linux-kernel - Re: [PATCH] x86, realmode: explicitly set ENTRY in linker script

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Date:   Tue, 24 Sep 2019 11:32:06 -0700
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     Nathan Chancellor <natechancellor@...il.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] x86, realmode: explicitly set ENTRY in linker script

On Tue, Sep 24, 2019 at 11:24 AM Nathan Chancellor
<natechancellor@...il.com> wrote:
>
> On Mon, Sep 23, 2019 at 03:24:02PM -0700, 'Nick Desaulniers' via Clang Built Linux wrote:
> > Linking with ld.lld via $ make LD=ld.lld produces the warning:
> > ld.lld: warning: cannot find entry symbol _start; defaulting to 0x1000
> >
> > Linking with ld.bfd shows the default entry is 0x1000:
> > $ readelf -h arch/x86/realmode/rm/realmode.elf | grep Entry
> >   Entry point address:               0x1000
> >
> > While ld.lld is being pedantic, just set the entry point explicitly,
> > instead of depending on the implicit default.
> >
> > Link: https://github.com/ClangBuiltLinux/linux/issues/216
> > Signed-off-by: Nick Desaulniers <ndesaulniers@...gle.com>
> > ---
> >  arch/x86/realmode/rm/realmode.lds.S | 1 +
> >  1 file changed, 1 insertion(+)
> >
> > diff --git a/arch/x86/realmode/rm/realmode.lds.S b/arch/x86/realmode/rm/realmode.lds.S
> > index 3bb980800c58..2034f5f79bff 100644
> > --- a/arch/x86/realmode/rm/realmode.lds.S
> > +++ b/arch/x86/realmode/rm/realmode.lds.S
> > @@ -11,6 +11,7 @@
> >
> >  OUTPUT_FORMAT("elf32-i386")
> >  OUTPUT_ARCH(i386)
> > +ENTRY(0x1000)
> >
> >  SECTIONS
> >  {
> > --
> > 2.23.0.351.gc4317032e6-goog
> >
>
> This appears to break ld.bfd?
>
> ld:arch/x86/realmode/rm/realmode.lds:131: syntax error
> make[5]: *** [../arch/x86/realmode/rm/Makefile:54: arch/x86/realmode/rm/realmode.elf] Error 1
> make[4]: *** [../arch/x86/realmode/Makefile:20: arch/x86/realmode/rm/realmode.bin] Error 2
> make[3]: *** [../scripts/Makefile.build:509: arch/x86/realmode] Error 2

Thanks for testing.  Strange, it seems that ld.bfd doesn't like it as
an ENTRY in the linker script, but will accept `-e <addr>`.  Not sure
if that's a bug in ld.bfd, or if ld.lld should error as well?
https://access.redhat.com/documentation/en-US/Red_Hat_Enterprise_Linux/4/html/Using_ld_the_GNU_Linker/simple-commands.html
v2 inbound.
-- 
Thanks,
~Nick Desaulniers