linux-kernel - Re: [PATCH v2] x86, realmode: explicitly set entry via command line

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Message-ID: <20190925102041.GB3891@zn.tnic>
Date:   Wed, 25 Sep 2019 12:20:41 +0200
From:   Borislav Petkov <bp@...en8.de>
To:     Nick Desaulniers <ndesaulniers@...gle.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        Jarkko Sakkinen <jarkko.sakkinen@...el.com>
Cc:     tglx@...utronix.de, mingo@...hat.com,
        clang-built-linux@...glegroups.com, x86@...nel.org,
        Tri Vo <trong@...roid.com>,
        Masahiro Yamada <yamada.masahiro@...ionext.com>,
        Rob Herring <robh@...nel.org>,
        George Rimar <grimar@...esssoftek.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] x86, realmode: explicitly set entry via command line

+ some more people who did the unified realmode thing.

On Tue, Sep 24, 2019 at 12:33:08PM -0700, Nick Desaulniers 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>
> ---
> Changes V1 -> V2:
> * Use command line flag, rather than linker script, as ld.bfd produces a
>   syntax error for `ENTRY(0x1000)` but is happy with `-e 0x1000`
> 
>  arch/x86/realmode/rm/Makefile | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/x86/realmode/rm/Makefile b/arch/x86/realmode/rm/Makefile
> index f60501a384f9..338a00c5257f 100644
> --- a/arch/x86/realmode/rm/Makefile
> +++ b/arch/x86/realmode/rm/Makefile
> @@ -46,7 +46,7 @@ $(obj)/pasyms.h: $(REALMODE_OBJS) FORCE
>  targets += realmode.lds
>  $(obj)/realmode.lds: $(obj)/pasyms.h
>  
> -LDFLAGS_realmode.elf := -m elf_i386 --emit-relocs -T
> +LDFLAGS_realmode.elf := -m elf_i386 --emit-relocs -e 0x1000 -T

So looking at arch/x86/realmode/rm/realmode.lds.S: what's stopping
people from adding more sections before the first

. = ALIGN(PAGE_SIZE);

which, with enough bytes to go above the first 4K, would cause that
alignment to go to 0x2000 and then your hardcoded address would be
wrong, all of a sudden.

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette