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Re: Can you add a signature to the kernel ELF image?

To: Adam Nielsen <a.nielsen@shikadi.net>
Subject: Re: Can you add a signature to the kernel ELF image?
From: Ralf Baechle <ralf@linux-mips.org>
Date: Tue, 24 Nov 2009 23:44:06 +0000
Cc: linux-mips@linux-mips.org
In-reply-to: <4B0C625B.5070408@shikadi.net>
Original-recipient: rfc822;linux-mips@linux-mips.org
References: <4B0C625B.5070408@shikadi.net>
Sender: linux-mips-bounce@linux-mips.org
User-agent: Mutt/1.5.19 (2009-01-05)
On Wed, Nov 25, 2009 at 08:46:51AM +1000, Adam Nielsen wrote:

> I'm trying to port the kernel to an NCD HMX X-Terminal (MIPS R4600), but 
> the one thing I have to do before I can actually boot the image is attach 
> a signature to it. I have the signature 'code' in assembly[1], but I'm 
> not sure how to link it so that it ends up as the first bit of code in 
> the ELF image (the very first instruction is a 'b' to jump over the 
> actual signature text.)
>
> Without this the boot monitor will refuse to boot the kernel.  Any 
> suggestions as to how I might accomplish this?

Take a look at arch/mips/kernel/head.S.  This file will be the first on
the final linker call's command line, that is head.S's .text section will
end at the lowest address.

In head.S there is this

#ifdef CONFIG_BOOT_RAW
        /*
         * Give us a fighting chance of running if execution beings at the
         * kernel load address.  This is needed because this platform does
         * not have a ELF loader yet.
         */
FEXPORT(__kernel_entry)
        j       kernel_entry
#endif

ifdef.  Add your own magic stuff there, something like

#ifdef CONFIG_NCD_HMX
        b       1f
        nop
        nop
        .word   0x20
        .asciz  "XncdHMX"
        .word   0, 0, 0
#endif

> [1] http://www.linux-mips.org/wiki/HMX

The wiki page says something about a CRC but just poking a 0x20 into a
constant address is not exactly a CRC calculation.  Not sure how this
really is meant.

  Ralf

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