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From: Borislav Petkov <bp@alien8.de>
To: Nick Desaulniers <ndesaulniers@google.com>,
	"H. Peter Anvin" <hpa@zytor.com>,
	Jarkko Sakkinen <jarkko.sakkinen@intel.com>
Cc: tglx@linutronix.de, mingo@redhat.com,
	clang-built-linux@googlegroups.com, x86@kernel.org,
	Tri Vo <trong@android.com>,
	Masahiro Yamada <yamada.masahiro@socionext.com>,
	Rob Herring <robh@kernel.org>,
	George Rimar <grimar@accesssoftek.com>,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH v2] x86, realmode: explicitly set entry via command line
Date: Wed, 25 Sep 2019 12:20:41 +0200	[thread overview]
Message-ID: <20190925102041.GB3891@zn.tnic> (raw)
In-Reply-To: <20190924193310.132104-1-ndesaulniers@google.com>

+ 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@google.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

  parent reply	other threads:[~2019-09-25 10:20 UTC|newest]

Thread overview: 13+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-09-23 22:24 [PATCH] x86, realmode: explicitly set ENTRY in linker script Nick Desaulniers
2019-09-24 17:30 ` Sedat Dilek
2019-09-24 18:24 ` Nathan Chancellor
2019-09-24 18:32   ` Nick Desaulniers
2019-09-24 19:33     ` [PATCH v2] x86, realmode: explicitly set entry via command line Nick Desaulniers
2019-09-24 19:37       ` Nick Desaulniers
2019-09-25 10:20       ` Borislav Petkov [this message]
2019-09-25 16:35         ` Nick Desaulniers
2019-09-25 17:02           ` Peter Smith
2019-09-25 17:10           ` Borislav Petkov
2019-09-25 18:09             ` [PATCH v3] x86, realmode: explicitly set entry via ENTRY in linker script Nick Desaulniers
2019-10-01 10:37               ` [tip: x86/boot] x86/realmode: Explicitly set entry point " tip-bot2 for Nick Desaulniers
2019-10-01 20:18               ` tip-bot2 for Nick Desaulniers

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