan 11, 2019 at 11:14 AM 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.
>
> "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@...ovin.in>
> Signed-off-by: Georgii Rymar <grimar@...esssoftek.com>

misspelled, and you may carry forward my tested by tag from v1.

> Signed-off-by: Tri Vo <trong@...roid.com>
> Tested-by: Tri Vo <trong@...roid.com>
> ---

So when making a change, you can put any text below this `---`
delimiter.  For LKML, it's common practice to put something like:
v1 -> v2: updated commit message
That helps code review so reviewers know what to expect "inter-diff"
when eyeballing patch files.  Internally when using gerrit, it will
highlight inter-version diffs (ie. what changed between v2 and v3 of a
patch), but since we do things the hard way on LKML, this can help a
lot.  After `git format-patch ...`, I usually add such a comment to
the patch file in vim before sending.

>  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
>


-- 
Thanks,
~Nick Desaulniers