Can you add a signature to the kernel ELF image?

Can you add a signature to the kernel ELF image?

[Adam Nielsen]

Hi all,

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?

Many thanks,
Adam.

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

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

[David Daney]

Adam Nielsen wrote:
> Hi all,
> 
> 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?
> 

Edit the linker script (arch/mips/kernel/vmlinux.lds.S)

David Daney

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

[Ralf Baechle]

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

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

[Adam Nielsen]

> 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.

Ah excellent, that combined with the linker script (to twiddle load addresses) 
is just what I'm after.

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

Perfect, thanks for the example, that makes it much easier.

>> [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.

As far as I'm aware 0x20 means "ignore the CRC", I think the CRC itself is 
covered by one of the zero words.

Cheers,
Adam.