Re: [PATCH][RFC] Add boot sector checksums to EDD

Re: [PATCH][RFC] Add boot sector checksums to EDD

[Matt Domsch]

On Wed, Nov 10, 2004 at 06:49:28AM +0100, Carl-Daniel Hailfinger wrote:
> Michael E Brown schrieb:

(copying lkml again)

> >    I'm curious, do you have a tool that uses the boot sector checksum? I
> > can think of a couple of places it could be useful, but all of my
> > use-cases are adequately covered by simple signature.
> 
> The tool is SUSE hwinfo. Currently its author does the checksumming
> in the bootloader which kills some BIOSes (1st EDD request in the
> bootloader is ok, second request by kernel EDD startup code hangs
> the machine).

I'd be curious to understand this better.  i.e. failure mode, why it
fails, for what systems, etc.
 
> I have some drives with identical mbr_signature (drives were wiped
> with dd before I got them). Using a checksum gives me at least some
> chance to identify them better because one has a bootloader, the
> other hasn't, they have different partition tables etc.

What wrote the bootloader to the first disk?  Can *that* write a
unique signature into the disk?  Red Hat's build of parted includes a
patch I wrote (not yet included in parted upstream IIRC) which has
parted write a unique signature to each disk, so that's how we solve
it there...

Thanks,
Matt

-- 
Matt Domsch
Sr. Software Engineer, Lead Engineer
Dell Linux Solutions linux.dell.com & www.dell.com/linux
Linux on Dell mailing lists @ http://lists.us.dell.com

[PATCH][RFC] Add boot sector checksums to EDD

[Carl-Daniel Hailfinger]

Hi Matt,

this first draft of a patch to implement boot sector checksums in EDD.
Since the comments in edd.S and edd.h discourage changing some offsets,
I made my code overwrite the /sys/firmware/edd/int13_dev*/mbr_signature
file contents. For a later patch I would suggest to add a mbr_checksum
file, so the two can coexist.
The patch is tested and didn't break anything on my machine so far.
Please comment.

Regards,
Carl-Daniel
http://www.hailfinger.org/

Signed-off-by: Carl-Daniel Hailfinger <c-d.hailfinger.kernel.2004@gmx.net>

--- ./linux-2.6.9/arch/i386/boot/edd.S~	2004-11-05 14:27:08.000000000 +0100
+++ ./linux-2.6.9/arch/i386/boot/edd.S	2004-11-06 23:55:23.000000000 +0100
@@ -5,7 +5,9 @@
  *   projects 1572D, 1484D, 1386D, 1226DT
  * disk signature read by Matt Domsch <Matt_Domsch@dell.com>
  *	and Andrew Wilks <Andrew_Wilks@dell.com> September 2003, June 2004
- * legacy CHS retreival by Patrick J. LoPresti <patl@users.sourceforge.net>
+ * disk checksum computation by Carl-Daniel Hailfinger
+ *	<c-d.hailfinger.kernel.2004@gmx.net> November 2004
+ * legacy CHS retrieval by Patrick J. LoPresti <patl@users.sourceforge.net>
  *      March 2004
  * Use EXTENDED READ calls if possible, Matt Domsch, October 2004
  */
@@ -88,10 +90,21 @@
 	movb	%dl, %bl			# copy drive number to ebx
 	sub	$0x80, %bl			# subtract 80h from drive number
 	shlw	$2, %bx				# multiply by 4
+	movw	%bx, %cx			# save bx
 	addw	$EDD_MBR_SIG_BUF, %bx		# add to sig_buf
 						# bx now points to the right sig slot
 	movl	(EDDBUF+EDD_MBR_SIG_OFFSET), %eax # read sig out of the MBR
 	movl	%eax, (%bx)			# store success
+	movw	%cx, %bx			# restore bx
+	addw	$EDD_MBR_CKSUM_BUF, %bx		# add to cksum_buf
+						# bx now points to the right cksum slot
+	xorl	%eax, %eax			# clear eax
+	movl	$0x7f, %ecx			# loop counter
+edd_mbr_compute_checksum:
+	xorl	EDDBUF(,%ecx,4), %eax		# compute checksum of mbr
+	decl	%ecx
+	jns	edd_mbr_compute_checksum	# until the checksum in complete
+	movl	%eax, (%bx)			# store success
 	incb	(EDD_MBR_SIG_NR_BUF)		# note that we stored something
 edd_mbr_sig_next:
 	incb	%dl				# increment to next device
--- ./linux-2.6.9/include/linux/edd.h~	2004-11-05 14:27:47.000000000 +0100
+++ ./linux-2.6.9/include/linux/edd.h	2004-11-06 23:43:20.000000000 +0100
@@ -50,6 +50,7 @@
 #define EDD_MBR_SIG_OFFSET 0x1B8  /* offset of signature in the MBR */
 #define EDD_DEV_ADDR_PACKET_LEN 0x10  /* for int13 fn42 */
 #define EDD_MBR_SIG_BUF    0x290  /* addr in boot params */
+#define EDD_MBR_CKSUM_BUF    0x290  /* addr in boot params */
 #define EDD_MBR_SIG_MAX 16        /* max number of signatures to store */
 #define EDD_MBR_SIG_NR_BUF 0x1ea  /* addr of number of MBR signtaures at EDD_MBR_SIG_BUF
 				     in boot_params - treat this as 1 byte  */